Table Of ContentExcerpts from the Archive.digest
AEROELECTRIC LIST
http://www.matronics.com/rv-list/
Courtesy of:
Matronics Email Lists
c/o Matt Dralle
PO Box 347
Livermore, CA 94550-7227
Frequently Asked Electrical Questions posted to:
The AeroElectric List
and answered by
ROBERT (Bob) NUCKOLLS
www.aeroelectric.com
"Imagination is more important than knowledge. It is a miracle that curiosity survives formal
education"
Albert Einstein
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Hell, there are no rules here-- we're trying to accomplish something.
Thomas A. Edison
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Too often we enjoy the comfort of opinion without the discomfort of thought. ~
John F. Kennedy
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Experience and common sense cannot be replaced with policy and procedures.
R. L. Nuckolls III
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There is a great difference between knowing and understanding:
you can know a lot about something and not really understand it.
C.F. Kettering
"In theory there is no difference between theory and practice. In practice there is."
Yogi Berra
Contents
1. Alternators 2. Antenna 3. Avionics 4. Batteries 5. Circuit Brkr
6. Coax 7. Connectors 8. Contactors 9. Diodes 10. Fuses
11. Grounding 12. Impedance 13. Instruments 14. LED’s 15. Mag / EI
16. Motor Cntr 17. Primer, Eltr 18. Relay 19. Starters 20. Strobes
1. Alternators 2. Antenna 3. Avionics 4. Batteries 5. Circuit Brkr
21. Switches 22. Tools 23. 24. Wiring 25. Misc
Voltage/Reg
Selected messages through Jan 2003. Edited by Louie Holt, RV8, Florida.
Notation:
> Questions
A Responses by Bob Nuckolls
S Supplementary comments by other List Participants
1. ALTERNATORS
Alternator Load Anaylsis
> Bob . . .I had asked about how to do a load analysis? I have listed all loads and breaker
sizes. Some things are on periodically, some all the time. On your CD of the Lancair IV
electrical there is a page with the 2 busses and place for names and loads. It is filled in with
info for the Lancair. Is there a blank copy of that same form?
A Actually, I'm working on an article for Sport Aviation on this topic. Yes, the forms I show
in those diagrams are an example and a blank for doing your own system. The notion is that
you need to categorize load conditions for all flight modes. You need to fill in currents that
represent continuous running loads . . . transient loads like transmit, landing gear motors, etc
are included but noted as intermittent and then not included as part of the totals.
What you need to convince yourself of is that the alternator has plenty of oomph to carry
worst case loads and STILL have about 20 to 25% of its capacity left over to recharge the
battery. Of course, you do a separate analysis for each system if you have a dual system.
Perhaps two more analyses for a situation where one alternator of a dual system is down and
you've closed the cross-feed contactor to keep both buses up. And last, an analysis for an
essential bus to show what things are on for normal ops and what loads are on only during
battery only ops . . . again, it's an exercise to KNOW how much you can depend on limited
resources of electrical energy under various operating conditions.
Of course, it's insufficient to just add up the breakers . . . .breaker size is only loosely
related to running loads. You need to get out the data for the equipment you plan to install and
look up the rated loads or actually measure it with an ammeter on your airplane. I'll see if I can
get a blank form published this evening that folks can use to facilitate the analysis tasks on their
projects. Bob . . .
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Load Analysis
A I've yet to see a single engine airplane homebuilt that won't run well on a 40A machine. The
largest full-up, night-ifr running load I've calculated so far is 27A which leaves 13A for battery
support. Bob:
> Using the above logic, the Z appendix drawings that call for awg 4 or 5 wire for the
alternator output seems to be more than required. Assuming even a 100% increase in load for
a short battery charging period, would not awg 8 be adequate? Do you recommend the larger
wire and 80 amp in line fuse to protect against a potential battery feed short in this wire?
A 4AWG was a one-size-fits all that was mated with the terminals supplied in our b-lead
fuse kit of some years past. It wasn't that 4AWG was necessary, just that it didn't hurt anything
and will mate with the hardware in the kit. 8AWG would be fine too. I'm going to be taking the
b-lead fuse kit off the website catalog and replacing it with ANL current limiters and the
appropriate mounting base. When this is done, customers will supply their own terminals to
attach to the mounting base and can size the alternator b-lead accordingly. Bob . . .
------
Load Anaylsis
> Bob, where or how do I find the current requirements for the different items on my electrical
diagram. Most items give a breaker size only that I can see. Radios must use much less when
receiving than when sending, for instance. Any direction on this matter? >Jim
A This information should be available from the manufacturer of the item or calculated
from it's rating. For example, a 55W landing light bulb would draw 55(w)/13(v)=4.2Amps. A
100W pitot tube will need 100/13 = 7.7A Or you get the data from the manufacturer's literature
for the product. Radios, pumps, etc should have information in the tech manual about how much
the device draws under various operating conditions. Bob . . .
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Load Analysis, Electric Heat
> Hi, Im building an RV-8 and there is a problem getting heat to the rear cockpit so im installing
an electric heater which draws 25 amps. I would like to wire the heater through the landing/taxi
switches so that the heater cannot be on when the lights are turned on to avoid over loading the
alternator and possibly blowing the current limiter at night close to the ground. Do you have any
suggestions as to what switch I should use or is the idea not feasable. thanks for your time, I
have sure enjoyed your book.
A The idea is feasible but not necessary. First, unlike generators, alternators are inherently
current limited in their ability to put be loaded severely beyond rated capacity. When an
alternator becomes overloaded, the bus voltage simply sags down to the point were the battery
makes up the difference. Sooo . . . an alternator is not capable of blowing it's own properly sized
b-lead protection. Should the condition you're concerned about come to pass, your low-voltage
warning light should begin flashing to let you know that some loads need to be shed. Bob . . .
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Re: Load Analysis, light watts/amps ratings
>Maybe this is nit-picky for the purpose of load analysis, but if I want to determine how many
amps a 100 W landing light will draw, do I use 12 V or 14 V in the equation? If I assume it's
rated at 12 V, then it will draw 8.33 A. Whereas if it's rated at 14 V which would be the norm for
a car or airplane when the alternator's working, it will draw 7.14 A which is enough difference to
be worth noting. Is there an industry standard for how bulbs are rated?
A Oh, sorta . . . take a peek at this catalog of lamps. Note that lamps for 14 volt vehicles
are rated at 12.0 12.8 and 13.0 volts.
http://www.lighting.philips.com/nam/product_database/pdf/mini_seal.pdf
Have no idea why the variable nor why they choose any particular value. It's nice that they
DON'T rate them at 13.8 or 14.0 volts because with larger lamps in particular, you don't want to
install wiring so large that the voltage drop to the lamp is near zero . . . Note that the 4509
lamps found on lots of single engine aircraft is rated at 100W at 13.0 volts. When in doubt, go
to the engineering data. Keep a copy of that lamp catalog .pdf file I cited above. I've had a
number of paper equals to it for years and now I have one on my hard drive . . .Bob . . .
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Re: Load Analysis Flap motor amps?
> Anybody know approx how many amps Vans flap motor draws? I know it's only an
intermittent load, but I'm just curious for fuse and relay sizing...
A I've not measured it but I've been told that it maxes out at 5A full load. Further, the way
it's used in the RV's doesn't begin to load it up severely. Bob . . .
S In the VANS supplied wiring loom they protect the flap circuit with a 5 Amp breaker so
clearly it is pulling less than that. Steve
S I looked it up on the comapany's web site one day and if memory serves correctly the
current draw was around 3A with a 100lb load. Should be less than that for an RV.
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Re:Load Analysis, Ray Allen trim servos
> The Lancair manual says that the trim system is 5amps. Is this for the whole trim system?
A The Ray Allen servos draw less than 0.5A full load. And, yes, much to my dismay they
do wire their product with ()#*$%)(U(%Z too small a wire. I've had numerous discussions with
them at their booth at OSH and over the phone. Seldom have I encountered such a lack of
understanding and unwillingness to accommodate from a supplier of parts to the OBAM market.
To install it in your airplane, you can ignore the wire size they use. Further, it's a good idea to
question anyone's recommendations for breaker size unless they're willing and able to explain
the rational for the recommendation.
First, I wouldn't recommend wiring anything in your airplane with less than 22AWG wire.
We recently included 24 AWG in production wire bundles and the folks on the production line
have nothing good to say about it. 22AWG is the practical lower limit for size strictly from
viewpoint of working with the wire. If you use 22AWG a 5A breaker is obviously appropriate for
protection of the wire. However, based on current draw of the actuator and the use of some
smaller wires in the system I recommend a 1A fuse.
Don't mean to imply that anyone would have "trouble" with properly installed wire of any
size. Just because the device is fitted with unusually small lead wires from the factory in no way
obligates the builder to use the same size wire to extend the leads for remainder of the
installation. Bob . . .
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Re: Load Analysis; Facet Electric fuel pump ?
What is the current draw on a Facet fuel pump?
S I have a Facet , the inrush current on switch-on is 1.2 Amps, it stabilises on 0.9 Amps
during continouse run (wet). I do not know, what your pumps type is, mine is a Facet 40108.
Hope it helpes Werner
S The 478360 draws 1 amp max. The 40108 draws 1.4 amps max. Regards Craig
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Re: Alternator fuse
>Bob, For the 40 amp alternator is the 80 amp fuse the right size to use or should I look for a 60
amp? Dave Ford
A The JJS/JJN series fuses are very fast acting and were deliberately oversized for
alternator b-lead applications to prevent nuisance tripping. An JJS/JJN-80 fuse is fine for
alternators from 35 to 60A. Given low cost availability of the ANL series limiters and in light of
their VERY robust overcurrent carrying behavior, you'll want to size the limiter closer to the
rated output of the alternator. ANL limiters are available in a variety of sizes which you can
check out by downloading this data sheet. http://www.bussmann.com/library/bifs/2024.pdf
Note these devices are designed to clear HARD faults in the hundreds of amperes . . .
exactly the type of fault to be expected in the alternator b-lead circuit. B&C stocks the ANL40
and ANL60 parts. Bob . . .
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Re: ANL Sizing
> Bob: I'm planning on using the B&C 40 Amp alternator. I notice that in the B&C catalog it
says that the 40 Amp ANL is appropriate for use with up to a 40 Amp alternator. Is this correct,
or should I use the 60 Amp ANL?
A 40 is fine. Bob . . .
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Re: Maxi Fuse for Alternator B lead
> Hi Bob, I am looking for a way around the $40 fuse set up. I found the Maxi Fuse holder with
a 60amp fuse, It comes with an 8-10awg wire. Can this setup be used reliably or am I asking
for trouble? My concerns are that I will be running the 4awg wire from the alternator and then
reducing to the 8-10awg wire to connect to the starter contactor. Have I restricted the load
carrying capabilities to the smaller wire? Is that bad? The person that I talked to said that if it
comes with a 60amp fuse it will certainly handle it. It made sense but I thought I should get your
opinion.
A Go for it. Keep the wire gage constant. Install the MAX60 and holder as close as
practical to the starter contactor. Extend the other lead of the fuseholder with the same size wire
and run it to the alternator b-lead. This is not quite as robust as the b-lead fuse kits or ANL
limiters . . . but I think it will probably perform well for you. Bob . . .
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Proposed E-bus loads using Z-13
I've been thinking about scrapping the idea of having pitot heat on my E-bus and going with the
SD-8 rather than the SD-20 for the second alternator. I wonder if folks could comment on using
the SD-8 with the following tentative load requirements. All information on current comes from
manufacturers' specs or from panel planner, although I used the same number for the typical
and maximum loads when only one number was given. My proposal:
Typical Maximum
UPS SL-15 Audio Panel .35 1.5
UPS SL-30 Nav/Com
Com Receiver (continuous?) .27 2
Com Transmitter (continuous?) 2.1 3.2
Nav Receiver .325 .5
UPS SL-70 Transponder .5 1.4
TruTrak Autopilot (1 axis on) 2 2
RC Allen Electric AI 1.2 1.2
Electric Turn Coordinator .35 .35
VM-1000 Engine Instruments .3 .3
EI Fuel gauge .1 .1
Panel & Reading Lights (guestimate) .2 .2
Totals 7.695 12.75
I could substitute the UPS GX-60 GPS/Com for the SL-30 with about a .2 amp increase, but I
figured I would have my battery-operated handheld GPS on board. Plus, I can live without the
GPS heading information because the TruTrak has its own DG. So the typical load on the SD-8
would be just under 8 amps, right near its rating, and the maximum load (if everything happened
at once I guess) would approach 13 amps, which is more than the alternator can handle.
Three questions: 1) Could I run this load on the SD-8 with the battery contactor open to save the
battery for the landing phase? 2) If the advice would be to run this with the battery contactor
closed, how fast would the battery draw down? 3) Would you advise against this? If so, why,
and what alternative would you suggest? Thanks in advance, Dan
A .35A continuous draw for the audio panel seems high. Also the 2.1A value for the
transceiver . . . Transceivers are just receivers except when the mic button is pushed so I would
expect the standby current to be on the order of .2 to .5A for any transceiver. The autopilot will
be pretty low unless you're in turbulence so all-in-all, the list you have presented looks
comfortably doable. If at any time one is running battery/SD-8 combination, watch take a peek
at the voltmeter (or have lv warning on the e-bus too). If you can't keep the bus high enough to
unload the battery while en route . . . shut some things off. I think you'll find that the SD-8 is
really an SD-10. It's got some headroom over the advertised 8A rating. Even if you CAN'T
keep the bus voltage up, then only the difference between SD-8 capability and loads will come
from the battery. If that's only an amp or two, you still have tremendous endurance.
You can't hurt it. The output simply sags and the battery takes up the slack. These are
transient conditions that do not figure into the total energy requirements to get where you're
going.
> Three questions: 1) Could I run this load on the SD-8 with the battery contactor open to
save the battery for the landing phase?
A This is the normal mode of operation for main-alt-out conditions.
> 2) If the advice would be to run this with the battery contactor closed, how fast would the
battery draw down? 3) Would you advise against this? If so, why, and what alternative would
you suggest?
A Contactor open until airport in sight. Then close the contactor to run whatever goodies
are deemed necessary or desirable to complete the flight. Bob . . .
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S Re: Maxi Fuse for Alternator B lead
>>In a message dated 1/10/02 you said “ Install the MAX60 and holder as close as practical to
the starter contactor.” Now you are confusing me (not hard to do BTW). I thought the idea was
to always locate the wire protector (fuse) as close as possible to the source of current, in this
case the alternator. What have I missed?
A The alternator is NOT the source of current that opens this fuse . . . Alternators are
physically incapable of putting out much more than their design limit with respect to current (not
so for voltage . . . you can get 100V+ from a runaway alternator). The current source that might
antagonize the alternator b-lead is the BATTERY . . . good for 700-1500 amps in a fault
condition through fat wires. Hence, the alternator b-lead protection goes at the end of the wire
opposite the alternator connection. Consider that the alternator b-lead breaker has always been
right at the bus even on the spam cans. Bob . . .
--------
Re: current limiter in b-lead
Bob, Your new Fig. Z-22 shows an ANL current limiter in the b-lead when using a Skytec
starter. I assume this wire run would only go from the alternator to the built-in starter solinoid
and thus be fairly short.(Reference your fig. Z-14, rev.8) Would the circuit protection still be
necessary and if so at which end to protect which device/length of wire?
A Yeah . . . we're concerned about shorted diodes in the alternator . . . here the source of
power that blows the fuse is the battery.
>> Without a PM starter system doesn't your circuit protection get put near the normal starter
solenoid?
A Yes, because the b-lead needs protection from battery. With the PM starter, the b-lead
is so short (rear of alternator to fat terminal on starter) that it's more a matter of mechanical
convenience as to how the fuse is installed. For this application, I think I'd stay with the fast
JJN/JJS series fuses covered with heat shrink . . . this combo is a bit easier to work with in the
small space and closer quarters behind the starter and alternator. Bob . . .
----------
Alternator Switch
>> Bob: Sorry if I'm the 10,000th guy to ask this, but on page 11-20 of your book you advise
using a 2-3 switch to control battery master contactor AND alt. field current simultaneously, but
you don't say why. It always made sense to me to be able to disconnect the alt. field coil
separately in order to avoid wasting power into it when the alternator fails and you're running off
the battery only. The Z-1 schematic shows the same switch.
A Most of the folks building MODERN single engine airplanes have an essential bus that is
powered independently of the main bus. If the alternator fails, one wants to shut down the main
bus which takes care of the alternator field loads -AND- battery contactor. If there's a need to
keep the main bus up via the battery contactor, then you can pull the alternator field breaker
while leaving the DC power master switch ON. This is most often done during maintenance ops
on the ground to minimize battery loads.
Another option is to wire the DC POWER MASTER with an S700-2-10 switch - two pole,
three position, progressive transfer. With this switch, one can emulate the features of the
mystical-magic split-rocker switch that's found on many certified ships and too many homebuilts.
The lower position is OFF, first position is BATTERY only, the second position adds the ALT
FIELD. This is the switch shown in the latest incarnations of the 'Connections power distribution
diagrams at: http://aeroelectric.com/articles/Rev10/z10.pdf
>Are you depending on the B&C voltage regulator to not power the field coil when the alt. dies?
A Nope, it's all done with switches as described above. Bob . . .
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Alternator Relays and Starter Contactors
>Why do we still use remote starter solenoids with modern PM starters that have a solenoid
built in? Anybody seen a modern car that had a second, remote solenoid? Nope. If the starter
solenoid were to stick, you could just kill the master solenoid to deactivate it.
A The built in starter contactors have VERY high inrush currents on the order of 30A or
more. You CAN use them as the sole control for starter operations but it's very hard on your
starter engagement switch. See http://aeroelectric.com/articles/strtctr.pdf
>About internally-regulated alternators: In looking at the wiring diagrams in my Aeroelectric
Connection book, I see that Bob recommends putting a relay in series with the output B-lead so
that the unit can be isolated from the rest of the wiring. Why? If you cut the power to the field
terminal, how can the alternator put out anything? Is it so that in case the internal regulator
shorts out somehow and full-fields the unit, it can still be isolated?
A The little wire going into the back of internally regulated alternators does not supply field
current. It's only a control lead to the electronics of the regulator. There are failure modes in
most internally regulated alternators that CANNOT be controlled externally . . . hence the need
for a disconnect relay in the b-lead.
> Also, what's the wisdom of putting a fuse in the B-lead? I thought that an alternator can only
put out as much current as it's rated for, since at that point it reaches maximum magnetic flux.
A The fuse is to protect the system and wiring from shorted diodes in the alternator which
can produce a hard fault to ground good for many hundreds of amps and much smoke. It's and
increasingly rare event with modern alternators . . . but I wouldn't bet the rate is zero. The limiter
is cheap insurance. Bob . . .
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Re: Protection for alternator field circuit and ammeter
>Drawing Z24 shows how to add an OV protection module for an internally regulated power
supply. It requires the use of a 5A circuit breaker and also of a fuselink attached to the main
connection of the main bus. The question: Can we replace the fuselink by using one of the
outputs on the main bus protected by a larger fuse (say 15amps)? I would like for sure the
breaker to let go before the fuse.
A Some 5A breakers will open a 15A fuse. Fuses are MUCH faster than breakers. This is
why I prefer the fusible link in this slot.
>On another subject, I will be using an ampere-meter, but I don't know where to connect the
shunt. Some of the Z diagrams indicate where to install a load-meter which would provide load
information for the alternator only when it is functioning.
Yup . . .
> An ampere-meter provides + and - amperes to/from the battery (I suppose). With a
dual-battery/battery bus installation, it seems there is no single place where it would make
sense to put the ampere-meter. If I install it between the battery contactors and the
alternator/main bus, it won't measure the load for the ignitions/fuel pumps/essential bus when
on alternate feed to e-Bus.
A Loadmeters only read 0 to some value either in AMPS full scale (or in the case of the
ones I sell, 0-100% of alternator output). See:
http://www.aeroelectric.com/Catalog/instrmnt/loadvolt.jpg
. . . . If you're down to battery only operations, the need for an ammeter (which was small to
begin with) goes to zero . . . you should plan in advance and KNOW what things you can leave
running in order to achieve what ever endurance you've chosen for yourself with battery only
operations. For myself, a "30-minute 'emergency' battery" is just that . . . a battery that is
incapable of preventing an emergency in all cases. A lightly loaded battery of known condition
will allow you to comfortably use all fuel aboard so that if you're going to have an "emergency" it
ain't gonna happen 'cause the panel went dark. Given that the loads under battery only ops are
predicted, the ammeter is unnecessary. It is, however, a useful tool for assessing the health of
the alternator and trouble shooting the system when things are not behaving as you wish. Bob .
. .
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Re: Question on Dual Alt/Single Battery
>> I am planning on using the SD-20 so I can have a little more juice in the event of main
alternator failure (to cover pitot heat, for example.) How should Z-13 be modified to wire in the
SD-20 instead of the SD-8?
A How about a variation on Z-14. Use S704-1 relays for a low current (30A) cross-feed,
and as the battery relay for a small (6 a.h. or so) battery to stabilize the SD-20 when the
cross-feed is open. Use the Aux Bus as you would the E-bus and put all the main-alt-out en
route loads on the Aux Bus. You wouldn't close the cross-feed during cranking in this case so a
single pole, single throw cross-feed switch would suffice. Bob . . .
-------
Alternator, capacitor
>> As long as you are using the same MFD rating, you should use a higher voltage rating to
keep voltage spikes from destroying the capacitor.
A "spikes" of the capacitor destroying variety don't exist in our systems. All capacitors have
a short duration "surge" rating that exceeds their rated operating voltage by about 25% but even
this capability is seldom tested in our systems. In fact, it is the capacitor's duty to FILTER off
what some folks refer to as "spikes" and other sources of noise.
> The physical size may be the deciding factor as you might have a limited space. I would use
the best cap with the right MFD but as at least 3 to 10 times the voltage rating for longevity as
the nominal rating of the device, (12 volts?)
>I'm confused and wanted to check on the required voltage rating for the electrolytic filter cap
for the Rotax (and other permanent magnet alternators). Rotax call for a 25V cap. Most of the
schematics in appendix Z also call for a 25V cap. Note 20 say's the voltage requirement ranges
from 15V to 50V. The 47,000uFd cap B&C sells (PN S251D479) and recommends for Rotax is
rated at 16V. Is a 16V capacitor acceptable in this application?
A You betcha! The general rule of thumb for selecting the capacitor size for a pm
alternator is 1,000 uF per amp of output . . . but having one larger than this can't do anything but
make the output smoother and quieter. Soooo . . . 10,000 uF or larger is okay for an SD-8,
18,000 or larger is appropriate to the 912/914 Rotax. 47,000 will work for any of them and is a
practical upper limit for physical size . . . bigger capacitors are harder to mount, more expensive
and will add only marginally to performance.
The biggest stress on capacitors of this type is RIPPLE CURRENT (the noise that is
inherent in the output of PM alternator) . . . increasing the voltage rating is of no assistance in
making the capacitor last longer . . . in fact, for the same size capacitor in uFD, the device's
internal impedance will go UP with voltage rating which in turn adds to internal heating. All
other things being equal, buy capacitors with smaller diameter and longer cases . . . small
diameters are easier to find clamps for and easier to mount. Smaller diameters also dump out
internal heat better. Aluminum electrolytic capacitors are life limited critters. I'd replace them
about every 200 hours of operation for best performance. Bob . . .
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Description:Re: Load Analysis Flap motor amps? > Anybody know In the VANS supplied wiring loom they protect the flap circuit with a 5 Amp breaker so clearly it is pulling
