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Wire sizing.

 
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dean.psiropoulos(at)veriz
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PostPosted: Mon Feb 06, 2006 10:32 pm    Post subject: Wire sizing. Reply with quote

Lectric Bob:

I've been doing the wiring diagrams for my all electric RV-6 and using some
of your autocad stuff on the AEC CD as examples and starting points. Since
I'm just hacking at Autocad these help get me going faster than if I had to
start from scratch, thanks for sharing your hard work with us. Couple
questions on wire sizing. I originally bought the Whelen strobe kit (A600)
that has both the side and aft position lights with strobe in each wingtip
mounted assembly (no tail position required). I later decided to buy the
combination tail position light/strobe combo as an addition in order to help
people see me better from the rear.

I now have 5 position lights at 2.2 amps each for a total of 11 amps of load
on the circuit. I would like to run them all from one switch and have ONE
wire running from the switch down through the center console of the airplane
through the spar to a distribution block under the floor. I've been
scratching my head a little and reading and re-reading the section in your
book about "Wire Selection and Installation". Using the chart for temp rise
vs wire ga vs current capacity, it looks like a 16 GA wire would have appx a
30 deg C temp rise with 11 amps flowing through it. If I convert that to an
appx Farenheit equivalent it becomes something like an 86 deg temp rise. If
ambient is a Florida summer with the OAT at 90 deg (F) and I add these two
temps together I get 176 degrees (F) temp in free air with 11 amps in the 16
GA wire, correct?

Running this wire down through the tight confines of my center console means
that it will be near several other wires and without lots of cooling air so
I would assume that one would see the full 176 degrees around that wire in
that confined space. This is not beyond the capabilities of the Tefzel
wire I'm using but still very hot to the touch and warmer than I feel
comfortable with. So if I drop down to a 14 GA wire it looks like the temp
rise would be appx 13 deg C or 55 F which gives about a 145 deg wire temp in
free air. The is still going to be pretty warm to the touch and I'm
thinking maybe it should be derated a bit more but don't really want to run
a 12 GA wire and am not 100% sure it's needed (although on a night flight
that wire will see the full 11 amps for the entire period of time).

Are my assumptions here correct and am I really going to experience 176 deg
wire temp with 11 amps in a 16 GA wire on a 90 deg day? If so do you think
dropping to 14 GA would be sufficient or should I make it larger? Since I
don't really want to run a 12 GA wire through the tight space would two 16
or 14 GA wires work equally as well or would the current shared by each be
unequal with the resultant heating of each wire also unequal (assuming they
are tied together at both ends)? Is there a good "rule of thumb" for wire
sizing with respect to temp rise (ie. Equipment that runs continuously
should not have wire temps higher that would be uncomfortable to touch or
some such thing). Since this wire is only about 6 feet long the voltage drop
through it shouldn't be a significant factor that I see (4/1000 ohms per
foot*6 feet*14 volts = .33 volts) so that leaves temperature.

Also, I see on your diagrams for landing and recognition lights you show a
resistor of unknown value between the ground connection and the light. I
assume this is simply there to remind us that there is some loss in lines
that long and also some loss in the return line (aircraft structure). Is
this assumption correct or is there a reason for the resistors there? Any
other word of wisdom would be most welcome. Thanks.

Dean Psiropoulos
RV-6A N197DM
Knee deep in Autocad and wire


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nuckollsr(at)cox.net
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PostPosted: Tue Feb 07, 2006 8:32 am    Post subject: Wire sizing. Reply with quote

At 01:28 AM 2/7/2006 -0500, you wrote:

Quote:

<dean.psiropoulos(at)verizon.net>

Lectric Bob:

I've been doing the wiring diagrams for my all electric RV-6 and using some
of your autocad stuff on the AEC CD as examples and starting points. Since
I'm just hacking at Autocad these help get me going faster than if I had to
start from scratch, thanks for sharing your hard work with us. Couple
questions on wire sizing. I originally bought the Whelen strobe kit (A600)
that has both the side and aft position lights with strobe in each wingtip
mounted assembly (no tail position required). I later decided to buy the
combination tail position light/strobe combo as an addition in order to help
people see me better from the rear.

<snip>

My apologies for running you thorough the 'rat race' with bad
data. The temperature rise data in Chapter 8 lost something in
the translation a few years ago when I crafted the figures. A number
of readers pointed this out over the years . . . in fact, I'd
prepared some corrected Figures for posting on the website but
never got them finalized and posted. Here they are:

http://www.aeroelectric.com/articles/Figures_8-3_8-4_Errata.pdf

With the correct data, you'll find that your 16AWG choice for
the position lights will be just fine.

Quote:
Also, I see on your diagrams for landing and recognition lights you show a
resistor of unknown value between the ground connection and the light. I
assume this is simply there to remind us that there is some loss in lines
that long and also some loss in the return line (aircraft structure). Is
this assumption correct or is there a reason for the resistors there? Any
other word of wisdom would be most welcome. Thanks.

Those are inrush limiters. See
http://www.ametherm.com/Inrush_Current/welcome.html

and

http://www.ametherm.com/Data%20Sheets/SL32%2010015.pdf

These a thermistors with strong negative temperature
coefficients that will greatly attenuate the 10-70x
inrush currents to large lamps. The part cited above
is a good one for lamps drawing 7-15 amps. You can buy
these from Digikey . . .

http://dkc3.digikey.com/PDF/T061/1328.pdf
I appreciate your bringing the topic up again. It prompted
me to get my homework done on the errata sheet. I'm putting
chapter 8 on the to-do list for Revision 12. Thanks!

Bob . . .


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william.p.dube(at)noaa.go
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PostPosted: Tue Feb 07, 2006 11:10 am    Post subject: Wire sizing. Reply with quote

Paralleling small gage wires is not a good practice. If one becomes
disconnected, the other gets too hot. No way to tell that this is
occurring (other than the resultant fire.)

16 gage wire would allow a 15 amp breaker. 11 amps would be 73% of the
breaker rating. Since the load is continuous, 80% maximum is prudent.

The voltage drop for 11 amps in a 20 foot 16 gage wire could be as much
as a volt. This could make the lights a touch dim.

If you run 14 gage wire, you can use a 20 amp circuit breaker, and
the voltage drop will be only 0.6 volts. Sounds like the better plan to me.

DEAN PSIROPOULOS wrote:

Quote:


Lectric Bob:

I've been doing the wiring diagrams for my all electric RV-6 and using some
of your autocad stuff on the AEC CD as examples and starting points. Since
I'm just hacking at Autocad these help get me going faster than if I had to
start from scratch, thanks for sharing your hard work with us. Couple
questions on wire sizing. I originally bought the Whelen strobe kit (A600)
that has both the side and aft position lights with strobe in each wingtip
mounted assembly (no tail position required). I later decided to buy the
combination tail position light/strobe combo as an addition in order to help
people see me better from the rear.

I now have 5 position lights at 2.2 amps each for a total of 11 amps of load
on the circuit. I would like to run them all from one switch and have ONE
wire running from the switch down through the center console of the airplane
through the spar to a distribution block under the floor. I've been
scratching my head a little and reading and re-reading the section in your
book about "Wire Selection and Installation". Using the chart for temp rise
vs wire ga vs current capacity, it looks like a 16 GA wire would have appx a
30 deg C temp rise with 11 amps flowing through it. If I convert that to an
appx Farenheit equivalent it becomes something like an 86 deg temp rise. If
ambient is a Florida summer with the OAT at 90 deg (F) and I add these two
temps together I get 176 degrees (F) temp in free air with 11 amps in the 16
GA wire, correct?

Running this wire down through the tight confines of my center console means
that it will be near several other wires and without lots of cooling air so
I would assume that one would see the full 176 degrees around that wire in
that confined space. This is not beyond the capabilities of the Tefzel
wire I'm using but still very hot to the touch and warmer than I feel
comfortable with. So if I drop down to a 14 GA wire it looks like the temp
rise would be appx 13 deg C or 55 F which gives about a 145 deg wire temp in
free air. The is still going to be pretty warm to the touch and I'm
thinking maybe it should be derated a bit more but don't really want to run
a 12 GA wire and am not 100% sure it's needed (although on a night flight
that wire will see the full 11 amps for the entire period of time).

Are my assumptions here correct and am I really going to experience 176 deg
wire temp with 11 amps in a 16 GA wire on a 90 deg day? If so do you think
dropping to 14 GA would be sufficient or should I make it larger? Since I
don't really want to run a 12 GA wire through the tight space would two 16
or 14 GA wires work equally as well or would the current shared by each be
unequal with the resultant heating of each wire also unequal (assuming they
are tied together at both ends)? Is there a good "rule of thumb" for wire
sizing with respect to temp rise (ie. Equipment that runs continuously
should not have wire temps higher that would be uncomfortable to touch or
some such thing). Since this wire is only about 6 feet long the voltage drop
through it shouldn't be a significant factor that I see (4/1000 ohms per
foot*6 feet*14 volts = .33 volts) so that leaves temperature.

Also, I see on your diagrams for landing and recognition lights you show a
resistor of unknown value between the ground connection and the light. I
assume this is simply there to remind us that there is some loss in lines
that long and also some loss in the return line (aircraft structure). Is
this assumption correct or is there a reason for the resistors there? Any
other word of wisdom would be most welcome. Thanks.

Dean Psiropoulos
RV-6A N197DM
Knee deep in Autocad and wire










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nuckollsr(at)cox.net
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PostPosted: Tue Feb 07, 2006 11:49 am    Post subject: Wire sizing. Reply with quote

At 12:01 PM 2/7/2006 -0700, you wrote:

Quote:


Paralleling small gage wires is not a good practice. If one becomes
disconnected, the other gets too hot. No way to tell that this is
occurring (other than the resultant fire.)

16 gage wire would allow a 15 amp breaker. 11 amps would be 73% of the
breaker rating. Since the load is continuous, 80% maximum is prudent.

The voltage drop for 11 amps in a 20 foot 16 gage wire could be as much
as a volt. This could make the lights a touch dim.

If you run 14 gage wire, you can use a 20 amp circuit breaker, and
the voltage drop will be only 0.6 volts. Sounds like the better plan to me.

The full current doesn't flow in the longest wire. Position lights
are an excellent example of a "branching" architecture where
a number of small loads added together determine the size of the
bus feeder.

His 5 x 2.2A lamps says that a 15A breaker/fuse feeding a switch
calls for a single strand of 16AWG. From the switch outward, the
circuit branches to various locations (left, right tail) such that
no conductor need carry full load past the switch. Voltage drops
will be VERY low.

Bob . . .


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william.p.dube(at)noaa.go
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PostPosted: Tue Feb 07, 2006 1:31 pm    Post subject: Wire sizing. Reply with quote

Well I guess as long as he can make it from the switch to the wingtip
using less than 23 feet of wire, he can stay within the allowed 0.5 volt
drop using 16 gage wire.

(4.81/1000)*4.4*23 = 0.5 volts

The wing is about 10 feet long. I'm not sure how much wire it will take
to get from the wing root to the switch, but 10 feet might be a good
guess. Probably would be OK.

The difference in weight between 16 gage and 14 gage for the whole
system would be something like 0.3 lbs.

Robert L. Nuckolls, III wrote:

Quote:


At 12:01 PM 2/7/2006 -0700, you wrote:



>
>
>Paralleling small gage wires is not a good practice. If one becomes
>disconnected, the other gets too hot. No way to tell that this is
>occurring (other than the resultant fire.)
>
>16 gage wire would allow a 15 amp breaker. 11 amps would be 73% of the
>breaker rating. Since the load is continuous, 80% maximum is prudent.
>
>The voltage drop for 11 amps in a 20 foot 16 gage wire could be as much
>as a volt. This could make the lights a touch dim.
>
> If you run 14 gage wire, you can use a 20 amp circuit breaker, and
>the voltage drop will be only 0.6 volts. Sounds like the better plan to me.
>
>

The full current doesn't flow in the longest wire. Position lights
are an excellent example of a "branching" architecture where
a number of small loads added together determine the size of the
bus feeder.

His 5 x 2.2A lamps says that a 15A breaker/fuse feeding a switch
calls for a single strand of 16AWG. From the switch outward, the
circuit branches to various locations (left, right tail) such that
no conductor need carry full load past the switch. Voltage drops
will be VERY low.

Bob . . .










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nuckollsr(at)cox.net
Guest





PostPosted: Tue Feb 07, 2006 7:00 pm    Post subject: Wire sizing. Reply with quote

At 02:25 PM 2/7/2006 -0700, you wrote:

Quote:


Well I guess as long as he can make it from the switch to the wingtip
using less than 23 feet of wire, he can stay within the allowed 0.5 volt
drop using 16 gage wire.

(4.81/1000)*4.4*23 = 0.5 volts

The wing is about 10 feet long. I'm not sure how much wire it will take
to get from the wing root to the switch, but 10 feet might be a good
guess. Probably would be OK.

Keep in mind too that the 0.5 volt value is a rule-of-thumb.
Consider that we expect things to perform in a useful way all the
way down to battery-only ops of 11.0 volts. That's a whole lot more
than 0.5 volts below a 14.2 volt bus when alternator is operating
normally. If your "thumb" should turn out to be 0.6 or 0.7 or even
0.8 volts long, it's not going to make a big difference in the
over-all scheme of things.

In fact, keep in mind that every 5% drop in voltage doubles the life
of a lamp. Run your lights with wire that allows a 10% drop will 4x
the lamp life . . . if that's your design goal, then so be it.
Just tell interested parties that your thumb is "1.4 volts long."

Bob . . .


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william.p.dube(at)noaa.go
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PostPosted: Wed Feb 08, 2006 10:24 am    Post subject: Wire sizing. Reply with quote

Robert L. Nuckolls, III wrote:

Quote:
Keep in mind too that the 0.5 volt value is a rule-of-thumb.
Consider that we expect things to perform in a useful way all the
way down to battery-only ops of 11.0 volts. That's a whole lot more
than 0.5 volts below a 14.2 volt bus when alternator is operating
normally. If your "thumb" should turn out to be 0.6 or 0.7 or even
0.8 volts long, it's not going to make a big difference in the
over-all scheme of things.

Actually, 0.5 volts it is a maximum limit for 14 volt devices under continuous operation set by the FAA.

AC-43.13-1B 11-66 b (Table 11-6)

You can read it yourself here:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2011.pdf

For incandescent position lights to put out their rated candlepower, they have to get the proper voltage. If there is excessive drop, your lights will be too dim, other folks might not see you as well, and .....

LED position lights (with regulators) are uneffected by any reasonable voltage drop, but they only draw about 1/2 Amp, so there is not much drop anyway.

Bill Dube'

http://www.killacycle.com/Lights.htm

Quote:
At 02:25 PM 2/7/2006 -0700, you wrote:



>
>
>Well I guess as long as he can make it from the switch to the wingtip
>using less than 23 feet of wire, he can stay within the allowed 0.5 volt
>drop using 16 gage wire.
>
>(4.81/1000)*4.4*23 = 0.5 volts
>
>The wing is about 10 feet long. I'm not sure how much wire it will take
>to get from the wing root to the switch, but 10 feet might be a good
>guess. Probably would be OK.
>
>






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