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Interpreting "Electric Wire Chart"

HFS

Well Known Member
I need some direction in the interpretation of this chart in the area at the lower right hand corner - covered up by the "Curves" description.

Can you extrapolate the "amperage" diagonal lines & "wire length" horizontal lines to their far axis and use that as a basis for wire sizing in that area - or?

Thank you - I have been looking at this chart since the middle '80's and have never figured out what to do in this area.

HFS
 

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The curves explanation area is entirely below curve 3 on the chart, which is for intermittent loads only. You would never want to be below curve 3 which is why the diagonal amperage lines do not extend below curve 3 and also why the explanation is placed in this area. The chart provides no useful information regarding wire sizing in the explanation area
 
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An Example

What if I have a starter load (150-200 A) from the firewall to the starter, about 30" - it seems like this situation would fall in the "no fly zone" - so what do I do?

Thank you.

HFS
 
I see your point. However, following the method in the first example in the explanation and using the 200 amp diagonal, where it meets curve 3 indicates a cable size of 2 is required. Checking the corresponding cable length on the Y axis of the chart shows that you are good for up to 12 feet without significant voltage drop (assuming 14V system). Obviously you would also be good for the shorter length you require. Use a 2 awg wire.
 
I have not seen this chart before and is looking to have no more than 1V voltage drop?

Looking at the "no fly zone" of the chart is counter intuitive for short runs of the same size wire with the same current requirements. Maybe after curve 3 In your example (200A) for the starter it could or should be a strait line down if all conditions are equal as you use same wire but shorter wire run?

In the chart you posted, the shortest run for 200A before the line stops is between #4 and #2, closer to #2, for 12' at 12V.

I tend to not use charts and stick to the math route. #2 AWG standard wire resistance is .000184 ohms per foot @ 149F. So at a 30" run of #2 wire (2.5'x0.000184)x200A=0.092 voltage drop.

There are tons of other things to consider, weight of wire, insulation type on the wire, strand thickness of wire cable, heat rise over time...etc. The starter motor is one of the few electrical items that may/may-not have a fuse to protect the circuit.

I like NASA's approach on wire and using a 150% safety factor when sizing wire and circuit elements such as switches, relays, fuses/breakers.

Back to the chart and the 200A line stopping near the #2 wire size at 12', if your are shorter on the wire run (no fly zone) you should be fine with #2.

What if I have a starter load (150-200 A) from the firewall to the starter, about 30" - it seems like this situation would fall in the "no fly zone" - so what do I do?

Thank you.

HFS
 
What if I have a starter load (150-200 A) from the firewall to the starter, about 30" - it seems like this situation would fall in the "no fly zone" - so what do I do?

Thank you.

HFS

There are two charts for figuring wire size in 43:13, Continuous and intermittent.

The proper usage is listed on the left side of the charts. Use the intermittent chart for engine start, flap usage etc...
 

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Get a copy of FAA AC43.13-2A has everything you will need to know how to "do it correctly".
DAR Gary
 
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