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Old 02-05-2018, 07:16 AM   #21
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When working on electrical systems , I never need to bother with "minimum wire sizes".

The bulk purchase of #10 allows the on hand wiring to be used almost everywhere , and the cost savings of bulk buying pays for the oversize wire.

Electric items do not suffer from almost no voltage drop.
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Old 02-05-2018, 07:41 AM   #22
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Quote:
Originally Posted by FF View Post
When working on electrical systems , I never need to bother with "minimum wire sizes".

The bulk purchase of #10 allows the on hand wiring to be used almost everywhere , and the cost savings of bulk buying pays for the oversize wire.

Electric items do not suffer from almost no voltage drop.
In my case this is not a question of price but really size. 10awg would be massive to wire my little led lights.

L
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Old 02-05-2018, 09:44 AM   #23
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As others have said, the required wire size is a function of the current, length of the wiring run, voltage and the acceptable voltage drop over the length of the wires. The formula for calculating the maximum length of a given size wire is:

L = (5 x x% x V)/(I x R)

where:
L is the maximum one way length in feet of the circuit
% is the acceptable voltage drop in the wires (normally 3% for lighting)
V is the voltage of the circuit (I use 12.5 for 12 volt systems)
I is the current to be carried
R is the resistance of 1,000 feet of wire.

Resistance of common wire sizes per 1,000 feet are

20 gauge 10.15
18 gauge 6.385
16 gauge 4.016
14 gauge 2.525
12 gauge 1.588
10 gauge 0.9989

The formula will tell you the maximum length of wire that you can use before the voltage drop goes over the desired percentage. Note that this formula is based on the one way length of the circuit in that the round trip length is built into it. The formula is based on electrical principles only and NOT ABYC suggestions.

Finally, the circuit breaker for the circuit should be based on the rated ampacity of the wire and the circuit should be fused based on the current requirements of the components the circuit supplies. So a circuit made from 16 gauge wire needs a 13 amp or less circuit breaker - typically a 10 amp breaker.
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Old 02-06-2018, 07:16 AM   #24
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"In my case this is not a question of price but really size. 10awg would be massive to wire my little led lights."

True, but there is no downside .
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Old 02-06-2018, 07:47 AM   #25
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I believe the 3 petcent drop recommended is for running lights, not any lighting cricuit.

As long as it is near 10 percent and not too close to ampacity limits, and you follow the engine room and bundled wire recommendations.... you should be fine, especially if still above the circuit protection amperage.

2 of my engineering friends have repeatedly told me the often suggested wire size for low amperage, 12V DC circuits is way larger than needed.

So I use the 3 percent rule for critical and wet area wiring, 10 percent for items that say they will run fine on 3 to 18 volts (or whatever the spread is), and somewhere above ampacity or above the 18g for signal wires or tiny draws like a few LED lamps.
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