Calculating wire size

[Davepar]

From reading the info in AC 43.13 chapter 11, this is how I interpreted the correct method for sizing wire for a circuit. Anybody correct me if I'm off track here. (Tables were lifted directly from AC 43.13-1B chapter 11.)

1. Choose a circuit breaker or fuse size. The manufacturer may recommend a fuse/breaker size, or use the device current rating, add 33% and round up to the next larger size. You want a size large enough so that you won't have false trips. However, you don't want a size too large (as you'll soon see).

2. Use table 11-3 to determine the minimum wire size. The purpose of the breaker or fuse is to protect the equipment, but also to protect the wire. If you choose too small of a wire, it may burn up before the breaker/fuse trips. Note that this table assumes some conditions that we don't get near in our planes, so these are very conservative values.


3. Measure the length of the wire. Start at the fuse/breaker bus and measure through the switch to the equipment and on to the ground point.

4. Calculate the voltage drop in the wire. Start with the minimum wire size from step 2 above. Look up the "Max. resistance" value in table 11-9. Calculate the voltage drop with this formula: V = (distance in feet) * (current in amps) * (max resistance). The current should be the current of the device, not the fuse/breaker rating. This value will be in millivolts.


5. Compare the voltage drop to the recommended maximums in table 11-6. For a 14v system, these are 500 millivolts continuous or 1000 millivolts intermittent.

6. If the voltage drop is over the maximum, step up to the next large size of wire and repeat steps 4-6.

Example:
Let's say I want to calculate the wire size for a 100 watt landing light. The current draw at 12 volts is 100 / 12 = 8.33 amps. (I used 12 volts here instead of 14, since that is the system voltage when running on batteries only and produces a higher current.) Adding a 33% cushion produces 8.33 * 1.33 = 11.1 amps. The next larger fuse size is 15A. Using table 11-3, the minimum wire size is 14 gauge. Let's say the length of the wire run is 20 feet. The voltage drop would be 20 * 8.33 * 3.06 (from table 11-9) = 510 millivolts. This is over the 500 millivolt recommended maximum. It may be close enough, or the safer bet is to bump up to 12 gauge wire.

Dave

[tomwebster]

I used the chart 'Wire Length VS Wire Size',
in section 5 of Van's manual under Electrical Wiring Notes.
It's on the 'RV Aircraft Sample Wiring Schematic'.
No math is needed and seems to give the same solution.
Might save some work,
Tom Webster
RV-7A

[szicree]

Originally Posted by Davepar

The purpose of the breaker or fuse is to protect the equipment, but also to protect the wire. If you choose too small of a wire, it may burn up before the breaker/fuse trips.

My understanding has always been that the fuse is to protect the wire, connectors and switches from burning in the event of a short. Consider the fuses in your house. Are they there to protect your doorbell, lamps and electric toothbrush? No, they're there to shut things down before the wires (and house) catch fire.

My method was to choose a wire size that could flow the needed current without getting hot, the fuse was then chosen based on the wire size.

Steve Zicree