What wire size is needed to run a 100 amp subpanel?

Answer 1

This is a voltage drop question. To answer this question a voltage must be stated.

This is because the NEC (National Electrical Code) suggests that a feeder have no more than 3% voltage drop. So in order to answer the question, we must ask 3% of what supply voltage?

For instance, suppose the subpanel in question is a 120/240V single-phase panel supplying 120V branch circuits (common in the US). According to NEC (2011) 215.2 (A) informational note #2, we should limit the voltage drop to 3.6V (0.03 X 120).

Now NEC Table 310.15 (B)(7) says that copper #4 conductors are the minimum size we can use for a 100A feeder, but this would result in a voltage drop greater than 3% for our 100 foot run.

The formula for voltage drop is E (voltage) = I (current) X R (resistance).

#4 copper wire has a resistance of 0.308 ohms per 1000 feet, or 0.0616 ohms per 200 feet (remember that our 100 foot run includes two conductors for the round trip). So the voltage drop at full load would be E = 100 X 0.0616 or 6.16 volts, higher than the 3.6 volts maximum we are looking for.

In order to find the wire that will meet our needs, we need to calculate the maximum resistance we can tolerate. We can rearrange the formula for voltage drop to solve for resistance. Rearranging E = I X R we get R = E/I, so:

R = E / I

R = 3.6 / 100

R = 0.036

Our 200 feet of wire can have no more than 0.036 ohms resistance, but the tables list resistance for 1000 feet of wire, so we can say our wire can have no more than 0.18 ohms per 1000 feet (1000 / 200 X 0.036 = 0.18)

Consulting chapter 9 table 8, we find that #1 copper wire has a resistance of 0.154 ohms per 1000 feet, it is the smallest conductor that will limit our voltage drop to less than 3% worst-case.

You can follow the same steps to figure wire sizes for other supply voltages using the same method. Of course the NEC may not be adopted in your part of the world (and the informational note is not mandatory, but just a suggestion), but limiting voltage drop is a good idea no matter where you live, just be sure to follow your local laws if they are more stringent.

Answer 2

#3 R90 or RW 90 degree copper wire. Its amperage rating is 105 amps.

Answer 3

A #3 copper conductor with an insulation factor of 75 degrees C is rated at 100 amps also, with an insulation factor of 90 degrees C a #3 conducor is rated at 105 amps.

Answer 4

A #3 copper wire with an insulation rating of 75 or 90 degrees C is rated at 100 and 110 amps consecutively.

Answer 5

#3 RW 90 wire

Answer 6

3 guage