ECN Forum Forums Code Related Discussion NEC & other Code issues amps for #14- #10 on motor circuits

I would suggest you look at this from the other direction.

Where does the NEC tell us we can not use table 310.16 for the ratings of conductors?

240.4(D) is the code section that many times requires us to use 14, 12, & 10 AWG for 15, 20 & 30 amp circuits. But notice it also says "Unless specifically permitted in 240.4(E) through (G)"


Now if you look at Table 240.4(G) you will find that motors along with many other items are permitted to forget about 240.4(D).

Not only can you use a 14 AWG for a motor load of 20 amps the breaker feeding this 14 AWG might end up being a 30, 40, 50 amp breaker once you follow the rules of Article 430.

Heres a quick example.

A 3 HP single phase 208 volt motor.

Table 430.148 shows this motor with a current of 18.7 amps.

Table 310.16 shows 14 AWG with a rating of 20 amps.

So we can use 14 AWG, now we look at Table 430.52 and we find that the Branch-Circuit Short-Circuit and Ground-Fault Protective device (lets say a Non time Delay Fuse) rating may be 300% of the motor load.

18.7 x 3 = 56.1 amps.

This would mean we could use a 50 amp non time delay fuse with the 14 AWG.

In this case we are not allowed to 'roll up' to the next standard size, as 430.52(C)(1) states;


The thing to remember or realize here is the fuse is not overload protection for the 14 AWG. The fuse only provides Branch-Circuit Short-Circuit and Ground-Fault protection.

The 14 AWG is protected by overload by the motor overload protective device.

Bob

[This message has been edited by iwire (edited 03-12-2005).]