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Joined: Oct 2002
Posts: 830
Another motor question: I've been use to the idea of only using #14 on 15 amp circuits, #12 on 20 amp. circuits and # 10 on 30 amp. circuits. Recently read where we could use the actual ratings in the 75 degree column for these conductors on motor circuits. #14 = 20 amps, # 12 = 25 amps, # 10 = 35 amps. The asterisk at the bottom of Table 310-16 refers me back to 240-3 which refers me back to 430-C,D,E,F,H I don't seem to find where it allows this, at this time. It's late and my batteries running out. Could anyone help me on the place in 430 where it says this? Thanks a lot... Steve

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Joined: Jan 2003
Posts: 4,391
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)"

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

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;

430.52(C)(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used.

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.


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

Bob Badger
Construction & Maintenance Electrician
Joined: Jan 2004
Posts: 1,507
Bob you need to look at that one again. 430.22 says 1.25%. so you'd need a #12 awg on that 3 hp motor [Linked Image]

George Little
Joined: Jan 2003
Posts: 4,391

Bob you need to look at that one again. 430.22 says 1.25%. so you'd need a #12 awg on that 3 hp motor


I did forget about that, thanks for pointing that out.

Thats what happens when posting a 'quick example'. [Linked Image]

OK use 12 AWG and a 125 amp Instantaneous Trip Breaker. [Linked Image] [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Joined: Nov 2000
Posts: 2,148
OK use 12 AWG and a 125 amp Instantaneous Trip Breaker.
Only if the breaker is part of a listed combination starter.

Joined: Apr 2003
Posts: 5
Junior Member
See 110.14 (C)1(a)4

Joined: Feb 2004
Posts: 133
In this case we are not allowed to 'roll up' to the next standard size, as 430.52(C)(1) states;

I believe you are allowed to 'roll up' to the next higher value standard size per 430.52(C)(1) Exception 1:
Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined
by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers,thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere
rating shall be permitted.

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