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Joined: Oct 2002
Posts: 830
I'm having a slow day(did not sleep well last night due to cough). When sizing conductors for a motor that can run on 120 or 240 do you have to size for 120 even though it is going to run on 240? I know that in figuring feeder size for swimming pool panel, I have had to figure that in, but I'm not sure about the branch circuit wires that go directly to the motor. Normally in the motors I have been hooking up for pool, it didn't go past a # 12 wire anyway, but the one I'm doing now will have to be wired in #10 if that's the case. I want to make sure because those wiring compartments on motors don't hardly give enough room for #12's, much less for # 10's. Thanks for the help. Steve..

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Joined: Jan 2004
Posts: 1,507
What ever the voltage is that the motor will be running at should be the size wire based on 1.25% of the corresponding ampereage in the tables for Article 430.

George Little
Joined: Jun 2005
Posts: 821
When sizing conductors for a motor that can run on 120 or 240 do you have to size for 120 even though it is going to run on 240?

I would decide what voltage I intend on running the motor at and then determine the wire size. Obviously, using a higher voltage will use less amperage, therefore a smaller wire. That seems to be the way to go in this case since you mentioned thee not being much room in the wiring box.

Good luck!

Joined: Jan 2005
Posts: 5,316
Cat Servant
For sizing conductors, you do not use the nameplate amps of the motor; you use the horsepower rating.

Using table 430.248, and the nominal voltage you plan to use, you use those amp values for your calculations.

If the motor is to be run 'continuously' (for over three hours), then the 1.25 multiplier applies.

The motor nameplate amps may be used for the sizing of your fuse or breaker ... and is certainly used for selecting your "heater" sizing.

The table values are, I'm convinced, based upon the worst 1930's motors they could find. I have yet to see a motor that, for the horses, required anything like the amps the tables specifies. I think the NEC is trying to 'worst case' your motor replacement.

[This message has been edited by renosteinke (edited 03-03-2007).]

Joined: Jun 2005
Posts: 821
Thanks for that simplified explaination, Reno.

[This message has been edited by ShockMe77 (edited 03-03-2007).]

Joined: Jan 2004
Posts: 1,507
Reno- 430.24 has me sizing the conductors for a General duty motor at 125% whether I run it continous or not. The numbers in the tables are used for starter numbers and they relate to the HP of the motor. If the running current showed on the name plate is higher than the numbers shown in the table then the higher of the two is used multiplied times 125% for sizing the conductors. The overload is sized based on the nameplate running current times the numbers in 430.32.

George Little
Joined: Jul 2004
Posts: 9,677
Likes: 9
I am with George on this but you do get a break since you can use 310.16, not 240.4(D)

Greg Fretwell
Joined: Dec 2000
Posts: 4,289
I find with Reno

430.6(A) (1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247,
Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings
of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the
motor nameplate.

The conductors are sized at 125% or greater regardless of whether it is a continuous load or not

The nameplate rating is used for sizing separate overcurrent protection

[This message has been edited by electure (edited 03-04-2007).]

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