Overload sizing

[Roger]

Ok I think what I would rather do here is just set up an example and show how I solved it. You guys can then discuss what I overlooked or if I am correct. I'm trying to make these question like those you would seee on a test not in the field. Thats what his forum is supposed to provide.

I have a single phase 3 HP 230 V continuous duty motor and I want to determine the maximum overload protection amps. No internal overload in the motor.

I look up my FLC from table 430.148 and get 17 amps. Now I go to 430.32 and since I have not listed a service factor I would use "all other motors" at a factor of 115%. So 17 x 1.15 = 19.95 amps.

[mdshunk]

I use actual nameplate data to size overloads.

[Homer]

Your FLA value of 17A from the tables should only be used for sizing the overcurrent devices (fuse/breaker) and the required conductor ampacity.

The nameplate FLA must be used for the overload selection. You would then use the factor of 115% to arrive at the actual overload relay size/setting.

In most cases the FLA on the nameplate will be a fair amount less than the NEC tables.

Homer

Edit: Gee, where do you find that pic?

[Roger]

Ok I see I didnt present the question correctly. So if the 17 amps had been the motor nameplate FLA I would have approached it correctly?

Ok...now I see something that causes me some interest an far as a testing format is concerned. They always are talking about "keywords" in a test. I highlighted the word maximum in the question. So lets assume that I posed the question correctly and used 17 amps as nameplate FLA. Do you suppose the tricky little narrow eyed test people would imply that to mean the motor wont start and want you to use 430.34....that that would really be your maximum overload?

By the way I'm not fixing to take any tests....dont need that kind of stress in my life anymore. Just trying to gain a little more knowledge.

Marc that picture is funny :D

[Wgoodrich]

Homer said it correctly. Overload is sized from name plate and duty rating.

Overcurrent and conductor size is sized by the tables.

If in a test question the words minimum or maximum is not present in the test question then you would use the minimum.

Only use 430.32.C when the words maximum overload or reference to hard start motors needing higher overload value is worded in the question.

Good Luck

Wg