Exam Question

[Dsm]

I hope some can help?My question is From my exam I took and got it wrong .Determine the minimum required conductor ampacity for an induction type motor 25 HP 460 volts 3 phase squirrel cage motor name plate full load 32 amp service factor 1.15?My answer was 34x1.25 =42.5A there answer was 43 A .The only thing I can figure is they rounded up.Any help would be greatly appreciated

 

[nickelec]

42.5a is not a standard breaker size , though neither is 43 .45 would be the best answer inmo as you can round up for branch ckts but not for feeders

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[JFletcher]

Hmm, doing the math off of that question, I would have gotten 40 amps (nameplate FLA x 1.25). Not sure what the annex of the NEC has to say about it.

Nickelec, the breaker can be up to 250 percent of the FLA. The motor could very well have a 60 amp breaker with #10 THHN

 

[infinity]

The question is about sizing the conductor so forget about the OCPD. You need to also ignore the nameplate current and find the appropriate value in T430.250 (as you did) and use that for the calculation. For rounding up or down (B) applies.

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347,480Y/277, 480, 600Y/347, and 600 volts shall be used.
(B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.

 

[charlie b]

Determine the minimum required conductor ampacity . . . .

As Rob pointed out, this question is not about the breaker. It is also not about sizing the conductor. It is about determining a specific number. I think your method is right, and the exam author's answer is wrong.

 

[infinity]

As Rob pointed out, this question is not about the breaker. It is also not about sizing the conductor. It is about determining a specific number. I think your method is right, and the exam author's answer is wrong.

Wouldn't 42.5 be rounded up to 43 which is the answer given?

 

[JFletcher]

43 amps is correct you round down fractions smaller than .5.

That said, I don't know why using the name plate is incorrect.

 

[kwired]

Wouldn't 42.5 be rounded up to 43 which is the answer given?

I didn't check the calculations, but if he did calculations correctly and came up with 42.5, then IMO 42.5 and 43 are both correct answers.

It says right in the section you quoted that you are permitted to round this up - not that you shall round it up.

 

[Besoeker]

43 amps is correct you round down fractions smaller than .5.

That said, I don't know why using the name plate is incorrect.

I would assume that the nameplate figure of 32A is correct for that particular motor assuming it is a real motor.
Making typical assumptions for effy and pf that's about what I come up with.

However, the tables probably have to be conservative.

 

[solis]

I thought ampacity of conductors was determined by the table values, not the nameplate.

 

[solis]

sorry you did use the table values

 

[kwired]

I would assume that the nameplate figure of 32A is correct for that particular motor assuming it is a real motor.
Making typical assumptions for effy and pf that's about what I come up with.

However, the tables probably have to be conservative.

NEC wants us to use values in some tables within the NEC instead of nameplate for determining minimum ampacity for conductors and other motor calculations. The values in those tables are the worst case efficiency/power factor motors you would ever find.

The idea I think is if you replace say a 10 Hp motor with another 10 HP motor - you shouldn't need to worry about conductor size - it was already sized for worst case 10 HP motor you would ever encounter.

 

[Besoeker]

NEC wants us to use values in some tables within the NEC instead of nameplate for determining minimum ampacity for conductors and other motor calculations. The values in those tables are the worst case efficiency/power factor motors you would ever find.

The idea I think is if you replace say a 10 Hp motor with another 10 HP motor - you shouldn't need to worry about conductor size - it was already sized for worst case 10 HP motor you would ever encounter.

That was kinda my point in post #9.
Conservative maybe should have been ultra conservative.

And then there is the vexed question of why the question would give nameplate current when you more or less have to ignore it and you are required to use NEC values?

 

[kwired]

That was kinda my point in post #9.
Conservative maybe should have been ultra conservative.

And then there is the vexed question of why the question would give nameplate current when you more or less have to ignore it and you are required to use NEC values?

Is common to give nameplate on such exams - Multiple choice answers will likely have a choice that includes using nameplate - exam taker is supposed to know to ignore the nameplate for such calculations - but does need to know nameplate for overload protection purposes.

 

[infinity]

I didn't check the calculations, but if he did calculations correctly and came up with 42.5, then IMO 42.5 and 43 are both correct answers.

It says right in the section you quoted that you are permitted to round this up - not that you shall round it up.

I agree, but when answering test questions some common sense needs to be applied if in the answer list 43 is there but 42.5 isn't, well you see where I'm going with this.

 

[Besoeker]

Is common to give nameplate on such exams - Multiple choice answers will likely have a choice that includes using nameplate - exam taker is supposed to know to ignore the nameplate for such calculations - but does need to know nameplate for overload protection purposes.

That wasn't asked. Conductor rating is what was.

 

[kwired]

That wasn't asked. Conductor rating is what was.

But to use NEC methods of determining minimum needed conductor ampacity - you must use the tables in the code and not the nameplate amps. About only exceptions are non standard sized motors or any type not included in the tables. NEC has tables for many standard sized motors including DC motors, single phase alternating current motors, three phase alternating current motors and believe it or not even two phase alternating current motors, and for most common voltages under 600 volts - three phase table does include 2300 volts.