1. Member
Join Date
Dec 2010
Posts
69
sorry you did use the table values

2. Originally Posted by Besoeker
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.

3. Originally Posted by kwired
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?

4. Originally Posted by Besoeker
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.

5. Originally Posted by kwired
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.

6. Originally Posted by kwired
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.

7. Originally Posted by Besoeker
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.

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