Mike Holt Masters Exam 2014 help

[sparky999]

I'm going through the calculations section of this test and I need help determining when to use the 60° column and when to use the 75° column for sizing conductors/OC device for welders.

At first I was following 110.14(C)(1) for under 100A with no terminal listing use 60°

I then ran into another question that the load was under a 100A , however the answer given used the 75° column

Question 36
A nonmotor generator arc welder has primary current rating of 40A with duty cycle of 50%. The branch circuit conductor for the welder must be rated at least _______

Answer is 30A based on 60°



Question 38
The maximum OC device rating for 10AWG conductors used for arc welder branch circuit is ______

Answer is 70A based on 75°


Does it state which degree column to use based under conditions?


Thanks for the help!

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[Dennis Alwon]

In general, if the conductors are rated 75C or 90C then you would use the 75C column. In the real world you hardly ever use the 60C unless you are using nm cable.

The reason is that all panels today are rated 75C or more (generally not more than 75C ) and if your wire is rated 75C or better than you use the 75C column

 

[sparky999]

Thank you for the response

...what caused my concern was that in the first question the answer was selected using 60° column and Im not sure why.

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[Dennis Alwon]

How can question 36 be based on 60C. I see 40 amps x .71 multiplier for 50% duty cycle== 28.4 amps or 30 amps. They aren't asking what size wire

 

[sparky999]

See photos

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[Dennis Alwon]

Yes, that is confusing. He is picking out the conductor size of #10 based on the 60C column. Here is why... Look at the asterisks after sizes 14, 12 , 10... Then go to the section mention below... 240.4(D) and it states you cannot use 10 any greater than 30 amps which is at the 60C column. You can use the higher rating if you had to do some de-rating .

You see that the 60C statement in the example is not really needed to answer the question.

 

[sparky999]

Agreed and that makes sense. What confused me was question 38 when he used 75° column.

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[Dennis Alwon]

Again he used 75C because the conductor was going to be larger than #10

 

[sparky999]

Can you explain your last response? I don't believe the conductor was going to be more then 10AWG , only because it states that it is 10AWG.

Just trying to understand. Thanks!

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[Dennis Alwon]

I sent you a pm

 

[sparky999]

Thanks for the help Dennis!

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