IAEI study guide questions

[wlancaster]

Thanks for all the previous help...here's another thing I'm confused about: Question # 46 pg. 92 -

A central heating and air conditioning unit has the following information on the name plate: Min. circuit ampacity 28 Amps, Max. fuse size 45 Amps. What is the minimum size Type THW conductor that may be used to supply this unit?

A. 12AWG
B. 10 AWG
C. 8 AWG
D. 6 AWG

Ok I get 28 amps x 1.25% = 35 amps. They say the correct answer is B. 10 AWG....I thought since it is 100 amps or less we have to use the 60deg C column? My answer would have been C. 8 AWG.

 

[infinity]

You need a minimum conductor size of 28 amps you do not need to add 25%.

 

[wlancaster]

In their solution explanation they add the 125% since it's the largest motor (it really doesn't explain the 125% in the solution). I made a typo above, should be 125%.

 

[infinity]

In their solution explanation they add the 125% since it's the largest motor (it really doesn't explain the 125% in the solution). I made a typo above, should be 125%.

You do not add 25% because the MCA already has the 25% already added in. The MCA by itself without alteration is used to size the conductors.

 

[wlancaster]

You do not add 25% because the MCA already has the 25% already added in. The MCA by itself without alteration is used to size the conductors.

Thanks I think I'm straight now, looks like they should not have added the 25%. And just so I'm straight on table 310.15B16- just because the question asks what size THW, that doesn't allow me to use the 75deg C column, you still have to use 60deg C if less than 100 amps, right?

 

[infinity]

Thanks I think I'm straight now, looks like they should not have added the 25%. And just so I'm straight on table 310.15B16- just because the question asks what size THW, that doesn't allow me to use the 75deg C column, you still have to use 60deg C if less than 100 amps, right?

For all intents and purposes you can use the 75° ampacity from the table because almost all modern equipment has 75° C terminals but since this is test question without referencing the "exception" in 110.14(C)(1)(a)(3) you would use the 60° C ampacity. Doesn't change the answer but that is the method that you would use.

 

[wlancaster]

For all intents and purposes you can use the 75° ampacity from the table because almost all modern equipment has 75° C terminals but since this is test question without referencing the "exception" in 110.14(C)(1)(a)(3) you would use the 60° C ampacity. Doesn't change the answer but that is the method that you would use.

Thanks!

 

[infinity]

Thanks!

Now you see that B. #10 AWG with a 60° C ampacity of 30 amps is the correct answer.

 

[Rock86]

For all intents and purposes you can use the 75° ampacity from the table because almost all modern equipment has 75° C terminals but since this is test question without referencing the "exception" in 110.14(C)(1)(a)(3) you would use the 60° C ampacity. Doesn't change the answer but that is the method that you would use.

To add to that, wouldn't you also need to take 240.4... paying attention to use 240.4(G).

 

[infinity]

To add to that, wouldn't you also need to take 240.4... paying attention to use 240.4(G).

Can you expand on how 240.4(G) would apply to this question?

 

[wlancaster]

Now you see that B. #10 AWG with a 60° C ampacity of 30 amps is the correct answer.

Yes that makes sense now thank you

 

[don_resqcapt19]

Can you expand on how 240.4(G) would apply to this question?

It lets you protect the 10 AWG with the specified maximum OCPD of 45 amps in place of the 30 amps maximum OCPD found in 240.4(D)(7).

 

[Rock86]

Can you expand on how 240.4(G) would apply to this question?

Wlancaster mentioned that by their calculations he would have choose #8. Even at their calculated 35A they could have used #10 if they applied 240.4(G) instead of saying " oh well, its greater than 30A, guess I have to use #8."

 

[infinity]

Wlancaster mentioned that by their calculations he would have choose #8. Even at their calculated 35A they could have used #10 if they applied 240.4(G) instead of saying " oh well, its greater than 30A, guess I have to use #8."

How could they use #10 for a 35 amp MCA when the question limits the conductor ampacity to 60° C which is 30 amps?

 

[Rock86]

How could they use #10 for a 35 amp MCA when the question limits the conductor ampacity to 60° C which is 30 amps?

Did it limit the conductor ampacity to 60C? isn't THW a 75C rated insulation? Cerrowire has it 75C.

 

[infinity]

Did it limit the conductor ampacity to 60C? isn't THW a 75C rated insulation? Cerrowire has it 75C.

Yes THW is a 75° C conductor but I mentioned in post #6 why you cannot use the 75° C ampacity.