NCEES 510

I am a bit perplexed with teh answer for 510.

I solve this by taking teh FLA which is 65A and multiply that value by 1.15. I get 74.75. I choose 80A as the answer but that is wrong. Where in the NCEES does it say we need to size down?

please share the question

Question: A 50HP 460V 3ph induction motor, svc factor 1.0, what is the initial maximum size of protection device? 

OP is correct about 65A full load current.

OP is correct about 1.15 multiplier based on 430.32(A)(1), giving 74.75A.

In 5-A increments, I'd want 75A, but that's not a choice. The closest two choices are 70A and 80A, and we are in between.

OP may not have noticed the wording just above the 1.15 multiplier:

"*No more than* the following percent of full load current"

80 would be more than, 70 would be less than. I choose 70 based on the words "No more than".

For fire pumps you round up the to the next largest size OCPD based on the LRC sum of the fire pump, jockey pump and accessories. 

Can someone explain to me why the NCEES solution to problem 510 uses the table value from 310.15(B)(16) to size the overload protection device? Shouldn't table values be used for conductor sizing? It clearly states in 430.6 (2) that nameplate values should be used to determine current ratings for separate motor overload protection. It also clearly states in 430.32(A)(1)  that the separate overload device shall be rated at no more than the following percent of the motor nameplate full-load current rating. If that was the case, I would solve for current given 50HP and 460V (efficiency and power factor not given so this would be an assumption at best), which doesn't seem to have a good answer. 

JohnMdd said:

Question: A 50HP 460V 3ph induction motor, svc factor 1.0, what is the initial maximum size of protection device?

OP is correct about 65A full load current.

OP is correct about 1.15 multiplier based on 430.32(A)(1), giving 74.75A.

In 5-A increments, I'd want 75A, but that's not a choice. The closest two choices are 70A and 80A, and we are in between.

OP may not have noticed the wording just above the 1.15 multiplier:

"*No more than* the following percent of full load current"

80 would be more than, 70 would be less than. I choose 70 based on the words "No more than".

Click to expand...

I made the same mistaken and overlooked the "No More Than.." text
Thanks for the explanation
Glad to be aware of this now in case this pops up on the exam!