danmueller
Member
Here is the problem from page 288 in Mike Holt's "Electrical NEC Exam Preparation" book based on the 2005 NEC:
If a dual-element fuse is used for the overload protection, what size fuse is required for a 50 hp, 460V, 3-phase motor, with a temperature rise of 39 degrees C, and a motor nameplate current rating of 60A (FLA)?
Answer: 70A.
Because the temperature rise is 40 degrees, or less in this case, the nameplate amps is multiplied by 125%. So, 60A x 125% = 75%. And it says in the problem that "The next size up rule does not apply when fuses are used for overload protection". So according to section 240.6(A), the next standard fuse size down from 75A is 70A, thus 70A is the answer given in the book.
So after multiplying the nameplate amperage by the proper percentage, whether it is 125%, 115%, etc., (whether it is a fuse, thermal o.l., dashpot, relay, etc.), and the answer does not correspond with a standard o.l. size, do you go to the next size down regardless of what type of o.l. it is? And where in the NEC is it stated? I know I am probably making this more difficult than it really it, but 2 friends and I have 3 different points of view, and we would like to resolve this matter before we have to take the state exam. Thank you.
If a dual-element fuse is used for the overload protection, what size fuse is required for a 50 hp, 460V, 3-phase motor, with a temperature rise of 39 degrees C, and a motor nameplate current rating of 60A (FLA)?
Answer: 70A.
Because the temperature rise is 40 degrees, or less in this case, the nameplate amps is multiplied by 125%. So, 60A x 125% = 75%. And it says in the problem that "The next size up rule does not apply when fuses are used for overload protection". So according to section 240.6(A), the next standard fuse size down from 75A is 70A, thus 70A is the answer given in the book.
So after multiplying the nameplate amperage by the proper percentage, whether it is 125%, 115%, etc., (whether it is a fuse, thermal o.l., dashpot, relay, etc.), and the answer does not correspond with a standard o.l. size, do you go to the next size down regardless of what type of o.l. it is? And where in the NEC is it stated? I know I am probably making this more difficult than it really it, but 2 friends and I have 3 different points of view, and we would like to resolve this matter before we have to take the state exam. Thank you.
