motor overload protection

[danmueller]

Referring to the Mike Holt NEC exam preparation for 2005 code, Page 288. The problem at the bottom left of the page and the corresponding graphic on the top right of the same page. It states that when using a fuse for overload protection, that the "next size up" rule does not apply--that you are supposed to size the fuse down. I have some friends who are studying for the journeyman exam saying diffferent things. One says that all overloads, after doing the calculation, are supposed to be sized up, and another person says that all ol's are to be sized down. None of us can find the rule in the 2005 NEC. What section of the NEC does it state that when using a fuse for o.l. protection, that it needs to be sized down? And also, where in the NEC does it say that other overloads are to be sized up after applying the correct percentage to the FLA? Thank you.

 

[don_resqcapt19]

Many of us do not have that book so you will have to give us more information. If you are talking about the motor overload protection in 430.32(A)(1), then there is no provision to use any overload device that has a rating or setting larger than what is shown in that section except for the provisions of 430.32(C). There is no automatic round-up to the next standard size of fuse.

 

[danmueller]

overload problem...

overload problem...

The problem in Mike Holt's book is:

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 emperature rise of 39 degrees C, and a motor nameplate current rating of 60A (FLA)?

Answer: 70A

60 x 125% = 75A, (125% is used because of the temperature rise being 40 degrees C or less) so it says in the book that the "next size up" rule does not apply when fuses are used for o.l. protection. So according to 240.6(A), the next standard size fuse down is 70A. So does that mean that if the calculation, after applying the appropriate percentage, comes out to be an answer that is not equal to a standard fuse size (or overload size if you are using other types of overloads such as bimetalic or dashpot type), that the correct o.l. has to be determined by taking the next size down?

Thank you!!!

 

[danmueller]

overload problem

overload problem

The question in the Mike Holt book is:

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 ratng of 60A (FLA)?

Answer: 70A. 60 x 125% = 75A, so the answer is 70A. (125% is used because the temperature rise is 40 degrees C or less). Mike Holt says that the "next size up" rule does not apply when fuses are used for o.l. protection. So does that mean, after doing the calculation using the appropriate percentage on the nameplate amperage, that you size the answer down to the next standard fuse size or other type of overload size if using bimetalic, dashpot, etc.

 

[one_eyed_jack]

I'm currently studying for my exam as well and this is a question I just did.

Article 430.32.A.1 says "This device shall be selected to trip or shall be rated no more than the following percent of the motor nameplate FLC rating:

Motors with a marked temp. rise 40C or less: 125%"

The shall be rated no more than is the part to pay attention to. You come up with 75A. This is the max and since 75A is not a standard fuse size, you need to go lower and use a 70A fuse. Stepping up to an 80A fuse here would violate the above Article as it would be rated more than 125%.

At least that's how I'm interpreting it.

 

[one_eyed_jack]

Oops. Didn't see your follow up post with the answer already.

 

[rbalex]

The question in the Mike Holt book is:

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 ratng of 60A (FLA)?

Answer: 70A. 60 x 125% = 75A, so the answer is 70A. (125% is used because the temperature rise is 40 degrees C or less). Mike Holt says that the "next size up" rule does not apply when fuses are used for o.l. protection. So does that mean, after doing the calculation using the appropriate percentage on the nameplate amperage, that you size the answer down to the next standard fuse size or other type of overload size if using bimetalic, dashpot, etc.

It is important to recognize there is no ?next size up? rule at all in this Section. It is a ?not to exceed? rule and it applies to all overload protective devices - fuses, circuit breakers and overload relays. You cannot use the ?next size up? (80A) standard sized fuse or fixed trip circuit breaker in this case. You may use adjustable trip circuit breakers or overload relays assuming they permit a setting that does not exceed 75A.