Help with Motors 430.32(A)-(D)

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evalencia01

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Some people are telling me that whenever the SF is 1.15-1.25, to size the overload device to 100% and whenever they're less than 1.15, to size it 90%. I was wondering if I should follow this rule or what's listed on the NEC. I'm asking this because I'm taking this course where they say the same thing and have their own exam, but I'm taking the exam elsewhere so I would like to know which rule to apply when taking the exam. I am for certain that they will ask a question regarding this and was wondering what is widely acceptable.

The NEC book says that at 1.15 or greater to size at 125% and all others 115%
 
Go with what the NEC says
Now the setting of the overload or sizing the heater, that depends on the manufacturers instructions, which is what "some people" may be referring to.
Sometimes its set the adjustable dial to motor FLA, sometimes its FLA x a %
 
Yeah it was my course that used the 100% and 90% ratings versus the 125% and 115% respectively. I am aware now that when in the field, use the manufacturer instructions. I just wanted to know which method to use if tested on, and I can see now that it depends on whether the manufacturer instructions are specified or not.

Thank you!
 
What people are telling you may be half true. When bi-metalic thermal overloads are used motor FLA is typically used depending on the temperature difference between the starter and motor.
 
The answer varies from mfr to mfr, you cannot generalize, that's why the only correct answer is to follow the instructions for the mfr you are using. As to why your test is not acknowledging this, that's a flaw in whatever organization designed the test. Their statement regarding 100% vs 90% is based probably on the ONE mfr they looked at, it's not true for all of them.

What it boils down to is this; The NEC makes a statement about the MAXIMUM level that a protection device must act upon. The mfr of the OL relay then has to decide how to approach it;
A. Incorporate the 115% into the "pick-up point" of the relay design, assuming most people will NOT use the SF and adjust UP if they do, or...
B. Incorporate 125% into the pick-up point of the relay, assuming most people WILL want to use the SF and adjust DOWN if not, or...
C. Incorporate no internal adjustments to the pick-up point and make the end user adjust it themselves based on how they intend on using the equipment.

The testis describing only scenario B; assume everyone WILL use the SF, so the basis is assuming a 125% pick-up point, making users adjust down for 1.0SF by selecting at 90% of FLA. That covers roughly 50% of the old style NEMA OL relays with replaceable heater elements out there. But it covers ZERO % of the adjustable IEC overload relays out there, which fall under A; you set the relay at the motor FLA and it ALREADY has the 115% built-in, so if you WANT to use the SF, you adjust UP. On a lot of Solid State OL relays, it's closer to scenario C in that they provide you with a way to SELECT the 125% or 115% pick-up point in programming, but you then still set the relay to the motor FLA.
 
Thank you both @Jraef and @MrJLH, you're both right on the dot as to why they used those ratings. I still have a lot more to learn through experience in order to get such a thorough view of it. For now, I'll just move according to the NEC unless specified otherwise on my exam's instructions. They may follow certain guidelines that I'll have to make sure I get myself beforehand
 
evalencia01 said:
Yeah it was my course that used the 100% and 90% ratings versus the 125% and 115% respectively. I am aware now that when in the field, use the manufacturer instructions. I just wanted to know which method to use if tested on, and I can see now that it depends on whether the manufacturer instructions are specified or not.

Thank you!
Click to expand...
I think the confusion here is, that the charts used by many manufacturers, do the "math" for you. Personally I wish they wouldn't "help". What I mean is that picking an overload heater with the full load current of the motor is 4 amps an using the heater rated at 4 amps, the heater is actually rated for 4 amps times 1.25. Now if your motor service factor requires a lower amperage protection you have to compensate for that math. During your test you are ONLY worried about the real math, not the compensations required from the manufacturer. Does this make sense?

I want to point out again, learn everything you can learn to be a better electrician. For the test, learn how to pass the test. THEY ARE NOT THE SAME THING.
 
Strathead said:
I think the confusion here is, that the charts used by many manufacturers, do the "math" for you. Personally I wish they wouldn't "help". What I mean is that picking an overload heater with the full load current of the motor is 4 amps an using the heater rated at 4 amps, the heater is actually rated for 4 amps times 1.25. Now if your motor service factor requires a lower amperage protection you have to compensate for that math. During your test you are ONLY worried about the real math, not the compensations required from the manufacturer. Does this make sense?

I want to point out again, learn everything you can learn to be a better electrician. For the test, learn how to pass the test. THEY ARE NOT THE SAME THING.
Click to expand...
Yeah you're absolutely correct, thank you. The test is one thing and I'll always follow the NEC for it, and even though I've yet to see this on any motor I've worked on, it's definitely out there.
 
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