# School question, overload relays for motors.



## Frasbee (Apr 7, 2008)

It's a mock journeyman exam for homework and this question has me stumped.

What is the maximum size overload relay permitted for protection against overload for single phase, 2 hp, 208 volt motor?

It's multiple choice and I have 4 answers in front of me, but that's not as important as much as finding where in the code book it is because I need to reference the article. I'm looking in 430.32 and 430.40.

Do I need to determine what the full load current/service factor is to figure this out?

430.32(A)(1) states: A separate overload device that is response to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater ------ 125%
Motors with a marked temperature rise 40*C or less -------125%
All other motors ------- 115%


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## NolaTigaBait (Oct 19, 2008)

frasbee said:


> it's a mock journeyman exam for homework and this question has me stumped.
> 
> What is the maximum size overload relay permitted for protection against overload for single phase, 2 hp, 208 volt motor?
> 
> ...


430.32 I'd say A


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## Frasbee (Apr 7, 2008)

NolaTigaBait said:


> 430.32 I'd say A


Would it help if I had the answers?

A. 17.16
B. 16.25
C. 18.15
D. None of these

I don't even know what the units are, this is over my head.


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## sbrn33 (Mar 15, 2007)

Total guess here but I would go with "B". 208 single phase is kinda stupid though


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## randas (Dec 14, 2008)

Frasbee said:


> What is the maximum size overload relay permitted for protection against overload for single phase, 2 hp, 208 volt motor?


Not that it helps you much, but in Canada you would go to table 45, which gives you 12A * 110% for 208v = 13.2A FLA
Then 28-306(1)(b) no marked service factor -> FLA*115% so, 13.2*1.15 = *15.18A *max overload setting

(if I remember correctly) :laughing:


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## A Little Short (Nov 11, 2010)

Frasbee said:


> It's a mock journeyman exam for homework and this question has me stumped.
> 
> What is the maximum size overload relay permitted for protection against overload for single phase, 2 hp, 208 volt motor?
> 
> ...





Frasbee said:


> Would it help if I had the answers?
> 
> A. 17.16
> B. 16.25
> ...


It doesn't sound like enough info is given in the question. If you use 430.32(A)(1) the answer would be 15.18 and that would be D. none of these.
If you use 430.32(C) the answer would be 17.16 which is A. 
Was there any other parts of the question that you didn't post?


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## Greg (Aug 1, 2007)

The key to that question is "maximum" so you use 430.32 (C) but little-lectric is right you need to know either its temperature rise or service factor. That will determine whether or not you use 130% or 140%. When sizing overloads you always use nameplate data, the motor tables are used for wire size and breaker size.

Now ironically, just last week I installed a 2hp 120/208-230 1Ph motor (wired for 208) for a customer. It was a replacement and the nameplate data said it drew 9.2A and had a service factor of 1.15 and a temperature rise of 40C, so I selected overloads rated at 12A(125%). I can go as high as 140% if the customer has nuisance tripping issues.

The numbers you see for the OL's are in amperage. The current through the OL causes heat when the current reaches a certain point the heat generated causes the OL's to open, when they cool down they be reset.


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## nitro71 (Sep 17, 2009)

It's a trick question. You're supposed to size the overloads off the nameplate full load amps. D is the answer because they don't give you the nameplate information.


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## JRaef (Mar 23, 2009)

nitro71 said:


> It's a trick question. You're supposed to size the overloads off the nameplate full load amps. D is the answer because they don't give you the nameplate information.


Bingo. You don't use the NEC to size the OL heaters, it only deals with MAXIMUMS. You size the OL heaters per the manufactures instructions. Some require that you apply multipliers, some require that you size them exactly per the FLA of the motor without using multipliers. You cannot pre-decide without knowing your brand and THEIR instructions.


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## starter (Oct 24, 2010)

I strongly agree with NITRO71


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