sizing thermal elements

nizak

Senior Member
I actually got a chance to put a meter on the motor that I hooked up.In my original post I was asking about thermal protection and sizing. Nameplate is 13.1A, motor is running at 10.8. I will assume that the load will probably not change since this is a air compressor motor and no additional burden should be added.After reading many of the posts, I guess I will just go by the nameplate since that is what NEC requires.
 

kwired

Electron manager
Location
NE Nebraska
I just replaced a 7-1/2 hp motor that turned out to have been a 10 (larger) motor that Baldor had 'relabled' for some reason. Something to keep in mind the next time you wonder about those wire size charts in the motor tables ...
Was it a special purpose motor of some kind?

I run into that a lot with fan motors that are intended to be located in the air stream. They can uprate the motor based on the fact it will have more cooling air flowing over it in that situation. Nameplate will also indicate that it is an air over application intended motor.
 

kwired

Electron manager
Location
NE Nebraska
I actually got a chance to put a meter on the motor that I hooked up.In my original post I was asking about thermal protection and sizing. Nameplate is 13.1A, motor is running at 10.8. I will assume that the load will probably not change since this is a air compressor motor and no additional burden should be added.After reading many of the posts, I guess I will just go by the nameplate since that is what NEC requires.
Air compressors can draw more when pressure in system is higher, like when nearing the cut out pressure. Normally it does not last for very long, but if the air usage was just right it could.
 

renosteinke

Senior Member
Location
NE Arkansas
Kwired, the motor was "down rated," not upsized.

Was it 'special?' Well, that becomes a mater of defining 'special.' The short version is that the machine required a motor to have a certain frame size, and the smaller motor was not / no longer made with the larger frame. The motor manufacturer had no problems supplying a replacement, though it was necessary to provide them with the motor serial #, shop order #, and some other information. Heck, the thing was even in stock!
 

iwire

Moderator
Staff member
Location
Massachusetts
Iwire, that comment regarding industrial electricians is unworthy of you.
When people put aside manufacturers instructions and codes to do there own fine tuning I call that shoddy work.

I have done such things myself and I feel the same way about my own actions, shoddy.

The industrial maintenance field is most definitely a field of trained professionals
Certainly they can be, and often are, but they also can be unprofessional and make decisions beyond there knowledge.



"Based upon" does not mean that you are limited to using only the number on the nameplate. Please note that the same language is not used regarding OCPD's. In this regard, "based upon" is no more than another way of saying 'should.' That FLA figure is nothing but a starting point. Start at FLA and work your way down.
You are free to under protect, I have agreed with that a number of times. I think it is a bad idea to do so. It is a band aid to a bigger problem.

Now, if you meant to say that putting LARGER heaters in was a bad idea, I'll agree with you.
That would be a code violation, what I meant to say is putting undesized heaters is a bad idea in most cases.

As an aside, the FLA figure is an important figure. Motor makers are extremely willing to fib (their term is 'relable') about the horsepower of their motors. I just replaced a 7-1/2 hp motor that turned out to have been a 10 (larger) motor that Baldor had 'relabled' for some reason. Something to keep in mind the next time you wonder about those wire size charts in the motor tables ...
You still have to follow the label, and the motor manufacturer is required to label the motor correctly.

You're hardly 'hacking' things, or making them less safe, by protecting the motor more closely.
We will have to remain in disagreement here, IMO there is no good reason to undersize the OLs.
 
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