# Motor overload/heater sizing- UGLY's



## MDShunk (Jan 7, 2007)

You can go up to 1.25, but 1.15 is more traditional for sizing heaters, is really all they're saying. 

I tend to pick overload heaters from the manufacturer's overload heater chart and not the book anyhow. There's a lot more choices.


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## Bird dog (Oct 27, 2015)

Has to do with NEC Art 430.32 whether the heaters are sized at 115%, 125%, 130% or 140%.


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## MotoGP1199 (Aug 11, 2014)

What is the PF of the motor?


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## MDShunk (Jan 7, 2007)

Not sure it would matter. For most of this overload stuff, you go by actual dataplate amp draw.


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## oliquir (Jan 13, 2011)

some motors are 1.15 service factor and others are 1.25


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## telsa (May 22, 2015)

Go with the manufacturer's numbers every time.

You can 'defend them' when questions come up.

Trying to argue from the basis of the NEC quickly turns into a hair-ball.

The Other Guys(r) will insist that their interpretation of the NEC is better than yours.

No-one is going to spend time arguing that the manufacturer doesn't know their own chit.

And, in any event, now the Other Guys(R) have someone new to argue with.

With the advent of the Internet, why would anyone try and figure things out when the NEMA players have it all spelled out some place on the Web?


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## paulengr (Oct 8, 2017)

Overload relays aren’t terribly accurate. So you size to 115% of rated load and Code limit is 125%. So it gives you a range of 105-125% if you have +/-10% accuracy.

Service factors can be anything but in practice I’ve only ever seen 1.0 and 1.15 based on thousands of motors. The 1.15 number is because one paragraph in NEMA MG-1 standard for motor manufacturers states that you get 200% of rated life of about 100,000 hours between rewinds or replacement if the service factor is 1.15 or basically oversizing it 15%. This is based on every 10 C degrees decrease in temperature doubles motor insulation life and 15% larger works out to 15%.


Sent from my iPhone using Tapatalk


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## JRaef (Mar 23, 2009)

Actually, the NEC statement of 125% is a reference point for the maximum value that can be used by the protective DEVICE, not your SETTING of the device. The actual SELECTION is what the OL relay mfr TELLS you to do. Some tell you to select for the motor nameplate FLA x 1.15, some tell you 1.25, some tell you the setting itself, NO adjustment factor because they ALREADY have that factored into their design! But if you use those that ALREADY factor in the 125% in that selection chart, and then YOU also add the 125%, the pick-up point of your OL relay (which is where it BEGINS to protect the motor) ends up at 125% OF 125%, so 156% of the motor nameplate FLA, and the motor is often damaged by the time the OL relay trips. 

So the only correct answer is... _*RTFM*_


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