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Thermal Overload Protection #177633 05/08/08 09:13 PM
Joined: May 2008
Posts: 13
dura101 Offline OP
I have always been confused by the code when it talks about overload protection.

If I have a 120v motor and it's full load current is 1amp, and it says on the nameplate that the motor is thermally protected, do I have to install an overload for this motor?

-not attended while in operation

I understand the code when it talks about larger motors but the smaller ones still confuse me.

Any help, and discussion on this matter would be appreciated.

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Re: Thermal Overload Protection [Re: sparkyinak] #177681 05/09/08 04:57 PM
Joined: May 2008
Posts: 13
dura101 Offline OP
Anybody out there?

Re: Thermal Overload Protection [Re: dura101] #177686 05/09/08 06:44 PM
Joined: May 2003
Posts: 1,155
dougwells Offline
I would say no that isnt much of a motor

Re: Thermal Overload Protection [Re: dura101] #177719 05/10/08 08:24 PM
Joined: Jul 2002
Posts: 8,390
Trumpy Offline
Hi there dura,
What is this motor running, in terms of a load on the shaft?

Most single phase motors of this size often have a simple circuit-breaker on the terminal box of the motor, it is meant to trip at a given value of current, not unlike the over-current trip on a multi-plug board, if the motor does trip, you just press the overload and the motor should be able to be re-started.
I've never seen external overloads used on fractional HP motors, but I could be wrong too.

With larger motors, however, they normally have thermistors embedded in the windings, the tails of which come out into the motor terminal box and are connected to an external control circuit and contactor that disconnect the motor, should the windings get too hot.

In this case though, I don't think this application warrrants a thing like that.

Last edited by Trumpy; 05/10/08 08:27 PM. Reason: Typo's
Re: Thermal Overload Protection [Re: Trumpy] #177730 05/10/08 11:48 PM
Joined: Mar 2004
Posts: 947
twh Offline
I think the confusion is about a $50 motor protected by $150 of overload devices.

Thermal protection doesn't meet the requirements of 28-302(b) for overload protection unless it is responsive to motor temperature and motor current. I'm not sure that it is.

28-308(c) provides that "a motor constructed so that it cannot be overloaded" does not need overload protection. An example would be a bathroom exhaust fan that has such high resistance that when it stalls, it doesn't draw much current. Low current motors tend to be like that.

I apply that rule so that I don't have to install overload protection on furnace motors. I've seen many thermally protected furnace motors stall and trip the thermal protection. The windings remain intact.

Apply the rule differently and we'll be putting overload protection on bathroom exhaust fans, furnaces, air exchangers, air conditioners, etc.

Re: Thermal Overload Protection [Re: twh] #177738 05/11/08 06:38 AM
Joined: May 2008
Posts: 13
dura101 Offline OP
Thanks TWH, that is what I was looking for.

Re: Thermal Overload Protection [Re: dura101] #177837 05/13/08 02:42 PM
Joined: Jun 2007
Posts: 106
mr_electrician Offline
No you don't. See 28-308(d) This talks about your overload protection allready installed and part of the motor. Probably a thermister.

Never trust an electrician with no eyebrows!!
Re: Thermal Overload Protection [Re: mr_electrician] #177856 05/13/08 09:46 PM
Joined: May 2008
Posts: 13
dura101 Offline OP
Hi, I can see why you would say that I guess. Lets just say for argument sake that this motor has a little controller and it must be started by pushing the start button, not automatic starting. Overheating protection (thermally protected) is not the same as overload protection.

I know that current thru the windings of the motor create a temperature rise but I always thought that the bi-metalic strip that opens the motor winding was calibrated for a specific temperature. Not the the temperature created specifically by 1.15 or 1.25 times the FLC.

But I do see 28-302(1)(b)- It says that thermally protected motors don't need overload protection just as long as they respond to temperature and current. They must also be able to protect the circuit conductors, control circuit conductors as well as the motor windings.

So if you had a small controller for this motor with a contactor the thermal protection would open the motor windings only. It wouldn't open the control cicuit like a NC contact that is in an overload relay would do.

Re: Thermal Overload Protection [Re: dura101] #177974 05/20/08 08:42 AM
Joined: Jun 2007
Posts: 106
mr_electrician Offline
Motors that I have seen that employ thermistors have had the terminals extend to the j box. You could probably use the thermistor wires in your control cct so that when the thermistor senses that the windings are to hot that it would open the control cct.

Never trust an electrician with no eyebrows!!
Re: Thermal Overload Protection [Re: dura101] #178062 05/22/08 01:59 PM
Joined: Jun 2006
Posts: 613
mikesh Offline
No you don't have to add an overload. Sorry code book is not handy but section 28 does mention thermally protected motors.

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