ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
What does your work place look like?
by NORCAL - 11/29/20 04:38 PM
Where is Everyone?
by gfretwell - 11/27/20 06:18 PM
Happy Thanksgiving
by gfretwell - 11/26/20 08:52 PM
Insulated Tools: A Waste of Money?
by HotLine1 - 11/26/20 12:28 PM
Ground fault remover :-)
by NORCAL - 11/26/20 08:13 AM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
1 registered members (Scott35), 21 guests, and 15 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 2 1 2
Thermal Overload Protection #177633 05/08/08 08:13 PM
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
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.





Tools for Electricians:
Re: Thermal Overload Protection [Re: sparkyinak] #177681 05/09/08 03:57 PM
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
Anybody out there?

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

Re: Thermal Overload Protection [Re: dura101] #177719 05/10/08 07:24 PM
Joined: Jul 2002
Posts: 8,346
Trumpy Offline
Member
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 07:27 PM. Reason: Typo's
Re: Thermal Overload Protection [Re: Trumpy] #177730 05/10/08 10:48 PM
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
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 05:38 AM
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
Thanks TWH, that is what I was looking for.

Re: Thermal Overload Protection [Re: dura101] #177837 05/13/08 01:42 PM
Joined: Jun 2007
Posts: 106
M
mr_electrician Offline
Member
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 08:46 PM
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
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 07:42 AM
Joined: Jun 2007
Posts: 106
M
mr_electrician Offline
Member
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 12:59 PM
Joined: Jun 2006
Posts: 613
M
mikesh Offline
Member
No you don't have to add an overload. Sorry code book is not handy but section 28 does mention thermally protected motors.

Page 1 of 2 1 2

Featured:

2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
HotLine1
HotLine1
Brick, NJ USA
Posts: 7,218
Joined: April 2002
Show All Member Profiles 
Top Posters(30 Days)
Trumpy 9
Popular Topics(Views)
272,099 Are you busy
205,995 Re: Forum
194,028 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3