Heater sizes for starters on larger motors

Normally when using heaters, I will get the FLA of the motor and look up a heater chart for the manufacturer of the MCC, usually a sticker in one of the buckets. And just cross reference it staight across. But several electricians at our plant are telling me to oversize the heaters one or two higher for heat. Even my supervisor was saying to take the service factor 1.2 and multiply it times the FLA and obtain correct size heater. But this is new to me, I have always figured most MCC's had the service factor calculated into their charts. Does anyone have any more information on this? I know that we are to oversize 125% of full load current for overloads, and I believe most MCC overload charts have that factored in already. So what does service factor have to do with this, or is service factor the reason why we are oversizing by 125%. I was just needing some clarification. Thanks
 

Isaiah

Senior Member
Location
Baton Rouge
Normally when using heaters, I will get the FLA of the motor and look up a heater chart for the manufacturer of the MCC, usually a sticker in one of the buckets. And just cross reference it staight across. But several electricians at our plant are telling me to oversize the heaters one or two higher for heat. Even my supervisor was saying to take the service factor 1.2 and multiply it times the FLA and obtain correct size heater. But this is new to me, I have always figured most MCC's had the service factor calculated into their charts. Does anyone have any more information on this? I know that we are to oversize 125% of full load current for overloads, and I believe most MCC overload charts have that factored in already. So what does service factor have to do with this, or is service factor the reason why we are oversizing by 125%. I was just needing some clarification. Thanks
With overload (thermal) heaters, they used to have 'compensated' and 'non-compensated' versions for ambient heat. Maybe this is why your supervisor is saying to upsize - is your MCC located outdoors? You're right, most vendors already have the 125% calculated, so you dont have to do the calc.
Most clients these days want to see their starters upgraded with Electronic, 'solid state overloads', such as a Eaton's C440 or Rockwell's E3 Plus, instead of the old style thermal heaters.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Normally when using heaters, I will get the FLA of the motor and look up a heater chart for the manufacturer of the MCC, usually a sticker in one of the buckets. And just cross reference it staight across. But several electricians at our plant are telling me to oversize the heaters one or two higher for heat. Even my supervisor was saying to take the service factor 1.2 and multiply it times the FLA and obtain correct size heater. But this is new to me, I have always figured most MCC's had the service factor calculated into their charts. Does anyone have any more information on this? I know that we are to oversize 125% of full load current for overloads, and I believe most MCC overload charts have that factored in already. So what does service factor have to do with this, or is service factor the reason why we are oversizing by 125%. I was just needing some clarification. Thanks
Do you mean the heater elements in the thermal overload relay?
I usually size these to motor FLC. The thermal delay allows the motor to operate at above rated current transiently like starting duty or the odd overload. But then we don't have that 125% requirement here.
 
With overload (thermal) heaters, they used to have 'compensated' and 'non-compensated' versions for ambient heat. Maybe this is why your supervisor is saying to upsize - is your MCC located outdoors? You're right, most vendors already have the 125% calculated, so you dont have to do the calc.
Most clients these days want to see their starters upgraded with Electronic, 'solid state overloads', such as a Eaton's C440 or Rockwell's E3 Plus, instead of the old style thermal heaters.
its indoors but in summer it can get hot in electrical rooms over 110 degrees
 

jap

Senior Member
To me when it comes to motors, full load is full load.

As others have said, the thermal delay allows some wiggle room in activation of the shutdown on temporary and most times unavoidable short time overloads.

Sizing overload protection above the full load current of the motor to me is defeating its whole purpose.

JAP>
 
Do you mean the heater elements in the thermal overload relay?
I usually size these to motor FLC. The thermal delay allows the motor to operate at above rated current transiently like starting duty or the odd overload. But then we don't have that 125% requirement here.
Yes I think I will. My supervisor said to go one over. But I don't recommend that usually. Usually they factor in service factor and ambient temperature already in the calculations when they make the heater charts. I think oversizing it can cause problems, it fails to be an over load at that point.
 

Isaiah

Senior Member
Location
Baton Rouge
Yes I think I will. My supervisor said to go one over. But I don't recommend that usually. Usually they factor in service factor and ambient temperature already in the calculations when they make the heater charts. I think oversizing it can cause problems, it fails to be an over load at that point.
Its not a huge concern, but 125% of FLA is also what drives the motor branch circuit cable size (430.22) since you're probably using MCP's for short circuit protection only. It is not wise to have OL setting exceed the ampacity of the cable.
 

kwired

Electron manager
Location
NE Nebraska
Most these starters if you look in catalogs for thermal element selection, you need to determine some characteristics of the application before you know which selection table to use.

For something like a MCC, a combination starter, or a simple enclosure for a single starter, the table they provide inside is the one the catalog would have pointed you to for the application.

All conditions can be same and yet you still will find there are different chart for one vs. two vs. three thermal devices being used in the application.

For motors with SF or 1.15 or higher you select value from chart based on nameplate amps.

The unit you come up with should have about 1.25% trip level factored into the table selections already.

If you have a motor with a SF or 1.0, many of them will tell you you need to multiply nameplate amps by .90 then make selection off that value.

I don't know that there are a lot of people that really know how to make the right selection for their application, and newer solid state overload devices don't help.

Other than quicker ability to pick up phase loss, I still think I'd rather see thermal overloads vs solid state. I have replaced many failed solid state overloads, but hardly ever have a thermal overload fail and if it does is usually more from being abused in some way vs just wearing out.
 

tom baker

First Chief Moderator
Staff member
Most these starters if you look in catalogs for thermal element selection, you need to determine some characteristics of the application before you know which selection table to use.

For something like a MCC, a combination starter, or a simple enclosure for a single starter, the table they provide inside is the one the catalog would have pointed you to for the application.

All conditions can be same and yet you still will find there are different chart for one vs. two vs. three thermal devices being used in the application.

For motors with SF or 1.15 or higher you select value from chart based on nameplate amps.

The unit you come up with should have about 1.25% trip level factored into the table selections already.

If you have a motor with a SF or 1.0, many of them will tell you you need to multiply nameplate amps by .90 then make selection off that value.

I don't know that there are a lot of people that really know how to make the right selection for their application, and newer solid state overload devices don't help.

Other than quicker ability to pick up phase loss, I still think I'd rather see thermal overloads vs solid state. I have replaced many failed solid state overloads, but hardly ever have a thermal overload fail and if it does is usually more from being abused in some way vs just wearing out.
Well stated, just follow the instructions on the starter door. I used to oversize thermals but then got smart and started using ambient compensated thermals.
If you oversize the thermal overload then the motor can pull more current and that will shorten its life. The service factor, normally 1.15 or 15% is a fudge factor in case the motor is not sized to the driven load correctly. NEMA allows service factor about the rated current, but this additional current generates heat which shortens motor life

This is an excellent resource by a Baldwin engineer well worth the read on motors
https://www.baldor.com/Shared/manuals/pr2525.pdf

And then I started using IEC overloads with NEMA starters, but thats a subject for a different topic
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
Heat, so the wire was also upped? Didn't think so.
Like everyone has said follow the door information, OR follow your boss and keep your job.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
To pile on about following the manufacturer's instructions:

MCCs in particular are required to be rated for 40C (104F) operation. So when the sticker inside of the door has the heater elements listed, that is the chart FOR THAT APPLICATION, based on the maximum ambient that the MCC is allowed to operate at.

Whether or not the OL manufacturer already has the 1.25 factor for a 1.15SF motor included in the sizing varies from mfr to mfr, you cannot generalize. A-B and Furnas/Siemens melting alloy and Square D NEMA OL relays for example are NOT inclusive of that, YOU must decide whether or not to allow for the SF amps or not and they give you instructions on adjusting the size if you want to. But Cutler Hammer and Furnas/Siemens bi-metal heater selections DO include the 1.25 factor already, so for them, if you have a motor with a 1.0SF, you must then size the heaters SMALLER. So if you then add 25% to the ones that ALREADY have it factored in, your motor protection is not going to start kicking in until the current is at 125% of 125%, or 156% of the motor nameplate FLA, allowing the motor to be damaged before it trips.

So again, you MUST follow the manufacturer's instructions, because only THEY know what you must do for THAT overload relay.
 

kwired

Electron manager
Location
NE Nebraska
To pile on about following the manufacturer's instructions:

MCCs in particular are required to be rated for 40C (104F) operation. So when the sticker inside of the door has the heater elements listed, that is the chart FOR THAT APPLICATION, based on the maximum ambient that the MCC is allowed to operate at.

Whether or not the OL manufacturer already has the 1.25 factor for a 1.15SF motor included in the sizing varies from mfr to mfr, you cannot generalize. A-B and Furnas/Siemens melting alloy and Square D NEMA OL relays for example are NOT inclusive of that, YOU must decide whether or not to allow for the SF amps or not and they give you instructions on adjusting the size if you want to. But Cutler Hammer and Furnas/Siemens bi-metal heater selections DO include the 1.25 factor already, so for them, if you have a motor with a 1.0SF, you must then size the heaters SMALLER. So if you then add 25% to the ones that ALREADY have it factored in, your motor protection is not going to start kicking in until the current is at 125% of 125%, or 156% of the motor nameplate FLA, allowing the motor to be damaged before it trips.

So again, you MUST follow the manufacturer's instructions, because only THEY know what you must do for THAT overload relay.
I am most familiar with Square D melting alloy units - every selection table I can ever recall using applied directly to motors with SF of 1.15 or 1.25. If you had a motor with a 1.0 SF you had to adjust your motor current - I believe to .90 before selecting a unit.
 

Isaiah

Senior Member
Location
Baton Rouge
I am most familiar with Square D melting alloy units - every selection table I can ever recall using applied directly to motors with SF of 1.15 or 1.25. If you had a motor with a 1.0 SF you had to adjust your motor current - I believe to .90 before selecting a unit.
Just a note, on the refinery side of things, we see 1.0SF usually for Explosion Proof motors in Class I, Division 1 locations. These motors invariably have embedded T'Stat(s) in the windings and they must be wired directly into the control circuit but not to be confused with motor overloads.
 

kwired

Electron manager
Location
NE Nebraska
Just a note, on the refinery side of things, we see 1.0SF usually for Explosion Proof motors in Class I, Division 1 locations. These motors invariably have embedded T'Stat(s) in the windings and they must be wired directly into the control circuit but not to be confused with motor overloads.
I can see those being 1.0 SF. Most the ones I see are usually a definite purpose motor.
 
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