Power transformers power loads directly - motors, heaters, lighting, other equipment. Control transformers power relay coils, contactor coils, pilot lights, PLCs, etc., the devices that might control the power loads.

xnuke
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Okay.. So if I have a "control" transformer in a panel and it controls relays and PLCs and DC power supplies and a motor?

Keith Cress
kcress - http://www.flaminsystems.com

"Control Power Transformers" (CPTs) have different characteristics in terms of overload capabilities and regulation under saturation conditions, hence the differences in allowable OC protection (and higher cost). If you are going to run a small motor from a CPT, that's fine. The bigger problem would be in trying to do controls behind a distribution type transformer that was not selected to handle the brief overloads and maintain voltage regulation. You likely would need a larger distribution transformer to avoid a significant VD when you start the motor, compared to a CPT.


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Okay gents. I think I have a better bead on the issue now.

Thanks!

Keith Cress
kcress - http://www.flaminsystems.com

I've always looked at this from the aspect of, is it bigger than a shoe box.
Bigger transformers tend to be optimized for cost of materials.
Smaller transformers tend to be optimized for cost of assembly.

I don't have NEC but I see 500% for transformers with <2A rated primary winding current everywhere else.

Dear Mr. itsmoked (Electrical)(OP)23 Mar 22 00:04
"...In sizing primary side protection for a transformer I'm seeing that if this transformer is in a power application the maximum protection allowed is 300% over the full load primary current. But then NEC goes on to say if it's used in a control application the maximum primary protection is limited to 500%! What is the definition of "power" verses "control"? "
1. In NEC: there is (mention) but [without definition] of "control circuit transformer" with rated current <2A; the maximum primary protection is limited to 500%!. The word "control circuit ..." not " control ..." explains. It is understood that there is a predetermined connected load. There is an initial high inrush of current, until the armature of the relay is sealed....
2. In NEC: there is definition of "Transformer" but not " Power transformer". The former is defined which intended to mean an individual transformer.....". It followed with requirements that is for "power utility" applications, which are different from the "control circuit transformer". Their characteristic/requirement differs, therefore the protection requirements.
Che Kuan Yau (Singapore)