Relay with 24V Internal Transformer for Unit Heater - Transformer primary protection?

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Avalon004

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Location
Renton, WA, USA
Howdy,

I have the following circuit:

  • 20A branch circuit breaker
  • Aube RC840T-120 (Electromechanical relay with 120V/24V transformer for a 24V control signal)
    https://customer.honeywell.com/en-US/Pages/Product.aspx?cat=HonECC+Catalog&pid=RC840T-120/U
  • 24V thermostats
  • 115V 0.8A hydronic unit heater

How do I comply with transformer primary protection in accordance with NEC 2017?
The internal transformer is a 24VAC, 1.2VA, class 2 transformer.
I read Article 430.72.
Article 430.72(A) states:​
"General. A motor control circuit tapped from the load
side of a motor branch-circuit short-circuit and ground-fault
protective device(s) and functioning to control the motor(s)
connected to that branch circuti shall be protected against
overcurrent in accordance with 430.72. Such a tapped control
circuit shall not be considered to be a branch circuit and shall
be permitted to be protected by either a supplementary or
branch-circuit overcurrent protective device(s) [...]"​

Article 430.72(C)(3) states:​
"Less Than 50 Volt-Amperes. Control circuit transformers
rated less than 50 volt-amperes (VA) and that are an integral
part of the motor controller and located within the motor
controller enclosure shall be permitted to be protected by
primary overcurrent devices, impedance limiting means, or
other inherent protective means."​
 
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I don't see Article 430 as applying here. This is an Article 725 issue and there are no special primary protection requirements for Class 2 power supplies. I also don't see anything in the instructions that require anything more than the branch circuit OCPD.
 
Is this all components within a listed unit heater or is the transformer/relay in question something field added (and not a listed accessory)?
 
hbiss said:
All such transformers include either an internal fuse or impedance protection. So the answer is the manufacturer of the transformer/relay has taken care of that.

-Hal
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Then why have I had to help HVAC guys straighten out their controls on several occasions after they burned up a couple transformers before finally asking for my help?
 
kwired said:
Then why have I had to help HVAC guys straighten out their controls on several occasions after they burned up a couple transformers before finally asking for my help?
Click to expand...
Burned up the transformer, or blew the internal secondary fuse that many Class 2 transformers have?
 
jap said:
And what control reworking did you do to resolve this issue?


JAP>
Click to expand...
It is always HVAC guys that aren't very good with control circuits - usually have something confused and connected wrong creating a short circuit in the control wiring. Sometimes cross connections between two different control systems or improper "twinning" of controls when there is a pair of units.
 
I just wanted to say, thank you all.
Yall have been helpful.

@kwired, the transformer/relay in question is not a listed accessory of the unit heater and would be field added/installed.

I agree with don_resqcapt19's response that the branch circuit OCPD is all that I need in this case.
I agree with hbiss that the manufacturer has taken care of providing transformer protection.
 
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