Control transformer

[hhsting]

I have project where for mechanical roof top units require control panel. The control panel is 120V. However voltage system is 480V.

So they are providing 480V to 120V transformer control. Would this kind of transformer separately derived or non separately derived? Would 250.30 apply? Also o any 240.21 apply?

 

[jim dungar]

Is this transformer internal to a UL Listed control panel?

If it is not, why do you think it would be treated differently than any other transformer?

 

[GeorgeB]

Before we were concerned with UL approval, we did conceptually similar panels 3 ways. Most common was a CPT as Jim mentions. Very common was a MCC bucket with an internal CPT, sometimes "extra capacity" if the panel needed it. Last, which does not apply here, was for the customer to supply "120". What we had INTERNAL to our panels was not subject to the NEC. We didn't consider loads on a MCC transformer any different from if we furnished it in our panels, but we may have been wrong, even in the 1980s.

 

[petersonra]

Is this transformer internal to a UL Listed control panel?

If it is not, why do you think it would be treated differently than any other transformer?

The rules are much the same for a xfmr inside a UL listed control panel. No GEC required if only a control xfmr and <= 1000 VA. Go over 1000 VA and a GEC is required.

 

[hhsting]

This transformer is provided by the customer outside the control panel

 

[petersonra]

This transformer is provided by the customer outside the control panel

It does not matter all that much. The rules are basically the same.

 

[augie47]

It does not matter all that much. The rules are basically the same.

agree,
Look at 250.30 exception #3 1000va is the key to your answer.