Transformer

[hhsting]

I have 120V to 24V Edwards #592 transformer. I was wondering if its separately derived and does it require grounding electrode conductor, system bonding jumper or not?

 

[augie47]

See 250.20

(this appears to be an outgrowth of your thread https://forums.mikeholt.com/threads/transformer.2564508/ where the answer was already "no" )

 

[hhsting]

See 250.20

So if my primary is 120V to ground supplied from panelboard 208/120V three phase with equipment grounding conductor and my secondary is 24 vac then

240.21(a) or 240.21(b) applies?

 

[augie47]

So if my primary is 120V to ground supplied from panelboard 208/120V three phase with equipment grounding conductor and my secondary is 24 vac then

240.21(a) or 240.21(b) applies?

Nope it's Art 725 ... see the transformer tech sheet ...
it's basically a doorbell transformer.. ever seen them with grounding or external OCP ??

 

[wwhitney]

So what's the technical reason for 250.20(A)(1)?

It says that with a 480V:24V transformer, you'd have to ground one of the 24V conductors. To get 24V ungrounded from a 480V (or 277V) supply, you'd have to use two transformers, e.g. 480V:120V (where one leg of the 120V is required to be grounded), and then 120V:24V.

Cheers, Wayne

 

[don_resqcapt19]

So what's the technical reason for 250.20(A)(1)?

It says that with a 480V:24V transformer, you'd have to ground one of the 24V conductors. To get 24V ungrounded from a 480V (or 277V) supply, you'd have to use two transformers, e.g. 480V:120V (where one leg of the 120V is required to be grounded), and then 120V:24V.

Cheers, Wayne

To provide some protection from a primary to secondary short in the control power transformer. Such a short is a bit more likely and more hazardous at higher voltages.
If you look at 480 volt AC equipment you will find that the factory has grounded the secondary of the Class 2 control power transformer.