Grounding a BB Transformer Used as a Gen. Purpose Transformer

[TommyDRC]

Hi all,

I usually wire motor control circuits and only work with small industrial control transformers. I'm now out of my element because I'm trying to wire a Buck-Boost transformer to be used as a general purpose transformer. More specifically, I'm having trouble figuring out how to ground this transformer. The transformer is a 1.5KVA BB transformer with a 240/480V primary and a 24/48V secondary. I'm using 2 three phase 230V legs on the primary side and tying X2 and X3 together on the secondary to generate 48V (I know that since I'm using two 3-phase 230V legs that I'm only getting 208V on the primary and about 40V on the secondary but that's okay). I'm then attaching two heating elements in parallel between X1 and X4. Per NEC 450.3(B) each element has a 2-pole secondary circuit breaker in series rated at 20A and the primary side has a 2-pole circuit breaker rated at 10A. Now that I've gotten that out of the way, here is my question...

Do I need to bond X4 to the case of the transformer and then bond the transformer case to my ground terminal strip in the cutoff enclosure? Also, how do I size these bonding wires? Do they need to be sized for the 50A that the primary circuit breaker would allow on the secondary side before clearing a ground fault? Thanks in advance for the help! I've read through NEC 250 and 450 several times plus multiple websites and feel thoroughly confused.

-Tommy

 

[augie47]

Welcome !

Double check 250.20(A) to make sure you don't fall within one of the "required" conditions. If not, as is probably the case, you do not need to ground the secondary as it is less than 50 volts.

 

[TommyDRC]

250.20(A)(1) says that if the transformer supply is above 150V then the 50V system must be grounded. My supply is 208V so I think I have to ground the 48V system.

 

[augie47]

Be sure to read the complete sentence .
I believe it says "over 150 volts to ground".
Unless you have an ungrounded supply you would not have over 150 volts to ground.
A "normal" 208 system would be 208Y/120.

 

[TommyDRC]

Yeah I realized I misread the sentence after I posted. If we assume that my supply voltage was greater than 150V to ground (e.g. I was using a 400V class supply on the primary side), then am I correct in saying that I would need to bond X4 to the case and ensure that I installed a properly sized ground path from the metallic surfaces around the heating elements to the case?