Delta Wye Transformer Wire Sizes

[carl.arnsdorff]

I am using a 24 kVA 480/240 VAC Delta-Wye transformer in a piece of equipment. I have a 65 A 3 pole breaker on the primary. The 65A breaker is routed inside the equipment to terminal blocks where line power is connected.

The secondary goes into a 4 pole 32 amp breaker where it goes into distribution blocks to the smaller single pole breakers to feed switching power supply loads, computers and other similar equipment.

I am specifying a 480 VAC, 30 Amp supply from the panelboard. The wiring and transformer are internal to the equipment. I plan to use 10 gage wire for the Panel wiring before the primary.

The wire(s) from the breaker(s) to the primary/secondary are 150C wire supplied with the transformer.

Shouldn't the incoming power wire be sized according to the breaker on the distribution panel (30A)?

How do you size the neutral wire and how do you size the ground wire for all three - 1) the equipment ground 2)the electrode ground and 3) the transformer shield?

What size

 

[augie47]

I may misundertand the question... but looking at it from an inspector/NEC standpoint, most of what I see you asking would be covered by the appropriate panel assembly codes like NFPA 79 and by the NRTL listing and labeling agency (UL, etc). The feed to the assembled unit would meet the nameplate requirements with grounding size accordingly.
I'm sure there are folks on the forum who can aswer the "internal" wiring questions if that is your question intent.

My opinion would be that since there is an internal SDS, there would need to be a #8 minimum gr. electrode conductor to appropriate building electrodes.(250.66/250.30)

 

[carl.arnsdorff]

Delta Wye Transformer supply and grounding wire sizes

Delta Wye Transformer supply and grounding wire sizes

thank you - this answers the grounding electrode conductor size. :smile:

A couple of other items remain. :-?

First is how to size the wiring to the transformer. Since the incoming feed is specified to be 30 amps but the installation sites may install larger panels/disconnects. For example it is possible for the particular site to supply a 100 amp supply) and my first breaker to the transformer is a 60 amp breaker (sized larger to ensure no nuissance tripping and that the secondary is protected at 125%) would the internal wiring to the transformer including the equipment ground conductor be larger? If so, how large - should it be beased on the 60 amp breaker?

Or if I am specifying a 30 amp supply should the internal grounding and wiring be sized for 30 amps up to and past the 60 amp breaker to the primary of the transformer?

Second is how do you size the transformer shield ground? Is there a requirement or is this sized per manufacturer recommendations?

 

[carl.arnsdorff]

Help with Delta Wye Transformer Wire Sizes

Help with Delta Wye Transformer Wire Sizes

Can anyone help with these questions?

 

[Smart $]

Can anyone help with these questions?

I think you have us confused a bit. You ask how to size, but yet seem to have already made the determination... yet it all seems a bit out of the ordinary.

First, a 240Y/139 transformer secondary is uncommon output voltage for 3?. Common would be 208Y/120 or 240/120.

With a load of less than 32-amperes per line, a 24kVA transformer seems a little large.

Why a 4-pole circuit breaker on the secondary? Is fourth pole for the neutral?

Is there any load-side equipment or device that requires 3? power?

Any motors?

...

A couple questions I can answer is: 1) the secondary neutral is required to be grounded with exception under few conditions, and 2) if this unit is considered an industrial control panel, it is a code violation for its feeder circuit to have a higher ampacity rating than the input overcurrent protective device.