Wire Sizing

[tarun]

I have a 45KVA transformer (480/240V) which will have maximum connected load of 36KVA. My breaker size for protection of transformer primary is 70A. The wires will be laid in conduit inside the building (temperature always maintained below 85F). There will be only 3 wires in the conduit. Wire length is 100 feet. Maximum allowable voltage drop is 3%

What should be my wire size?

 

[Buck Parrish]

It could be as low as #4 cu. awg Depending on some other circumstances.

 

[tarun]

It could be as low as #4 cu. awg Depending on some other circumstances.

Thanks. Can you suggest the wire size on secondary of this transformer which will run to a 3 phase panel in a conduit of 20 feet length.

 

[tarun]

It could be as low as #4 cu. awg Depending on some other circumstances.

What other circumstances may need higher size wire?

 

[Smart $]

You'll earn the title of electrical Houdini for wires laid in conduit... :happyyes:

Need some clarification. Is the transformer 1Ø or 3Ø? Respectively, is the secondary configuration 120/240V 1Ø 3W or 240V 3Ø 3W?

Have to ask because 70A primary ocpd is quite low for 1Ø but good for 3Ø primary-only protection. And if 1Ø, you'd need secondary conductor protection.

Process of elimination says it's a 240V 3Ø 3W secondary, but want to make sure...

EDIT: I see answer was posted while I was typing.

 

[Carultch]

Thanks. Can you suggest the wire size on secondary of this transformer which will run to a 3 phase panel in a conduit of 20 feet length.

See 240.21(C) for transformer secondary conductors.

 

[Smart $]

You're looking at 1/0 copper, minimum... with no consideration for voltage drop at 100'.

 

[Carultch]

And if 1Ø, you'd need secondary conductor protection.

I thought single phase and delta-delta 3-wire ungrounded, were allowed to have the primary OCPD protect the secondary conductors by proxy.
It is usually all other 3-phase configurations (WYE, high leg delta, corner grounded delta etc) that require OCPD on both sides.

The idea is whether or not fault current is guaranteed to line up winding-to-winding. And if it is, then only primary OCPD is required. If not, then OCPD on both sides is needed.

 

[Smart $]

I thought single phase and delta-delta 3-wire ungrounded, were allowed to have the primary OCPD protect the secondary conductors by proxy.
It is usually all other 3-phase configurations (WYE, high leg delta, corner grounded delta etc) that require OCPD on both sides.

The idea is whether or not fault current is guaranteed to line up winding-to-winding. And if it is, then only primary OCPD is required. If not, then OCPD on both sides is needed.

It's 1Ø 2W and 3Ø 3W that can get away with primary protection.

I'm gathering this will be either 240V 3Ø 3W ungrounded or corner grounded.

 

[don_resqcapt19]

The secondary conductors can be any size you want as long as they are protected by one of the rules in 240.21(C).

 

[Carultch]

It's 1Ø 2W and 3Ø 3W that can get away with primary protection.

I'm gathering this will be either 240V 3Ø 3W ungrounded or corner grounded.

Thanks for the clarification.