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Joined: Mar 2003
Posts: 849
Y
Member
Can a 480/277 Sub panel be feed with straight 480 plus ground??? No neutral loads are required. All loads straight 480 v 3 phase.

2017 / 2014 NEC & Related Books and Study Guides
Joined: Mar 2003
Posts: 849
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Found my answer , (NO) 240.85 the breakers that fit in a 227/480 volt panel are rated 277 to ground and they must be rated 480 volt to ground incase the hookup feeding the panel is a 480 volt Delta not a Y.
Yoopersup

Joined: Dec 2004
Posts: 94
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tkb Offline
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But the power will still be a Y system, just without the neutral connection.
It will still be 277 to ground unless you have a real 480 delta.
This is not uncommon in many industrial facilities.
I would just remove the neutral bar and re label the voltage on the panel.
Most likely would voilate the listing of the panel though. I would still be able to sleep at night though. [Linked Image]

Joined: Mar 2003
Posts: 849
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Not if fed with a True Delta secondary not a Y which is common in many Industral applications /

Joined: Nov 2000
Posts: 2,148
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tkb,
A 3 phase wye secondary without a connection to XO is an ungrounded system and would require 480 volt breakers. It only becomes 277v to ground when you bond XO. This system behaves exactly like an ungrounded delta and there is no way to tell the difference without looking at the transformer itself.
Don


Don(resqcapt19)
Joined: Jan 2005
Posts: 354
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pdh Offline
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Just to add to resqcapt19's post, a wye secondary could be bonded to ground at an end, such as X1, and then it would behave much like a corner grounded closed delta. That could be a suitable replacement transformer for an existing corner grounded delta system.

Now a question is, if the supply really is 480Y/277 at the source (X0 bonded), and the conductors brought to the panel are only X1,X2,X3 and G (not neutral), can you still safely and legally use 480/277 ratings? One thing you obviously cannot do is connect any 1-pole 277 volt loads. But 2-pole and 3-pole breakers (such as EDB series for Square-D NF panels) would be rated at 480/277 volts, so I would think they can be used even though the neutral is absent.

If this is an ungrounded or impedance grounded system, then it has to be treated as a pure 480 volt system since under a single-fault condition, it actually will be. And 240.85 limits 480/277 breakers to only solidy grounded systems with (not more than) 277 volts to ground. I assume Yoopersup's "straight 480 plus ground" could be a corner grounded delta where you cannot use 480/277 rated breakers (unless you know for sure otherwise, assume the worst). But can you use them if the system is definitely a 480Y/277 even if the neutral is not present?

Joined: Jul 2001
Posts: 599
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JBD Offline
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Voltages ratings for panelboards are line->line and line->ground. The presence, or absence, of a grounded conductor is not germane. As long as the system is a solidly grounded wye the "slash" ratings of the equipment may be used.


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