480/277V Rated Breakers on a Delta System

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templdl said:
It is the voltage to ground that is the issue. 277v doesn't qualify as the voltage to ground is 480v. A straight 480 or 600v rated breaker does.
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Either the delta is corner grounded (in which case he has 480 to ground) or it is ungrounded, in which case NEC says to use the line to line voltage instead (480 again), or he actually has a solidly grounded wye with the neutral missing, in which case the 480/277 would be OK.
 
GoldDigger said:

Either the delta is corner grounded (in which case he has 480 to ground) or it is ungrounded, in which case NEC says to use the line to line voltage instead (480 again), or he actually has a solidly grounded wye with the neutral missing, in which case the 480/277 would be OK.
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Good point and I apologize as I was addressing a delta. Yes the '/' rating is the voltage to ground. Then there is the 240/120 3ph4w that is grounded which often times is misapplied.
 
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My breaker is 480/277 rated. After going over this post, and others, It is my understanding that these are safe to use on a 480Y system. They are also safe to use on a 480 Delta system, as long as it is not a corner grounded system. Is this correct?

Also, they are safe to use on any 277V or below system as long as it is not coming from a center tap derived from a 480 corner grounded Delta system? Are these accurate conclusions?
 
fifty60 said:
They are also safe to use on a 480 Delta system, as long as it is not a corner grounded system. Is this correct?
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NO.

The quick and very simple explanation:
Slash rated breakers can only be used on wye systems.

A more detailed explanation:
Slash rated breakers can only be applied on systems where the nominal Line-Ground voltage does not exceed the lower voltage rating. This condition only occurs on solidly grounded wye systems and on the center-tapped leg of some delta systems.

It seems the more detailed and thorough explanations often lead to confusion.
 
Thank you sir. So if I am using the breakers on a 240 volt single phase supply, even though it is single phase, I should list the supply voltage as 240VAC Y ?

Also, this only applies to the electrical service, and not the load. I can still use the breakers on 3 phase delta loads right?
 
fifty60 said:
Thank you sir. So if I am using the breakers on a 240 volt single phase supply, even though it is single phase, I should list the supply voltage as 240VAC Y ?
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NO.

The designations Delta and Wye only apply to 3-phase systems



fifty60 said:
Also, this only applies to the electrical service, and not the load. I can still use the breakers on 3 phase delta loads right?
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NO.
Equipment must be applied based on its ratings regardless of its location in the system.

Again keeping it simple:
The electrical system is defined by its source, which is usually the utility or a transformer secondary.
It is quite common to have circuits which are subsets of the original system, such as 2 wires + ground, when the service is a 3-phase 4-wire.

After while there comes a time when slang and simple explanations cause more confusion than they solve.
this is the primary reason I suggest the use of the voltage rating and descriptions per the NEC (i.e. Art 100 definition of Voltage, Nominal) which are based on IEEE/ANSI C84.1.
 
A 480/277 Y system has 277 volts from each ungrounded conductor to the neutral - a 277/480 rated breaker can be used on any pole of this system.

A 480 volt corner ground delta does not have any portion of the supply that operates below 277 volts to ground (aside from the grounded conductor itself) - you can not connect any load in a way that gives you less then 277 volts between conductors or ground on this sytem - a 277/480 rated breaker can not be used on this system.

A 480 volt delta with the midpoint of one phase grounded (leaving one high leg of 415 volts) can use a 277/480 rated breaker on all but the high leg, as the other two legs are 240 volts to ground. These systems are common out here on lower capacity irrigation services supplied by open delta configurations, but seem to be rare in most other places. We hardly ever are supplying circuit breakers on these systems either though, usually a 30 to 100 amp fused disconnect for service and they are usually limited to only one or two pieces of equipment that need a branch circuit - also usually supplied by fused disconnects instead of circuit breakers. More common yet is they are supplied by a feeder/feeder tap and the branch circuit device is already installed in the control panel for the equipment.
 
Thanks for the explanations. For 208V 3phase systems, there should not be any situation where the voltage to ground exceeds 277V. Is that correct? So any system below 277VAC, whether single phase or three phase, should be OK?

We are really only worried about 3 phase systems above 277V, and then we need to designate the supply as a Wye..
 
fifty60 said:
Thanks for the explanations. For 208V 3phase systems, there should not be any situation where the voltage to ground exceeds 277V. Is that correct? So any system below 277VAC, whether single phase or three phase, should be OK?

We are really only worried about 3 phase systems above 277V, and then we need to designate the supply as a Wye..
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Will we be getting royalties, for helping you design and apply your product?
 
rlundsrud said:
Since there isn't a code requirement for which phase the high leg is on, the breaker would have to be designed to handle the 480 volts phase to phase regardless of which of the phases the high leg was on. Given that bit of logic I would say yes, but that is an unqualified statement. When it comes down to it the manufacturer is who you would need to contact so you can verify that it is designed for that purpose.

Bob
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There are very few (if any) 480V high leg systems.
 
Jraef said:
It's only because there is a feature on the slots for B phase on a 120/240 delta panel so that 1 and 2 pole slash rated breaker frames cannot plug into the B phase, and that for the 2 pole that are delta rated they have a feature that allows them to be plugged in anywhere. ...
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Hmmm... First time I've heard of a B-phase, slash-rated-breaker-rejection feature.

Last I heard the only rejection feature was mental. :blink:
 
jim dungar said:
... The quick and very simple explanation:
Slash rated breakers can only be used on wye systems.
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Perhaps you mean this with respect to 3-pole slash-rated breaker, because otherwise it would not be true.

...

It seems the more detailed and thorough explanations often lead to confusion.
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Yes, and this thread is empirical evidence to that fact.

The matter is really quite simple...
A slash rated breaker cannot be used on any pole where:​
VLL > higher number​
VLN > lower number​
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...period. Any attempt to try and explain otherwise just confuses the issue.
 
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Smart $ said:
Perhaps you mean this with respect to 3-pole slash-rated breaker, because otherwise it would not be true.
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No, I mean what I said.

For maybe 90 times out of a 100, when a slash-rated breaker is going to be used it involves a 2-pole breaker and a 3-phase source. The simple 100% safe response is: if the source is not a wye do not use a slash-breaker.
The people that understand the actual issues are not the ones confused by the answers to the application question.

Keep it simple, answer the OP question.
Ask for feedback from the OP then go into more detail.
 
Carultch said:
There are very few (if any) 480V high leg systems.
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I can promise you they exist, and can show you probably a hundred or so within 50 miles of where I live.

Local POCO's no loner supply corner grounded delta systems - too many unqualified farmers messing with corner grounded systems creates too many hazardous situations. Most of these systems are supplied by open delta banks and supply limited single load applications - particularly irrigation equipment around these parts.
 
jim dungar said:
No, I mean what I said.

For maybe 90 times out of a 100, when a slash-rated breaker is going to be used it involves a 2-pole breaker and a 3-phase source. The simple 100% safe response is: if the source is not a wye do not use a slash-breaker.
The people that understand the actual issues are not the ones confused by the answers to the application question.

Keep it simple, answer the OP question.
Ask for feedback from the OP then go into more detail.
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Might be a safe response... but inaccurate.

For example, a 480/240V 3? 4W supply with 2-pole 480/277 breaker in spaces 5 and 7.
 
Smart $ said:
Might be a safe response... but inaccurate.

For example, a 480/240V 3? 4W supply with 2-pole 480/277 breaker in spaces 5 and 7.
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What part of keep it simple are you having problems with? Other than traffic lights and irrigation systems, this voltage system can be considered rare if not outright esoteric.

A general question can be given a 100% safe answer even if it does not cover 100% of the possible applications.
 
One esoteric point not yet mentioned is that for an ungrounded system the nominal line to ground voltage is taken to be the maximum line to line voltage.
 
jim dungar said:
What part of keep it simple are you having problems with? Other than traffic lights and irrigation systems, this voltage system can be considered rare if not outright esoteric.

A general question can be given a 100% safe answer even if it does not cover 100% of the possible applications.
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What part of inaccurate do you not comprehend?

I did not say your 'guideline' wasn't 100% safe.
 
Smart $ said:
What part of inaccurate do you not comprehend?
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It may be incomplete but it is not inaccurate, based on my stated premise of a quick and very simple answer.

We have taken a topic that should have been addressed in maybe 4 or 5 postings and stretched it to +40 posts. It it reasons like this that inexperienced posters rarely come back back into the discussion.
The OP appears to be building a piece of equipment that will be used in commercial or light industrial applications. I would hazard a guess that the odds are 1 in a million that one will be installed on a 480/240 3-phase 4wire center tapped delta.
Again, the very simply answer: if the design uses slash-rated breakers the instructions should call for a wye supply system, with a note to contact tech support for other voltage systems. If the equipment must work on any possible system, slash-rated breakers should not be used.
 
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