480V Wye Vs Delta Distribution Panels 6

[engineerithink]

Please help. I am a new engineer which explains the simple question. When purchasing a distribution panel to be fed by 480V delta 3 phase 3 wire is it okay to buy one rated for 480V wye 3 phase 4 wire? Are they always interchangeable? What differences would I have to look out for?

Thanks,

 

[whycliffrussell]

Yes. that is okay. the only difference is if you buy a 4 wire panel the fourth wire (neutral) does not get connected b/c you don't have a neutral on your system (3 wire ungrounded system). however i would specfiy a 3 wire panel if this is a new installation (just becuase it makes sense and that's what you need).

 

[itsmoked]

I could easily see some serious confusion a few yeas down the road if someone thinks they can get wye out of the panel.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[alehman]

I agree with Keith. If you have a 4-wire panel, the neutral should be connected.

 

[rcw retired EE]

A panel and breakers rated for 480/277 3-phase, 4-wire, may not be suitable for 480 V 3-phase, 3-wire service from a delta system, if the delta is ungrounded, or corner grounded. The breakers may only be rated for the 277 volts to ground on each phase. A ground fault on a delta system could place full phase-to-phase voltage on the panel and breakers.

But I believe the design and certification testing of the panel is the same for a grounded wye or ungrounded delta system. So the panel is not different. But some breakers may only be listed for 480/277V.

You can buy a 3-phase, 4-wire panel and use it on a 3-phase, 3-wire system. Just make sure the neutral bar is removed and the circuit breakers are rated for full voltage.

 

[itsmoked]

Excellent idea rcwilson. Remove the bar and you will remove the confusion too.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[whycliffrussell]

yes. so none-the-less the panel can be used. and here's another brilliant idea - Label the panel!

 

[BJC]

"Remove the bar and you will remove the confusion too."
You also remove the UL listing and may get the installation red tagged.

 

[engineerithink]

Thanks all for the info. Best move ever was to register for this site.

 

[waross]

Is this an old system? I haven't seen a new 480 volt delta system installed for decades. If there is a chance that the system may be upgraded to a wye system any time in the future a four wire panel will be reusable.
respectfully

 

[engineerithink]

Yea because of my lack of experience I didn't know that this was hardly done. There is no way that the system will ever be changed.

 

[jraef]

rcwilson is dead on.

With 480V systems, SOME breakers are rated for 3ph 3 wire installations, some are not. Those that are not are called "Slash" breakers, because the ratings will say "480/277V", meaning that their interruption capacity was tested based solely on 4 wire systems where the maximum voltage to ground will be 277V. So therefore, SOME panelboards are rated for 3 Ph 3 wire installations, some are not. Breakers that are rated for 3 wire can be used on 4 wire installations, but not the other way around. Because of that, panels that are ordered for 3 wire installations have provisions that reject the installation of Slash breakers, and conversely when you buy a panelboard with the coding for a 4 wire bus, you cannot change it to be used on a 3 wire installation by removing the bus because you will be able to plug in Slash breakers that cannot be connected to 3 wire. Actually you CAn physically do it, but you are violating it's UL listing and subjecting yourself to liability.

So make sure of what you have and need, then specify it carefully.

 

[itsmoked]

Thanks Jeff for the reason.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[alehman]

One point of clarification - the discussion of 3W vs. 4W rated breakers pertains to the system grounding, not the number of wires in the panel. A 3W panel connected to a 4W solidly grounded system via a 3W feeder can use "slash rated" breakers. If the system is truly 3W (ungrounded, resistance grounded or corner grounded), slash rated breakers cannot be used.

 

[jraef]

Good point, although that would be a sub-panel not a main distribution panel.

 

[Adamito]

As I am just out of school, I still don't understand a few things:

As I understand it, a slash breaker cannot be used for 3 wires equipment because it has to support line-to-line Voltage instead of just line-to-ground.

If I am right, then why do a 3W panel connected to a 4W solidly grounded system via a 3W feeder can use "slash rated" breakers?

 

[alehman]

If the system is 4W solidly grounded, the highest voltage that can appear across one pole of a breaker is the L-N voltage (277V), regardless of whether or not there is a neutral carried to the panel.

Think about a line-to-ground fault. If the system serving the panel is 4W solidly grounded, the highest voltage from phase to ground is 277V, regardless of whether or not there is a neutral carried to the panel. There is still a neutral-ground bond at the source. That situation doesn't change if there is a neutral carried to the panel on not.

 

[waross]

I understood that a two pole or three pole breaker feeding a 480 volt load experienced the same maximum voltage issues on either a delta or wye system.
I was under the impression that slash rated breakers were only available in single pole configuration for this reason.
Comment?
respectfully

 

[alehman]

Think about a 480V delta system with B-phase grounded. Normally one would use 2-pole breakers on such a system, wired in the A and C phases. Say there is a fault from phase A to ground. The A phase pole on the breaker will see 480 volts.

On a grounded 4W system, the maximum voltage across any one breaker pole is the L-N voltage.

 

[jraef]

waross,
It used to be that way, but IEC manufacturers trying to sell MCBs (Miniature Circuit Breakers) here in the US exploited that slash rating issue to make their products more "acceptable" to 480V users. You probably don't see it happening much in Canada because they don't have a 600V "slash" rating at all.

So they (we, because I now work for Siemens) are now selling 3 pole MCBs with 480/277V ratings on them. I for one try to educate the users on just what this means, but 99% of them don't want to hear it because they want the cheap price. I make my salespeople walk away from those customers, but most of my competitor's salespeople have no clue what it means and just sell them willy-nilly. I have even started seeing them being used as Main circuit breakers on control panels recently because several manufacturers have now come out with thru-door interlocking handle operators. It's a disaster waiting to happen in my opinion, but nobody seems to be listening.