277v 480 4wire or 480v 3wire

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mannyb

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Electrician
I understand the difference between 4wire and 3 wire 480v one has neutral the other one doesn't but why would the 3 wire application but why would power company ask which one as we requesting. We are consulting a POCO enginner and they asked which one are we requesting. I was just wondering why?
 
You already know why. One has a neutral, one doesn't. You should know what type of loads you will be supplying. My money is on this post getting flagged

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WarrMann said:
You already know why. One has a neutral, one doesn't. You should know what type of loads you will be supplying. My money is on this post getting flagged
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What does that mean?

mannyb said:
I understand the difference between 4wire and 3 wire 480v one has neutral the other one doesn't but why would the 3 wire application but why would power company ask which one as we requesting. We are consulting a POCO engineer and they asked which one are we requesting. I was just wondering why?
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Because you can choose either type based on your loads.
 
Will you have 277 volt loads? If so you will need the 480/277 (which is a wye secondary) or else separately derive it on site, but given choice up front you are better off getting what you need.

If you have limited load application, for example a pumping station, you may be more likely to desire the 480 volt three wire system (which is from a delta secondary).

Note that using typical 277/480 volt panelboards such as Square D's NF panels - they can't be used on a 480 volt delta, those breakers are 277/480 rated and can not be used when there is more than 277 nominal to ground, which means you need higher rated panels such as I-Line or use fuses witch is also more common for limited load applications.
 
kwired said:
Will you have 277 volt loads? If so you will need the 480/277 (which is a wye secondary) or else separately derive it on site, but given choice up front you are better off getting what you need.

If you have limited load application, for example a pumping station, you may be more likely to desire the 480 volt three wire system (which is from a delta secondary).

Note that using typical 277/480 volt panelboards such as Square D's NF panels - they can't be used on a 480 volt delta, those breakers are 277/480 rated and can not be used when there is more than 277 nominal to ground, which means you need higher rated panels such as I-Line or use fuses witch is also more common for limited load applications.
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That makes sense. There is alot of 480v 3phase 3 wire in my area but like you mentioned mainly in oldet industrial part of town. I was just asking why would anyone want 3phase 3 wire. Thanks.
 
We put in a LOT of 480v services, we've never been offered anything other than WYE as far as I know.

Since curiosity has got the best of me, I'll have to look at the power company service request sheets we have to see if 480v Delta is even an option, I doubt it though.
 
Good answers from others, but to add that if you will be using inverter variable speed drives, the wye will be preferred.
 
mannyb said:
I understand the difference between 4wire and 3 wire 480v one has neutral the other one doesn't but why would the 3 wire application but why would power company ask which one as we requesting. We are consulting a POCO enginner and they asked which one are we requesting. I was just wondering why?
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Is there a cost difference? If so, is it big difference?
If not, I'd be inclined to go for the 4-wire option.
 
texie said:
In today's world, it would be a rare POCO that would supply anything but a wye for a new 3 phase 480 volt service.
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Around here there is a fair amount of 480 volt open delta systems for limited load applications - in particular for irrigation services. One particular POCO no longer has any corner grounded delta systems, if it is a delta system they will ground midpoint of one phase so there is 240/480 and a 416 volt high leg.

Besoeker said:
Is there a cost difference? If so, is it big difference?
If not, I'd be inclined to go for the 4-wire option.
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For a full delta arrangement no significant cost difference to the POCO.
 
Ungrounded delta

Ungrounded delta

Depending on what they make some companies may still want an ungrounded delta with ground fault monitoring. If the operation needs to keep running and shut down it a orderly fashion under fault. We had a chemical reaction product that once you added water it had to mix for 45 minutes or it was $$$$$ money for cleanup.

Cowboy
 
just the cowboy said:
Depending on what they make some companies may still want an ungrounded delta with ground fault monitoring. If the operation needs to keep running and shut down it a orderly fashion under fault. We had a chemical reaction product that once you added water it had to mix for 45 minutes or it was $$$$$ money for cleanup.

Cowboy
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Float glass production lines are a bit like that. Stop it and the liquid glass solidifies in the furnace. Takes weeks to clear it out.
The few we did were DC drives with massive battery back up and mechanical arrangements that allowed two inputs to the gearboxes.

Sorry mods - a bit off topic,
 
just the cowboy said:
Depending on what they make some companies may still want an ungrounded delta with ground fault monitoring. If the operation needs to keep running and shut down it a orderly fashion under fault. We had a chemical reaction product that once you added water it had to mix for 45 minutes or it was $$$$$ money for cleanup.

Cowboy
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Exactly, it's an older concept still used in some industries. I call it the "Damn the torpedoes, full speed ahead!" philosophy. In an industrial facility if you have an ungrounded delta* system and you get a ground fault anywhere on it, the system goes from ungrounded delta to "corner grounded delta" for all intents and purposes, so the plant keeps on chugging along because no breakers trip. It's not ideal and you will want to KNOW that this has happened, hence the requirement for a "ground monitoring system", but again, nothing automatically shuts down. I used to work at a steel mill, that's what we had. If the arc furnaces shut down because a ground fault somewhere else caused the main breaker to trip, it would cost millions to rebuild. Ungrounded delta systems were common at one time, especially during WWII when war production took precedence over life and limb. But it's fallen out of favor now, especially with more modern ways to detect and isolate a fault at the lowest possible level, which also prevents that major shutdown. In addition as mentioned earlier, anything with power electronics will NOT like being connected to a delta system, so VFDs, DC drives, UPS systems, any large 3 phase power electronics. Most utilities now will either not even offer ungrounded delta any more or if they do (which is apparent in this case), they usually make it onerous.

* aka 3 wire in this context, but I agree it's not the best way to refer to it. In a lot of cases you may have a 4 wire secondary on the transformer and ground the X0 point, but if there are no loads that need a neutral, for instance an MCC feeding all 3 phase motor loads or other small single phase transformers, nobody runs a neutral wire out of the service entrance device. So everything after that can be referred to as "3 wire" but it is not a delta system, it's 3 wire from a solidly grounded wye system.

Besoeker said:
Float glass production lines are a bit like that. Stop it and the liquid glass solidifies in the furnace. Takes weeks to clear it out.
The few we did were DC drives with massive battery back up and mechanical arrangements that allowed two inputs to the gearboxes.

Sorry mods - a bit off topic,
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Not really off topic. He did ask why anyone would use it, you gave an example. :thumbsup:
 
Jraef said:
* aka 3 wire in this context, but I agree it's not the best way to refer to it. In a lot of cases you may have a 4 wire secondary on the transformer and ground the X0 point, but if there are no loads that need a neutral, for instance an MCC feeding all 3 phase motor loads or other small single phase transformers, nobody runs a neutral wire out of the service entrance device. So everything after that can be referred to as "3 wire" but it is not a delta system, it's 3 wire from a solidly grounded wye system.
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One other thing worth noting: I have heard that sometimes an "ungrounded delta" service is actually fed from a bank of transformers with a wye secondary and the wye point floating. This allows standard transformers to be used.
 
electrofelon said:
IMO the use of the term "3 wire" is a bit ambiguous. I assume they mean "ungrounded" yes?
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Not necessarily. Corner grounded delta is still a three wire system. Ungrounded or corner grounded systems you still need to run an equipment grounding conductor with your branch circuits and feeders, but there is still only three conductors that carry normal operating current.

electrofelon said:
One other thing worth noting: I have heard that sometimes an "ungrounded delta" service is actually fed from a bank of transformers with a wye secondary and the wye point floating. This allows standard transformers to be used.
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It still acts pretty much the same way as the ungrounded delta.

Ungrounded delta could have a mid point on each individual transformer that gets left floating and is in many ways the same thing, except you have three floating points in the system instead of one, but in either case such points are not extended beyond the source.

Also about the same thing as having a multi-volt single phase transformer secondary and utilizing two taps and insulating the remaining taps. That would still be considered a two wire system if connected in that manner.
 
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