Feeding a Single Phase 480 Transformer from a 3 Phase 3 Wire Panel

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bohdand

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Hi,

I am looking at supplying a single phase 240x480 to 120/240 15kva transformer from an existing 3 pole breaker in a 3P 3W 480 panel board (ultimate upstream source is a grounded 480Y transformer) using only 2 phase wires with no neutral. The question I have is, does the code prevent me from doing this? I've been looking at section 250 and do not see where it would preclude doing this. What seems odd is that this new transformer will not have a neutral/grounded conductor on the high side (the low side would). Am I missing anything here?

Thanks,

Bohdan
 
Is it going to be utility owned (your profile says you are a utility engineer) or privately owned?
If utility owned, is it part of the utility's distribution infrastructure, or for use in their offices, etc.?
 
The code does not have an issue with that installation. It is just like any other feeder....you install the required circuit conductors and an EGC from the source to the load.
Of course you will need a grounding electrode and grounding electrode conductor for the secondary of the transformer.
 
Just be aware that there are a few breakers that require power flowing through all legs, which makes it difficult to use a 3 pole breaker for a 2 pole operation when the breaker is installed in a panelboard. Just make sure you ask this question of the breaker manufacturer first.
 
Jraef said:
Just be aware that there are a few breakers that require power flowing through all legs, which makes it difficult to use a 3 pole breaker for a 2 pole operation when the breaker is installed in a panelboard. Just make sure you ask this question of the breaker manufacturer first.
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I know that there are overloads that work that way but I am not aware of any breaker that works that way.
The UL White Book for "Circuit Breakers, Molded Case and Circuit Breaker Enclosures (DIVQ) says the following:
3-pole circuit breakers are suitable for use on 3-phase systems. A 3-pole breaker used in place of a 2-pole breaker on a 3-phase system, such as a 2-pole breaker used in a branch circuit that is actually two legs of a 3-phase system, is acceptable without the 3-pole breaker being specifically marked.
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This applies to breakers listed under UL 489.
 
don_resqcapt19 said:
I know that there are overloads that work that way but I am not aware of any breaker that works that way.
The UL White Book for "Circuit Breakers, Molded Case and Circuit Breaker Enclosures (DIVQ) says the following:

This applies to breakers listed under UL 489.
Click to expand...

I have not seen one in a long time but I did run across a MCCB once that had some kind of single phasing protection built into it. Might be what Jraef is talking about. I don't recall if it was something tied into a shunt trip or not though. Was a long time ago.
 
don_resqcapt19 said:
The code does not have an issue with that installation. It is just like any other feeder....you install the required circuit conductors and an EGC from the source to the load.
Of course you will need a grounding electrode and grounding electrode conductor for the secondary of the transformer.
Click to expand...

Thanks. You refer to running the EGC. Since this is in a Generating Plant and we have a Ground Grid, is there any reason I cannot just attach to the nearest ground grid connection? In the 50 year old plants I typically work in, there are no EGCs run between panels or to motors, just the three phases. The conduits are all bonded and grounded.
 
petersonra said:
I have not seen one in a long time but I did run across a MCCB once that had some kind of single phasing protection built into it. Might be what Jraef is talking about. I don't recall if it was something tied into a shunt trip or not though. Was a long time ago.
Click to expand...

It's IEC designed breakers that do this, because in other parts of the would they use breakers as Manual Motor Starters by leaving the thermal trips adjustable instead of fixed. So as starters, the thermal elements are the OL and have the same differential current biassing of the trips as their bi-metal OL relays do. So they will require "looping through" all 3 poles. The thing is, a number of the breaker mfrs are now owned by EU based corporations (Siemens, Schneider, Eaton, ABB), who have been slowly integrating their IEC designs into their product mix here as "World Class" breakers.
 
Jraef said:
The thing is, a number of the breaker mfrs are now owned by EU based corporations (Siemens, Schneider, Eaton, ABB), who have been slowly integrating their IEC designs into their product mix here as "World Class" breakers.
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If the breaker is listed to UL489, it makes no difference if it is a EU design.

Panel breakers, often used internal to equipment, are rarely encountered as feeder and branch breakers in Switchboards and Panelboards.
 
bohdand said:
Thanks. You refer to running the EGC. Since this is in a Generating Plant and we have a Ground Grid, is there any reason I cannot just attach to the nearest ground grid connection? In the 50 year old plants I typically work in, there are no EGCs run between panels or to motors, just the three phases. The conduits are all bonded and grounded.
Click to expand...
If you are installing to the NEC, 250.30(A)(4) specifies the required grounding electrodes.

As far as the EGCs, metallic conduit is permitted to serve as the EGC.
 
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