240v 3 phase Corner grounded (Grounded B Phase) wiring

[Research1]

Subject: 240v 3 phase Corner grounded (Grounded B Phase) wiring
I need to confirm the usual way a 240v 3 phase corner grounded B phase system is wired.
Two methods seem to be permitted by code.
1. Using a 2 pole 240v rated main breaker with 2 pole branch breakers.
Connecting the service Phase A and Phase C wires to the main breaker and the service grounded B phase wire to the neutral bar in the panel. Connecting the load to the phase A and phase C with the phase B load wires to the neutral bar.
Of course using full rated wire for all 3 wires and a green equipment ground.

2. Using a 3 pole 240V main breaker with 3 pole branch breakers.
Connecting all 3 service wires to the breaker, phase A phase B and phase C, and connecting the Phase B wire to the neutral bar.
Connecting all three wires from the branch breakers to the load.

Jerome

 

[big john]

What are your expected loads and how is this set up?

For a bunch of 240V 3? loads coming off an SDS I would do my grounding and bonding in the transformer and then bring the conductors into a three phase main breaker. It simplifies wiring the loads because you can bring each circuit straight to a common-trip 3 pole breaker.

If you're making your bond in the panel then it's easiest just to have the neutral and 2P main. Just be aware you cannot use standard 2 pole breakers, they need to actually be rated 1?/3?. See 240.85

Also, in either case you cannot use slash-rated breakers and each pole must be rated for the full 240V.

 

[kwired]

I think you will find three pole breakers are easier to come by then double pole breakers that are rated for the application at least for miniature series type panelboards/loadcenters, they may or may not cost much different then a double pole rated for the application either.

Remember that grounded phase is not a neutral but still is a grounded conductor and needs to be white/gray identified, and must be separated from equipment grounding conductors beyond the main/system bonding jumper just like a grounded neutral does.

 

[Research1]

240v 3 phase Corner grounded (Grounded B Phase) wiring

Thank you for the reply. Very helpful.
There was a question on setup and loads. Here is the answer.

The utility has a 75kVA 240V 3 phase pole mounted transformer feeding into a meter box and 200Amp meter socket.
The utility lists the service as a 75 kVA 240V grounded B phase.
I will check to verify if the utility has grounded the transformer at the pole.
The loads are:
one 240V 3 phase 17 amp full load amp air conditioner
one 240V 3 phase 32.9 amp full load amp kiln
one 240V single phase 48 amp full load amp kiln
one 240V single phase 45 amp full load amp kiln


From the replies it looks like the panel can be either 3 pole mains and branches or 2 pole mains and branches. But the 3 pole needs to have the transformer B phase grounded. The 2 pole approach needs to have the service panel grounded.
Is this correct?

 

[scott thompson]

Two ways to use the Grounded Conductor on 3Ph 3W Delta

Two ways to use the Grounded Conductor on 3Ph 3W Delta

Check out these Schematics, regarding a 3 Phase 3 Wire Corner-Grounded Delta, and the Two ways the Grounded Conductor may be connected.

http://www.electrical-contractor.ne...orner_Grounded_3_3_Wire_Delta.html#Post148737

BTW, I have the Grounded Conductor on Phase "A" in these drawings... "Mentally Swap" Phase "A" and Phase "B" to correct the issue!

--Scott (EE)

 

[big john]

...From the replies it looks like the panel can be either 3 pole mains and branches or 2 pole mains and branches. But the 3 pole needs to have the transformer B phase grounded. The 2 pole approach needs to have the service panel grounded.
Is this correct?

With either approach you have to ground and bond one phase. On pole-mount transformers the utility will ground a phase at their end and that doesn't remove your responsibility to bond the service on your end and tie your equipment grounds to that point.

It's just that if the transformer were under your control, it would be acceptable to do your bonding there and bring a neutral and equipment ground from the transformer into your service equipment.

I suppose you could bring the grounded phase into the neutral bar of your service equipment, and then jump back up to the line side of your main breaker. This would allow you to use correctly rated 3? breakers on your loads and would make branch circuit terminations easier.

 

[Research1]

3 Phase 3 Wire Corner-Grounded Delta

3 Phase 3 Wire Corner-Grounded Delta

Check out these Schematics, regarding a 3 Phase 3 Wire Corner-Grounded Delta, and the Two ways the Grounded Conductor may be connected.

http://www.electrical-contractor.ne...orner_Grounded_3_3_Wire_Delta.html#Post148737

BTW, I have the Grounded Conductor on Phase "A" in these drawings... "Mentally Swap" Phase "A" and Phase "B" to correct the issue!

--Scott (EE)

Scott, Thank you for your reply. The disgrams were super. It explained the wiring of the corner grounded Delta perfectly.
Best regards
Jerome

 

[Research1]

With either approach you have to ground and bond one phase. On pole-mount transformers the utility will ground a phase at their end and that doesn't remove your responsibility to bond the service on your end and tie your equipment grounds to that point.

It's just that if the transformer were under your control, it would be acceptable to do your bonding there and bring a neutral and equipment ground from the transformer into your service equipment.

I suppose you could bring the grounded phase into the neutral bar of your service equipment, and then jump back up to the line side of your main breaker. This would allow you to use correctly rated 3? breakers on your loads and would make branch circuit terminations easier.

John,
Thank you for your reply. Your suggestion of connecting the grounded B to the neutral and back wiring to the breaker and then treating it like any other 3 phase service, using all 3 pole breakers seems like the simplest approach.
Regards
Jerome