3 phase service 1 phase sub panel

[Kessler4130]

Topic basically says it all, I have a job where the Project manager submitted a single phase panel as a sub panel from an existing 3 phase panel.

1) My argument is that supplying a single phase sub panel from a 3 phase Y regardless of the immediate load can potentially be used to severely unbalance the load at some point down the road.

2) Also as far as I know there is no panel board rated as 120/208v Y 3wire, only 4 wire.

Aside from the NEC requiring all equipment be rated and listed for the purpose I cannot think of anything else to possibly tell him. He also believes this sub panel which is in the very next room requires a main breaker, I see 408.36 allowing me to use the OCPD at the main panel board as my only disconnecting means.

Any information is appreciated.

 

[charlie b]

You can install such a panel, and it doesn't have to have a main breaker. The question of imbalancing the load is worth looking into, but much depends on the nature of the loads on the subpanel, and the nature of the loads on the main panel. My biggest concern would be if the subpanel were to use 2-pole breakers to serve single loads. This would give you 208 volts, and most dryers, water heaters, and ranges that use 200+ volts are rated for 240, not 208. So they won't work as well.

 

[Kessler4130]

You can install such a panel, and it doesn't have to have a main breaker. The question of imbalancing the load is worth looking into, but much depends on the nature of the loads on the subpanel, and the nature of the loads on the main panel. My biggest concern would be if the subpanel were to use 2-pole breakers to serve single loads. This would give you 208 volts, and most dryers, water heaters, and ranges that use 200+ volts are rated for 240, not 208. So they won't work as well.

Thank you sir,

I still feel it should be a 3 phase main lug only panel, the proposed loads are all 120v and I do not foresee the installation ever seeing a 2pole load.

 

[LarryFine]

2) Also as far as I know there is no panel board rated as 120/208v Y 3wire, only 4 wire.

Any standard 120/240 1ph panel can be used.

He also believes this sub panel which is in the very next room requires a main breaker, I see 408.36 allowing me to use the OCPD at the main panel board as my only disconnecting means.

You're correct.

 

[LarryFine]

I still feel it should be a 3 phase main lug only panel, the proposed loads are all 120v and I do not foresee the installation ever seeing a 2pole load.

I agree with this, at least in principle, and it's likely what I would do, too.

In reality, the supply-end breaker, the feeder, and the panel all cost more.

 

[Kessler4130]

I agree with this, at least in principle, and it's likely what I would do, too.

In reality, the supply-end breaker, the feeder, and the panel all cost more.

Understood, and thank you for your input.

My only remaining question is, the submitted panel board says...
Rating(s):
120/240 single phase 3 wire
120/208 three phase 4 wire

I assumed this meant it was intended to be used with 3 phase voltage only in a 4 wire termination, obviously I am being to literal and will take a look at it again in a bit just to make sure. I do see where you are coming from thought and most likely his point of view as well, single phase equipment is much cheaper plus you are losing a wire.

It is possible the document I was looking at was referring to the single phase panel board and not its available orientation, so I stand corrected.

Thanks again for the help, regards.

 

[mcclary's electrical]

I see this done alot around here. It comes from people hiring the cheapest contractor around. Three phase panel, fully loaded, so they set a single phase panel right beside it and feed it with a two pole from the three phase panel. The reason I sya it's the cheap contractors doing this is they can't get three phase panels at Lowes and home depot, so they use single phase. I see it all the time, I correct it when I can, but more often than not, they leave it just like it is.

 

[skeshesh]

Is the only issue the cost of the subpanel? Why dont you propose using a 4 wire subpanel instead but just connecting single phase loads? That would take care of the load balance issue.

Charlie already addressed the issue about equips rated at 240V and I agree with him. It's not the end of the world though, bottom line its no absolutely crazy or unsafe but as you stated its not a great practice.

 

[Kessler4130]

Is the only issue the cost of the subpanel? Why dont you propose using a 4 wire subpanel instead but just connecting single phase loads? That would take care of the load balance issue.

Charlie already addressed the issue about equips rated at 240V and I agree with him. It's not the end of the world though, bottom line its no absolutely crazy or unsafe but as you stated its not a great practice.

Well the immediate load is a few LED landscape lights and 4 receptacles, so the current state of the proposed panel will not unbalance the load of the remaining phase at all really, I was more or less shooting for practicality. If they decide down the road..... "hey theres a nice convenient sub panel here lets use that" or as Charlie mentioned they think because it appears to be single phase it is 240 volts only to find out their new dryer is really just a machine that spins clothes while barely warm, or even god forbid someone with barely if any knowledge at all manage to throw in a 3pole QOB for some motor load regardless of the size of it.

The cost of the panel is the issue, and the extra wire would just add to that, if it is code compliant I will install it as specified, I was just hoping I had means to require a 3 phase panel by panel rating or ul listing.

 

[iwire]

There is absolutely nothing wrong with installing a single phase panel from a 3 phase source. It is not my personal choice for best design in most cases but there is nothng wrong with it. We wire for today unless the customer asks us to wire for tomorrow. Saying it is wrong to use a single phase panel from a three phase source is the same as saying you can not connect single phase loads to a three phase source. In either case the potential for unbalanced loading is there if the installers do not think about it.

Did you know that almost all large multi-family dwelling unit buildings are supplied with three phase services but each unit is single phase? In that case you simply disbute the loading by placing 1/3 of the panels on A and B, another 1/3 on A and C and the final 1/3 on B and C.

My suggestion is just go ahead and do what the PM wants and stop putting your personal thoughts ahead of the codes.

 

[iwire]

BTW, I have never seen a disconnect switch rated for 208 volt, they all seem to be rated 250 volt or 600 volt. Has that stopped you from using either for 208 volt?

 

[Kessler4130]

There is absolutely nothing wrong with installing a single phase panel from a 3 phase source. It is not my personal choice for best design in most cases but there is nothng wrong with it. We wire for today unless the customer asks us to wire for tomorrow. Saying it is wrong to use a single phase panel from a three phase source is the same as saying you can not connect single phase loads to a three phase source. In either case the potential for unbalanced loading is there if the installers do not think about it.

Did you know that almost all large multi-family dwelling unit buildings are supplied with three phase services but each unit is single phase? In that case you simply disbute the loading by placing 1/3 of the panels on A and B, another 1/3 on A and C and the final 1/3 on B and C.

My suggestion is just go ahead and do what the PM wants and stop putting your personal thoughts ahead of the codes.

Well if I just went ahead and did what everyone said regardless of its poor design or efficiency I would just be another mindless idiot, where I work if you have a better idea, be it safer, cost effective or reduction in man hours they will always look at and consider it.

The NEC was designed for personal safety the introduction makes that perfectly clear, it is intended to provide safety and not convenience, efficiency, or good service. It is our job to do what we see as right and NEC compliant to meet the customers needs.

If we all did what the drawings tell us to all the time, we would be doing alot of work twice, and those of us who save the consumer money down the road for our careful planning and considerations are usually rewarded.

And to answer this:
"BTW, I have never seen a disconnect switch rated for 208 volt, they all seem to be rated 250 volt or 600 volt. Has that stopped you from using either for 208 volt?"

This was not what I was asking, the submittal says the panel is rated to be used as 120/240 single phase 3 wire, or 120/208 three phase 4 wire, the question had nothing to do with its 250 or 600v rating.

Either way the NEC does not absolve us from using our heads and making changes to installations for the sake of efficiency. We would not have RFI's or RFP's of that were the case.

Thank you for your input.

 

[jim dungar]

This was not what I was asking, the submittal says the panel is rated to be used as 120/240 single phase 3 wire, or 120/208 three phase 4 wire, the question had nothing to do with its 250 or 600v rating.

The 120/240V 3W is a maximum rating for the panel when used on a 3 wire system. This includes 120V 2W, 120/208 3W, and 120/240 3W.

The 208Y/120V 4W rating is the maximum rating for the panel when used on a 4 wire system. It may not be used on 240/120V 4W (wild leg).

 

[Kessler4130]

The 120/240V 3W is a maximum rating for the panel when used on a 3 wire system. This includes 120V 2W, 120/208 3W, and 120/240 3W.

The 208Y/120V 4W rating is the maximum rating for the panel when used on a 4 wire system. It may not be used on 240/120V 4W (wild leg).

Thank you sir, I simply took the rating much to literally looking at the situation with a biased opinion.