120/240V Delta Question

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Kidd

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Hey guys,

I came across a scenario that I was hoping to get some input on. Currently there's an existing delta-delta (center tapped) 75kVA transformer that feeds a single phase panel. Because this panel has 120V loads, I'm assuming it's wired across the A-N-C phases (assuming B phase is high-leg) since that's the norm.

We're looking to install another 120/240V single phase panel, but with the existing loads and proposed new loads, the 25kVA windings across A-C will not have enough capacity to handle all that load.

I'm thinking about adding the single phase panel across B-A (or B-C), but also bring the neutral along to this panel. I would install a label saying no 120V loads on the B phase, since that's still the high leg, but across A-N we should still get 120V. I've never really seen it done this way, so I just wanted to be sure this was possible. The more I think about it, the more I can't find a reason why not, so thought I would ask here.

Has anyone come across this? Any help/input would be appreciated!

Thanks guys!
 
"Don't to it, Johnny!!"

If nothing else, you'll still be loading the A/N winding so if that's already close to or at capacity, they're little left to give. It'll also be extremely confusing to anyone else who comes along.

You could, maybe, use the added panel for only 240v loads, but you'll need full 240v rated breakers; 120/240 slash-rated ones won't do (since there's a 208v to ground lead).

If you have to, get a 240v - 240/120v transformer and connect the primary to A/B or B/C and get what's now an SDS off the secondary.
 
You need to explore the limitations of that transformer a bit more closely.

It can probably handle more than 25 kVA of load from A to C because all 3 coils support all L-L loads. My guess is that this 75 kVA transformer could supply 50 kVA of 240V A-C loading.

But the transformer is probably rated for 5% neutral loading.

Meaning that if you have mostly 240V loading you can probably add more to the A-C terminal, but if you have mostly 120V loading without careful balancing, the transformer might already be overloaded.

If you want to use the high leg to feed 240V loads, you should probably install a 3 phase panel and use 3 pole breakers. Often 2 pole breakers are not rated for the 208V L-G voltage.

Don't create a situation where a future electrician has the opportunity to accidentally use the 208V leg to feed 120V loads.

Jon
 
zbang said:
"Don't to it, Johnny!!"

If nothing else, you'll still be loading the A/N winding so if that's already close to or at capacity, they're little left to give. It'll also be extremely confusing to anyone else who comes along.

You could, maybe, use the added panel for only 240v loads, but you'll need full 240v rated breakers; 120/240 slash-rated ones won't do (since there's a 208v to ground lead).

If you have to, get a 240v - 240/120v transformer and connect the primary to A/B or B/C and get what's now an SDS off the secondary.
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Thanks @zbang
I do think that adding a small 240V - 240/120V transformer would probably be a better/safer way of doing this, but the material and install cost would go up as well.

Yea, I was hoping the loads were only 240V loads, but unfortunately, they're planning on installing about four single phase circuits. It's not really a whole lot that's needed, maybe about 10 amps.

But lets say we knew for sure the A/N winding had enough capacity to handle the load. Would there be any code violation in wiring a new panel as B-A-N, and just adding a placard stating B to ground is 208V? I know this is not the norm and can be confusing to the next person working on it, but it sounds like it could work?
 
winnie said:
You need to explore the limitations of that transformer a bit more closely.

It can probably handle more than 25 kVA of load from A to C because all 3 coils support all L-L loads. My guess is that this 75 kVA transformer could supply 50 kVA of 240V A-C loading.

But the transformer is probably rated for 5% neutral loading.

Meaning that if you have mostly 240V loading you can probably add more to the A-C terminal, but if you have mostly 120V loading without careful balancing, the transformer might already be overloaded.

If you want to use the high leg to feed 240V loads, you should probably install a 3 phase panel and use 3 pole breakers. Often 2 pole breakers are not rated for the 208V L-G voltage.

Don't create a situation where a future electrician has the opportunity to accidentally use the 208V leg to feed 120V loads.

Jon
Click to expand...

Hi @winnie
So I did speak to the transformer manufacturer a month or so ago. he did confirm, if we're using a single phase panel, we should get 1/3 of the rated kVA. I'm assuming this is because they're really three 25kVA single phase transformers banked together in the delta configuration, so across any single set of windings you'd only be using that one transformer.

I did not ask him about the 5% neutral loading, but I also did not see that on the transformer nameplate the way it's typically listed. I will say, there are currently only about 3 120V circuits on the existing panel which serve convenience outlets, so I dont think we'd be going over the 5%.

Not sure i understand what you mean by using the highleg to feed 240V loads. Really, after looking at the loads we need, they're only four single phase, 120V circuits, so the B phase in our scenario wouldnt be used at all. If we were to use it, it would be strictly for single phase 240V loads (across A-B).

Yea, that was my main concern, was creating a scenario where they could use the 208V leg to feed 120V loads. The more I thought about it though, the more I kept thinking as long as we have a placard stating that panel gets 208V when connected B to ground then we should be good. I mean, on other designs/installs where we have a main panel on a 3 phase delta high leg system, that's what we do anyways. We have a placard that states only A and C phases are to be used for 120V loads and No 120V loads are to be installed on B phase. B to ground is 208V. So i figured if we did that on this panel we're proposing, we should be good.

I don't see a code violation, but I figured I'd ask the question on here to see if anyone does see a violation.

Thanks for your input guys!
 
Can you move existing 240v loads to a new panel and put the new 120's on the old? (If the A-N winding has unused capacity, why put the new loads on another panel? If it doesn't, then a new panel won't help.) Or is this mostly a matter of space?
 
When I said 'use the high leg to feed 240V loads', I was being loose with the language. I meant to say 'use the B leg with A or C, L-L, to feed 240V loads', what you are planning to do.

As far as I know, there is no code problem with what you plan. Code will require high leg marking and proper breaker voltage rating.

As a practice for keeping things simple, I would put all 120V circuits in the original panel, moving pure 240V loads to the new panel. IMHO there is too great a chance to have an off by one error and put a 120V circuit on the high leg, even with a placard. This is not a code requirement, just what I think is a good practice.

A code requirement mentioned by others is that your 2 pole breakers for 240V loads will need to be rated for 208V to ground. Most single and two pole breakers for a 120/240V system will be rated only 120V to ground. But the three pole breakers are usually rated for the full 240V to ground.

Jon
 
@zbang
I forgot to mention, this panel is for a separate tenant, so they want to install a private sub-meter to monitor their power consumption. That's why we're trying to keep them separate, but I might look into this. there might be a way to shuffle some loads around. I think the meters can accept multiple circuits, so maybe we meter only those four circuits and feed the circuits from the existing panel. My only concern with that, would be if that tenant needs to add more circuits down the line.
 
winnie said:
When I said 'use the high leg to feed 240V loads', I was being loose with the language. I meant to say 'use the B leg with A or C, L-L, to feed 240V loads', what you are planning to do.

As far as I know, there is no code problem with what you plan. Code will require high leg marking and proper breaker voltage rating.

As a practice for keeping things simple, I would put all 120V circuits in the original panel, moving pure 240V loads to the new panel. IMHO there is too great a chance to have an off by one error and put a 120V circuit on the high leg, even with a placard. This is not a code requirement, just what I think is a good practice.

A code requirement mentioned by others is that your 2 pole breakers for 240V loads will need to be rated for 208V to ground. Most single and two pole breakers for a 120/240V system will be rated only 120V to ground. But the three pole breakers are usually rated for the full 240V to ground.

Jon
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@winnie
ok, gotcha, thanks!

ok, yea, I don't think it's not the best option, but if it's not a code violation, then i think the client would want to work with what we've got. Really, I dont know why the original people used a delta-delta for this. delta wye would've made things easier.

Yea, ideally, that's how we typically design out delta highleg systems when we're starting a new service. We typically have a dedicated single phase panel and a dedicated 3 phase panel. In this case, the new panel is for a separate tenant that will be metered separately. That's the reason they want all new loads on this new panel.

Ok, thanks! I did see the mention of that. Currently I dont see any 240V loads, but you never know, they might want to add some later.

Thanks for all the help guys!
 
So you have existing tenant with only 120V loads, on a panel with A-N-C.

You have a new tenant with a combination of 120V and 240V loads, and you want to put them on a panel with A-N-B, so that the 240V loading goes to the B leg?

This will increase the potential for unbalance on the N, which might be a problem.

What about giving the new tenant a full 3 phase panel, so that their 120V loads could be balanced, A-N and C-N. Meanwhile 240V loads could be installed A-B or C-B.

-Jon
 
@winnie

So, really, the system was installed incorrectly from the get go. The original plans were calling for a single phase transformer, but whoever did the install, put in a 3 phase transformer. There is 3 phase to the building, but it should have been a single phase transformer feeding a single phase panel.

The existing tenant is has a single phase 120/240V panel. It has mainly 240V single phase loads, with about (3) 120V receptacle circuits.

The new tenant, is currently only asking for (4) 120V, 15 amp circuits. so really, I only need a 120V panel, but I dont think those exist right?? I actually have never seen one, but I might google that real quick. The only reason I want to use the Bphase is in case they ever want to add 240V loads in the future, they have the option to do so, without putting so much strain on just one set of windings (A-C). because in reality, with the (4) 120V circuits they're planning to add right now, they're only going to add load to that A-C winding anyways.

Hope this makes sense.
 
IMHO if they only want 120V circuits, you are better off adding a single phase panel connected A,C, N.

As you note, 120V circuits on an A,B,N panel will load the A-C leg, and on top of this you will have poorer neutral balance.

The only benefit of an A,B,N panel is for future potential 240V loads.

So installing an A,B,N panel is worse for current loading but might be better with future loading.

Jon
 
Kidd said:
@winnie

So, really, the system was installed incorrectly from the get go. The original plans were calling for a single phase transformer, but whoever did the install, put in a 3 phase transformer. There is 3 phase to the building, but it should have been a single phase transformer feeding a single phase panel.

The existing tenant is has a single phase 120/240V panel. It has mainly 240V single phase loads, with about (3) 120V receptacle circuits.

The new tenant, is currently only asking for (4) 120V, 15 amp circuits. so really, I only need a 120V panel, but I dont think those exist right?? I actually have never seen one, but I might google that real quick. The only reason I want to use the Bphase is in case they ever want to add 240V loads in the future, they have the option to do so, without putting so much strain on just one set of windings (A-C). because in reality, with the (4) 120V circuits they're planning to add right now, they're only going to add load to that A-C winding anyways.

Hope this makes sense.
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Is there any three phase loads?

Is the mostly 240 volt loads you mentioned have any connected to the high leg at all right now?

If answer to both questions is no you sort of only have a 25kVA supply transformer as earlier mentioned.
 
winnie said:
What about giving the new tenant a full 3 phase panel, so that their 120V loads could be balanced, A-N and C-N. Meanwhile 240V loads could be installed A-B or C-B.

-Jon
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This is the solution, If it were my building that's what I would want.
If its like a small office / retail building where there can be turnover that gives lots of flexibility.

One strategy to manage the neutral on a 240 delta xfromer is to gradually move the main OH lighting to 240V as each tenant space is remodeled. Lots of LED troffers I see these days are labeled for 100-277 Volts, unlike the florescent ballasts where you had a black lead for 120 orange lead 277V.
And if you have a lot of IT equipment being added like server racks see if they can run off also run off 240V.
Then you can balance the single phase (240) loads over all three phases.
 
Thanks for the help guys! Sorry about the late follow up.

I reached out to the equipment manufacturer that is supplying the equipment for the new tenant. it turns out all the equipment will operate at 240V, single phase, no neutral required.

Knowing that, we're going to propose just installing a 100 amp, single phase 240V (no 120V available) across A-B and label the panel stating no 120V is available/allowed. Maybe we'll fill the panel with 240V spare breakers too.

I dont like this design, but I also don't see a code violation. I tried to convince them to re-do the design with a 3 phase panel, but it was a no go because of cost.

Thanks guys!
 
Kidd said:
Thanks for the help guys! Sorry about the late follow up.

I reached out to the equipment manufacturer that is supplying the equipment for the new tenant. it turns out all the equipment will operate at 240V, single phase, no neutral required.

Knowing that, we're going to propose just installing a 100 amp, single phase 240V (no 120V available) across A-B and label the panel stating no 120V is available/allowed. Maybe we'll fill the panel with 240V spare breakers too.

I dont like this design, but I also don't see a code violation. I tried to convince them to re-do the design with a 3 phase panel, but it was a no go because of cost.

Thanks guys!
Click to expand...
Might be easier to acquire and maybe even less cost to go with a 277/480 panel and breakers than to try to get straight 240 rated two pole breakers for the typical 120/240 series panels. I'd at least check into it. May also lessen the chance of someone trying to add a 120 volt circuit later on.
 
kwired said:
Might be easier to acquire and maybe even less cost to go with a 277/480 panel and breakers than to try to get straight 240 rated two pole breakers for the typical 120/240 series panels. I'd at least check into it. May also lessen the chance of someone trying to add a 120 volt circuit later on.
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I don't think his intent is to find the proper breakers. Only to limit the installation of 120 volt loads by signage.
I see a 120v load fed from a 2 pole breaker with the feeder EGC being used for a neutral, in the future.
 
Kidd said:
Thanks for the help guys! Sorry about the late follow up.

I reached out to the equipment manufacturer that is supplying the equipment for the new tenant. it turns out all the equipment will operate at 240V, single phase, no neutral required.

Knowing that, we're going to propose just installing a 100 amp, single phase 240V (no 120V available) across A-B and label the panel stating no 120V is available/allowed. Maybe we'll fill the panel with 240V spare breakers too.

I dont like this design, but I also don't see a code violation. I tried to convince them to re-do the design with a 3 phase panel, but it was a no go because of cost.

Thanks guys!
Click to expand...

Will this tenant space have any 120V convenience receptacles? Will it have any single pole controlled lighting? Does the owner want these loads paid for by the tenant or are they supplied from another panel?

Remember that this 240V panel will require full 240V rated breakers, not the more common 240/120V 'slash rated' breakers. Thus @kwired commenting about using a nominal 480/277V panel, as it might be cheaper than a 240V panel with straight 240V breakers.

You are setting things up from some trunk slammer to try to run 120V circuits using ground as a neutral.

But IMHO your plan doesn't violate code if you get the details right.

IMHO going with a 3 phase panel _might_ be cheaper than doing the A-B 240V panel correctly. And if it permits moving 'incidental' 120V loads to the tenant for metering that will also help with long term costs to the owner.

-Jon

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