240/120 3 phase panel to feed 240/120 1 phase panel

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Cartoon1

Cartoon1

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Florida
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Electrical Engineer
I have a 240/120 3 phase high delta panel (4 wire system) that is feeding things like 5-10hp motors, the load is low on this panel but i need more circuit breakers to feed 120v receptacles throughout. I plan to add a sub-panel 240/120 single phase fed from the 240/120 3 phase panel. Taking two hot wires and the neutral and ignoring the highleg. So a 2 pole breaker in the 240/120 3 phase to feed the 240/120 1 phase sub-panel to feed around 20 circuits 120v circuits receptacles. Is there any issues with this setup? Will this cause an inbalance in the system?

Thank you
 
This will, of necessity, cause an imbalance in the system. You will be loading the 120V to N legs more than the 208V to N leg.

This may be a problem depending upon the supply for the system.

For example, having a large single phase (120/240V) center tapped transformer with a smaller transformer to develop the third leg is a common utility arrangement to provide 'high leg delta'. This system is _intended_ to supply lots of single phase loading with a bit of 3 phase, and would be perfectly happy with what you propose.

On the other hand, most small 'delta' dry type transformers are designed for limited L-N loading, and might be very unhappy if you do what you propose and then have unbalanced 120V single phase usage.

-Jon
 
winnie said:
This will, of necessity, cause an imbalance in the system. You will be loading the 120V to N legs more than the 208V to N leg.

This may be a problem depending upon the supply for the system.

For example, having a large single phase (120/240V) center tapped transformer with a smaller transformer to develop the third leg is a common utility arrangement to provide 'high leg delta'. This system is _intended_ to supply lots of single phase loading with a bit of 3 phase, and would be perfectly happy with what you propose.

On the other hand, most small 'delta' dry type transformers are designed for limited L-N loading, and might be very unhappy if you do what you propose and then have unbalanced 120V single phase usage.

-Jon
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The "208V" leg is not a L-N load, it can only be used as a L-L load.
 
norcal said:
The "208V" leg is not a L-N load, it can only be used as a L-L load.
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Sorry that I didn't make that clear. I simply intended to name the leg (using its nominal L-N voltage), not imply that this was intended to be used.

I don't believe there is a restriction prohibiting the use of the 208V leg for L-N, it would simply be very bad practice for a bunch of reasons.

-Jon
 
winnie said:
I don't believe there is a restriction prohibiting the use of the 208V leg for L-N, it would simply be very bad practice for a bunch of reasons.
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One of which is that in an open delta configuration the B phase transformer is often much smaller than the A-B center tapped tranny. In that case an imbalance is desirable and unavoidable.
 
winnie said:
On the other hand, most small 'delta' dry type transformers are designed for limited L-N loading, and might be very unhappy if you do what you propose and then have unbalanced 120V single phase usage.
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Then why would they sell such an item? The center point of the one side of the delta is clearly for a 120/240 single phase feed.
 
They sell these transformers to supply mostly balanced 3 phase loads with a few incidental single phase loads. It is just a design optimization that the OP needs to be aware of which happens in some situations. The center point of one side of the delta is clearly for this use, but it just might be a _limited_ use.

 
winnie said:
They sell these transformers to supply mostly balanced 3 phase loads with a few incidental single phase loads. It is just a design optimization that the OP needs to be aware of which happens in some situations. The center point of one side of the delta is clearly for this use, but it just might be a _limited_ use.

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I have seen high leg services with most of the loads on the A and C phases with a neutral and a few loads on all three phases delta. In those cases the B phase transformer was a lot smaller and the B phase fuse and service conductor was likewise smaller. This is a potential issue for designing PV systems because if three phase inverters are used on high leg services one has to be careful not to overload the B phase.

This is of course with open delta services.
 
ggunn said:
I have seen high leg services with most of the loads on the A and C phases with a neutral and a few loads on all three phases delta. In those cases the B phase transformer was a lot smaller and the B phase fuse and service conductor was likewise smaller. This is a potential issue for designing PV systems because if three phase inverters are used on high leg services one has to be careful not to overload the B phase.

This is of course with open delta services.
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But isn't that the utility's problem and they will certainly be aware of it at the application process and do whatever they need to do? I don't ever recall worrying about or engineering utility's equipment for a PV system.
 
electrofelon said:
But isn't that the utility's problem and they will certainly be aware of it at the application process and do whatever they need to do?
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To make up some numbers, if you have a 240/120 3 phase delta service that's 50 kW, and you want to interconnect a 20 kW PV inverter, your equipment choice should depend on the details of the existing service. E.g. if the available power on the 3 phase delta is 50 kW, and on the 240/120 single phase is only 10 kW, you pick a three phase inverter. Conversely, if the available power on the 240/120 single phase is 50 kW, and on the 3 phase delta is only 10 kW, you pick a single phase inverter.

If you choose the wrong way, then presumably the utility is going to come back and say they have to upgrade the transformers, which will cause needless delay or expense. I would think, I've never dealt with this.

Cheers, Wayne
 
wwhitney said:
To make up some numbers, if you have a 240/120 3 phase delta service that's 50 kW, and you want to interconnect a 20 kW PV inverter, your equipment choice should depend on the details of the existing service. E.g. if the available power on the 3 phase delta is 50 kW, and on the 240/120 single phase is only 10 kW, you pick a three phase inverter. Conversely, if the available power on the 240/120 single phase is 50 kW, and on the 3 phase delta is only 10 kW, you pick a single phase inverter.

If you choose the wrong way, then presumably the utility is going to come back and say they have to upgrade the transformers, which will cause needless delay or expense. I would think, I've never dealt with this.

Cheers, Wayne
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I admit I never have either. Although many times, we don't even know what the utility is serving a building with, say if it's an underground system. Say you come across a site for proposed PV system that has a 120/240 3 phase high leg service, you would have three options of how to arrange the inverters. You can go single phase 240 all on two phases, you could distribute the inverters across all the phases in a Delta configuration,. Or you could go with three phase inverters. From my experience, I wouldn't have much luck "picking anyone's brain" at the utility before submitting an application so I would probably have to just choose and let them kick it back if they wanted.
 
electrofelon said:
I admit I never have either. Although many times, we don't even know what the utility is serving a building with, say if it's an underground system. Say you come across a site for proposed PV system that has a 120/240 3 phase high leg service, you would have three options of how to arrange the inverters. You can go single phase 240 all on two phases, you could distribute the inverters across all the phases in a Delta configuration,. Or you could go with three phase inverters. From my experience, I wouldn't have much luck "picking anyone's brain" at the utility before submitting an application so I would probably have to just choose and let them kick it back if they wanted.
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When we have to interconnect with a 240/120V high leg service we always inquire as to how the system is configured and if it's open delta we make sure that our three phase inverter kVA/3 is not more than the B phase transformer capacity.
 
ggunn said:
When we have to interconnect with a 240/120V high leg service we always inquire as to how the system is configured and if it's open delta we make sure that our three phase inverter kVA/3 is not more than the B phase transformer capacity.
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What if that is significantly smaller than the initially desired PV system size? Do you just put in the application as you want it and see what they come back with on construction charges?
 
electrofelon said:
What if that is significantly smaller than the initially desired PV system size? Do you just put in the application as you want it and see what they come back with on construction charges?
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No, we downsize the system, switch to a split phase inverter, or punt. Replacing a utility transformer is usually too expensive. We found out the hard way that we cannot depend on the utility to either recognize that the transformer is too small or to pay for replacing it when it is discovered.
 
ggunn said:
No, we downsize the system, switch to a split phase inverter, or punt. Replacing a utility transformer is usually too expensive. We found out the hard way that we cannot depend on the utility to either recognize that the transformer is too small or to pay for replacing it when it is discovered.
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Yeah I think I remember that story. I would think the POCO would look at that during the application process and give a yay or nay, but apparently not 🤬
 
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