120v/240v 3 phase load calculation

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Smart $

Esteemed Member
Location
Ohio
... I guess my question is do I keep my 3 phase loads separate when converting to kva from my single phase loads?
Depends...

First, as kingpb pointed out the possibility of the POCO wanting to convert the system to wye...

Second, whether the high leg can deliver full service rating current. No one has yet mentioned the possibility that POCO may be supplying power via an open delta configuration with a lower rated transformer for the high leg. If this is the case, you will have to apportion three phase, two-pole, and split phase loads to determine loading on each line.

Finally, barring any additional post I may make to this thread, by all means, utilize demand factors in your calculations.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
No one has yet mentioned the possibility that POCO may be supplying power via an open delta configuration with a lower rated transformer for the high leg. If this is the case, you will have to apportion three phase, two-pole, and split phase loads to determine loading on each line.

That's a good point. I'm just getting started with a project with this type of setup. In our case, due to the new construction, the utility pole has to be relocated so we assumed that they will require switching to a balanced three-phase transformer. The utilities here certainly try to get rid of high leg deltas when they get the chance, however, if the 800A service is existing and there's no need to alter it I'm not sure if it's likely that the utility will do anything about it.
 

kwired

Electron manager
Location
NE Nebraska
You really need a full equipment list first. The 120 volt equipment can only be connected to A or C phase, which sometimes means that the service ampacity will need to be bigger than assuming a balanced load. And yes, you should use the allowed demand factors.

Agreed, you may need to calculate all the A & C phase loads (plus the three phase portion of three phase loads) and somewhat disregard B phase unless you have a lot of load that can be placed on the B phase because it will likely have less load than the other phases.

Are you sure he's not referring specifically to the lighting load? For instance, if the restaurant was 5000sf, T220.12 would give a minimum lighting load of 10,000VA. However, if the connected lighting load was actually 13,000VA, you would need to use 13,000VA for lighting in your calculation.
The lighting load could be peanuts compared to restaurant equipment which is calculated differently than lighting load.

The building is gutted and has an existing 800 amp service. We are about to put a restaurant in the existing space. We are trying to avoid upgrading the service due to obvious cost issues. The problem is it is a 120/240v 3 phase service. I have all of the loads but am having trouble converting them. I guess my question is do I keep my 3 phase loads separate when converting to kva from my single phase loads?

You can not total the KVA and then balance it across all three phases - especially with the 120 volt loads. You will essentially need to calculate the current of each phase as you intend to connect the loads. I don't see why you couldn't apply demand factors to each phase for a net result.

As far as dealing with the high leg delta system, as mentioned briefly dkidd, you need to be careful as the unbalance caused by the 1-phase loads may call for a larger transformer than you would need in case of a balanced load.
As far as part of your original question, I'm still a bit confused: what do you mean by connected load vs. design load? Reading Charlie's response, I also started thinking about some sort of measured or documented load but it seems that it will be a new setup altogether so please clarify.

Have you contacted the utility yet? If not, you may want to do so, just to confirm the availability of that existing service. I don't know your location, but some utilities use every opportunity to get rid of the high leg delta systems. All you hard work may be a mute point, if the utility dictates that you have to switch service anyway.

If transformer supplies multiple customers the Delta system may have to remain unless owner wishes to possibly spend more $$ to get them to supply him with a wye service. If it only supplies this one service then I would see what it will take to get them to change it to a wye system.
 

Smart $

Esteemed Member
Location
Ohio
...

You can not total the KVA and then balance it across all three phases - especially with the 120 volt loads. You will essentially need to calculate the current of each phase as you intend to connect the loads. I don't see why you couldn't apply demand factors to each phase for a net result.

...
I believe "can not" makes your statement too absolute. While 120V loads are limited to A and C to N connections, 2-pole loads (if any) can be connected AB or BC to offset A>N<C loading. So "may not be able to" seems more appropriate... IMO.
 
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