Trying to understand a 240/120V, 3 Phase system

[Machiavelli999]

I am working on a tenant improvement project with a 240/120V, 3 Phase system. I assumed this was a high-leg delta system. Existing panels in the building had markers on them that say "HI LEG PHASE B". There is a main disconnect in the electrical room through which power enters. The disconnect is labeled 400 amps. Today we opened up the disconnect and noticed that there are 400A fuses on two legs of the service and a third 200A fuse.

I am now confused as to what my electrical capacity is. I had assumed I had 240V * sqrt(3) * 400 = 166.4 KVA

Now knowing that what I assume the B phase is fused at 200A, am I right to assume my capacity now is:

400A * 120V = 48KVA+
400A * 120V = 48KVA+
200A * 120V = 24KVA
120KVA

Additionally, I also think I am limited to a maximum of 96KVA for my single phase 240V or 120V loads.

And just 83.1KVA for my three phase loads.

But in total the loads cannot exceed 120KVA.

Are my assumptions correct? Am I using the right formulas to calculate capacity on such a system?

 

[GoldDigger]

Your numbers and formulas look good to me.
You are correct in not calculating any single phase load on the B wire, only 1/3 of a three phase load. Assuming the POCO transformer is configured properly you could run 24KVA from A to neutral and from C to neutral on top of the 83KVA three phase load.
Chances are good that POCO is supplying you from set of pots that cannot deliver 400A to B even if the fuse were changed.

 

[pv_n00b]

I analyze these assuming they are two services superimposed on each other. It's a 3ph 240V 200A service and a split-phase 240/120V 400A service. The 3ph and split phase service share the A and C legs so the capacity trades off. If you have 400A of 240V split-phase load then you won't have any capacity for any 3ph loads. If you have 200A of 3ph load then you can have up to 200A in additional split-phase load.

These are common commercial services where the load is a predominantly a split phase load with maybe some 3ph HVAC and an elevator. Most likely an open delta with a large 240/120V transformer and a small 240V transformer.

 

[Johnhall30]

I believe you can calculate the single-phase load for phase B by: 200A x 208V, because the line to neutral voltage is 208, not 120, on phase B,

This does not increase your total 3-phase load capacity, it only raises your total load on the B phase.

 

[kwired]

If you only use the hi leg for 3 phase loads then you can kind of do two calculations, one for the 120/240 single phase and one for three phase loads and in a way superimpose the three phase load onto the single phase load.

It is possible to supply 240 single phase loads off the high leg though and if you do so will complicate things to some degree if you don't have such loads balanced across all three phases - where you can sort of treat them similar to three phase loads.

Not impossible but also not all that likely to supply single phase 208 volt loads off the high leg, but if you did that would also complicate load calculations.

 

[pv_n00b]

Interesting turn on balancing loads. I've always heard that on open delta services all 240V single phase and 120V loads go on the A-C legs because that is the large transformer. Only 240V 3ph loads use the B leg and we are not supposed to use 240V single phase loads on A-B or B-C to try to balance the loads since the single transformer that creates the open delta is not as large as the split phase transformer. Is this not right?

 

[LarryFine]

Interesting turn on balancing loads. I've always heard that on open delta services all 240V single phase and 120V loads go on the A-C legs because that is the large transformer.

L-L loads that do not use the neutral can be supplied by any two lines. For 120v and 120/240v loads, you need the A and C legs.

Only 240V 3ph loads use the B leg and we are not supposed to use 240V single phase loads on A-B or B-C to try to balance the loads since the single transformer that creates the open delta is not as large as the split phase transformer. Is this not right?

Actually, as I said above, 240v 1ph loads can be supplied from the high leg and either of the other two.

 

[jim dungar]

....since the single transformer that creates the open delta is not as large as the split phase transformer. Is this not right?

Kind of.
For the majority of installations the center tapped transformer is the larger one because it needs to carry the entire 1-phase load as well as its share of the 3-phase load. However, it is not a requirement that the transformers be different sizes, especially if the 1-phase loading is relatively insignificant.

 

[kwired]

Interesting turn on balancing loads. I've always heard that on open delta services all 240V single phase and 120V loads go on the A-C legs because that is the large transformer. Only 240V 3ph loads use the B leg and we are not supposed to use 240V single phase loads on A-B or B-C to try to balance the loads since the single transformer that creates the open delta is not as large as the split phase transformer. Is this not right?

Depends on the source.

For an older retail store that maybe only had three phase for an air conditioner and everything else is 120/240 single phase - chances are good that there is a two transformer system with a smaller high leg pot.

If it is supplying industrial or other heavy three phase load application it has a good chance of being a full delta with three same sized transformers and was intended to have heavy load on all three of them.

 

[victor.cherkashi]

If the system is 3 phase closed delta, I believe you get wattage as in below picture. There is chance, it's open delta. What leg is grounded in your system?

Sent from my ONEPLUS A6013 using Tapatalk

 

[kwired]

If the system is 3 phase closed delta, I believe you get wattage as in below picture. There is chance, it's open delta. What leg is grounded in your system?

Sent from my ONEPLUS A6013 using Tapatalk

The midpoint of one phase, now per NEC the opposite line is supposed to be the middle or "B" phase in switchboards/panelboards if the neutral is also present regardless of what phase you may call it at the source.

 

[pv_n00b]

Kind of.
For the majority of installations the center tapped transformer is the larger one because it needs to carry the entire 1-phase load as well as its share of the 3-phase load. However, it is not a requirement that the transformers be different sizes, especially if the 1-phase loading is relatively insignificant.

But if the fuse on the B leg is half the rating of the fuses on the A and C legs that would seem to indicate either the person who last replaced the fuse only had a 200A available or the service has an open delta with a smaller transformer making up the 3ph. If it were a closed delta with balanced transformer ratings then having unbalanced fuse ratings would not make much sense.

 

[jim dungar]

But if the fuse on the B leg is half the rating of the fuses on the A and C legs that would seem to indicate either the person who last replaced the fuse only had a 200A available or the service has an open delta with a smaller transformer making up the 3ph. If it were a closed delta with balanced transformer ratings then having unbalanced fuse ratings would not make much sense.

You are correct if the transformer are the same size. However, 240/120V 3PH 4W systems can be obtained from both open-delta and closed-delta configurations made from multiple 1-phase transformers of different sizes. But, we typically do not size each one differently partly because of circulating currents which could be 'magnified' by the unbalanced impedances in a delta-delta connection. This circulating current issue is one reason that open-delta or wye-delta configurations are preferred for these systems.

 

[kwired]

But if the fuse on the B leg is half the rating of the fuses on the A and C legs that would seem to indicate either the person who last replaced the fuse only had a 200A available or the service has an open delta with a smaller transformer making up the 3ph. If it were a closed delta with balanced transformer ratings then having unbalanced fuse ratings would not make much sense.

The thing with fuses is it is also possible to run smaller conductor and give proper overcurent protection than it is for typical three pole circuit breaker.