Reading from 120V/208V 1 phase panel - HELP

[febee]

Hi, I am wondering if you can help me out.

There is an existing panel. According to the existing documentation, the panel is 400A 120V/208V 1 phase. The existing panel has been modified for several times. The existing panel schedule is no longer up to date with all current loads except the location and maybe the discription. We took a reading for each Phases. We got for Phase A, 50A and Phase B, 25A. This is the first time I encountered this situation so I need help.
How do I calculate my total loads for the panel? Do I need to add the load from Phase A and Phase B?

Thanks for your help.
Febee

 

[waross]

Assuming that all the loads have unity power factor (Not likely but bear with me) part of the phase current will be in phase with the line to neutral current and part of the current will be in phase with the line to line voltage. The sum of the line to neutral based current and the line to line based current will be more than the measured current. Now throw in various power factors and it starts to get a little complicated.
But why do you need to know the loads? Usually for the sake of transformer loading. So forget about the actual loads and just work with current. Whether the transformer is line to neutral, phase to neutral to phase, three phases to neutral, single phase or three phase. Each phase winding of the transformer will see the measured phase current. If you base your transformer rating on the measured phase current you should not overload the transformer. Trying to add load currents at various, unknown power factors and phase angles is mostly a waste of time. The calculations will seldom match the measured total current and the measured total current is what the transformer sees.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[febee]

Thanks Bill.

The reason I am asking the load, I am not sure how it is done correctly. There was an existing transformer upstream of the panel, 112.5 kVA 480V 1 phase. Then another panel was tap on the same existing transformer. This information is part of the existing documentation. Now, I need to split them with their own transformer. I guess since I know the panel amps, I can figure out the transformer for each panels.

Febee

 

[waross]

For transformer sizing, don't use the total load, use the load on the highest loaded phase. In your case:
A phase 50 amps, B phase 25 Amps, use A phase 50 amps- B phase 50 amps.
If you have 120/208 Volts this is not single phase, it is two phases of a three phase system, usually supplied from a three phase transformer.
If you feed this panel from a single phase transformer there will be no change in your 120 Volt loads but your 208 Volt loads will now see 240 Volts. The current will probably change. Most of the equipment on 208 Volts is probably rated at 240 Volts but it is well to check that there is not any equipment rated at 208 Volts or 200 Volts on your panels. Your main breakers or fused switches may need to be changed to protect the transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[febee]

Thanks Bill.

I just went back and checked the existing documentation. It indicates that it is 120/240V, 1 phase.

 

[waross]

That makes it a lot easier. Use the highest phase load (A phase at 50 Amps for both phases for transformer sizing.

Bill
--------------------
"Why not the best?"
Jimmy Carter