Understanding Panel KVA loading per Phase

designer82

Member
Location
Boston
This 1 phase panel has a little over 15kva per bus. See pic

Question is, to get the total kva do you add them and get 30kva or is it 15kva? Basically, 15kva/208v=72amps or 30kva/208v=144amp

So the real question is, in real life, if you put a ammeter to the panel feeder, will it read 72 amps or will it say 144 amps?

Appreciate it

picture
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Add them up.
think of it like this..
Say you take a 480/120V 15kVA transformer for the 120V loads.
you need two of them to feed these loads.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
To a close approximation you just add up the per phase kVA to get the total kVA.

As victor.cherkashi notes, the phase angles of a 120/208V system screw this approximation up; if you only had 120V loading and the kVA numbers were accurate, then the amp reading you would have on leg A would be 15.7kVA / 120 = 131A, and on leg B would be 15.49 / 120 = 129A. Because you have both 120 and 208V loading the calculation is more complicated but would give near the above numbers.

In real life, if you put an ammeter on the phases I'd expect something like 30-40A. :) :) This looks sort of like a residence calc, but not quite; and residential calcs are known to be severely conservative. If the kVA numbers are accurate than the 100A main breaker is overloaded, however there are almost certainly diversity factors to consider.

-Jon
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
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Retired PV System Designer
This 1 phase panel has a little over 15kva per bus. See pic

Question is, to get the total kva do you add them and get 30kva or is it 15kva? Basically, 15kva/208v=72amps or 30kva/208v=144amp

So the real question is, in real life, if you put a ammeter to the panel feeder, will it read 72 amps or will it say 144 amps?

Appreciate it

picture
The right question is if you put two ammeters in the two panel feeders will the sum be 144A. If it is truely a single phase panel there is no 208 volts present anywhere. If you read 208V phase to phase you have a two out of three phase panel, not single phase. Also, if you measure the current in each leg you still do not know the neutral current unless you make assumptions about the power factor of the loads.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
The right question is if you put two ammeters in the two panel feeders will the sum be 144A. If it is truely a single phase panel there is no 208 volts present anywhere. If you read 208V phase to phase you have a two out of three phase panel, not single phase. Also, if you measure the current in each leg you still do not know the neutral current unless you make assumptions about the power factor of the loads.
Is this a 120-0-120 V system?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Is this a 120-0-120 V system?
This is two legs of a 3 phase 208/120 system. So you have 120-0-120 but with 120 degrees between the two 120-0 legs.

It is officially called 'single phase 120/208V' but is not a true single phase with center tap system.

Makes for fun vector math and 240V heating elements that put out 75% of rated power.

-Jon
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
This is two legs of a 3 phase 208/120 system. So you have 120-0-120 but with 120 degrees between the two 120-0 legs.

It is officially called 'single phase 120/208V' but is not a true single phase with center tap system.

Makes for fun vector math and 240V heating elements that put out 75% of rated power.

-Jon
Well, yes............
designer82 said it was single phase..........

We Brits just have it so much simpler. Single phase 230V L to neutral, three phase LL 400V.
 

Carultch

Senior Member
Location
Massachusetts
This is two legs of a 3 phase 208/120 system. So you have 120-0-120 but with 120 degrees between the two 120-0 legs.

It is officially called 'single phase 120/208V' but is not a true single phase with center tap system.
I like the term "Open Wye" as a name for this kind of system, because it comes from two out of three phases of a wye grid. It is called single phase for historical reasons, but I feel that is a bit of a misleading term.
 

designer82

Member
Location
Boston
This is part of a 3 phase system but it's a single phase panel 2 pole. So forget about the 3 phase. I want to just know what will be the current reading on this single panel's feeder only.

If you add up the legs, you get a total of 30+ kVA. 30kVA/208=144+ Amps

The engineer from the picture... https://imgbb.com/h9JyR5P .... has sized the Main CB for this panel at 100A.

So either he is accounting for demand or no one knows what the heck they're doing
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Oh, we all have clue. The answer to your basic question is that if your loads actually follow the load schedule you have something north of 125A at 208V. This answer was given in several posts above

The exact value depends upon the phase angles of the load compared to that of the source, and the differences in our answers all stem from pinning down those differences.

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
This is part of a 3 phase system but it's a single phase panel 2 pole. So forget about the 3 phase. I want to just know what will be the current reading on this single panel's feeder only.

If you add up the legs, you get a total of 30+ kVA. 30kVA/208=144+ Amps
Pretty much. In the detailed calculation you cannot ignore that it is part of a 3 phase system, but the answer will be somewhere between 125A and 144A.

The engineer from the picture... https://imgbb.com/h9JyR5P .... has sized the Main CB for this panel at 100A.

So either he is accounting for demand or no one knows what the heck they're doing
I agree. There are some fishy things in that sheet. Circuit 15/17 is for an AC? But with only 100VA of load? Is this one of those thermo electric mini fridges? A good number of the values appear to be for a commercial situation, but the overall layout appears residential.

The total calculated load is greater than the panel rating and breaker. But _if_ this were a residential install than the _actual_ current would likely be much lower than the value shown in the panel schedule. You would need to see the article 220 load calc.

-Jon
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
From reading the responses it appears no one has a clue as to how it works.
You had a direct answer starting at post 2 and continuing on.
The amps will be determined by the loads, pure and simple. From reading your response you appears you don’t understand the answers.
 

topgone

Senior Member
That should be a simple exercise! One line will read 130.7A and the other line will read 128.99A (15,000/120= 130.7A; 15,490/120=128.99A.
But since, the first phase contains spares (not loaded), the line amps will be 12,520/120=104.3A while the other phase stays at 130.7A.
 
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