120/208 panel calcs question

[chrisjhn41]

Say you have a 30 ckt 120/208 panel, with the A,B,C phase KVA each totaling 14KVA, would that make the total KVA for the panel 42KVA (14KVA x 3phase) and the panel need to be sized at 150A, or would the total KVA be around 25KVA ((14KVAx3phases)/sqrt of 3) and then the panel would be sized at 100A.

In relation to that question, if the above mentioned panel is a sub panel from a 400A 120/208 panel what size does the CB in the 400A panel feeding the above panel need to be?

It seems to me that if each phase is carying a 14KVA load that the panel and breaker would need to be sized for at least 150A and each phase feed wire would also need to be sized that way, but i have been told to size the breaker and feeders for the 25KVA calculated as shown above and feed size by the 25KVA @ 208 3phase for 100A.

Can anyone shed some light on this situation so i have a better understanding as to how the KVA per phase related to the total panel KVA?

 

[tryinghard]

You need to calculate your load as per Article 220, notice examples in Annex D1 through D10.

Yes your example will be 42KVA and your amperes will be VA/(Volts x 1.732) so 117A = 208/360 meaning you need a 125A minimum panel, feeder & sub panel breaker.

 

[mayanees]

one thought...

one thought...

Don't forget to CONSIDER future loads, and perhaps round up sizing, based on wire ampacity and panel bus offerings.
i.e. In this case I would consider rounding up to 200 amps, and use a 200A MLO panel depending on possible future additions to the panel.
JM

 

[kingpb]

A 120/208 V panel with phases A, B C is not physically possible. This panel is, according to IEEE C84.1 a single phase panel.

Perhaps you meant to say 208Y/120V ?

 

[rattus]

A 120/208 V panel with phases A, B C is not physically possible. This panel is, according to IEEE C84.1 a single phase panel.

Perhaps you meant to say 208Y/120V ?

King,

Maybe you could post these definitions for those of us who don't have that document.

 

[charlie b]

This panel is, according to IEEE C84.1 a single phase panel.

You are not going to gain many followers on this forum. The vast majority are not IEEE members (I am a Senior Member, myself), and have no interest in IEEE standards. They go by what the NEC says, and very little else. The NEC does not make the distinction that you are attempting to describe. "120/208" and "208/120" and "208Y/120" are going to be treated as meaning the same thing, regardless of what the IEEE has to say on the subject.

There is a difference between being right and being understood. So long as you are not wrong, there is nothing wrong with simply being understood. :grin:

 

[kingpb]

King,

Maybe you could post these definitions for those of us who don't have that document.

I would be happy too:

View attachment 920

Pertinent portion of Table (the rest is acceptable voltage variations)

View attachment 921

Notes:
View attachment 922
View attachment 923

 

[peter]

tryinghard,
"your example will be 42KVA and your amperes will be VA/(Volts x 1.732) so 117A = 208/360 "
This is a little confusing. Maybe it should be: 42,000/(208 X 1.732). This indeed works out to slightly less than 117A.
~Peter

 

[kingpb]

You are not going to gain many followers on this forum. The vast majority are not IEEE members (I am a Senior Member, myself), and have no interest in IEEE standards. They go by what the NEC says, and very little else. The NEC does not make the distinction that you are attempting to describe. "120/208" and "208/120" and "208Y/120" are going to be treated as meaning the same thing, regardless of what the IEEE has to say on the subject.

There is a difference between being right and being understood. So long as you are not wrong, there is nothing wrong with simply being understood. :grin:

I'm sorry, but I don't believe that folks in this "Electrical Calculations/Engineering Forum" (NEC Forum is located elsewhere) have no desire or interest in learning about industry standards. I think most are here to learn new things and get a better understanding about things they don't know.

Frankly, I would think many are taking insult to your suggestion that all they go by is the NEC and care very little about anything else ( I know I would), especially when the NEC is not a design guide, it is minimum installation guide related to safety.

I guess NFPA 70E making reference to IEEE 26 times should be edited since apparently, no one cares about anything except NFPA 70. For those that don't know, NFPA 70 is the NEC.

 

[tryinghard]

tryinghard,
"your example will be 42KVA and your amperes will be VA/(Volts x 1.732) so 117A = 208/360 "
This is a little confusing. Maybe it should be: 42,000/(208 X 1.732). This indeed works out to slightly less than 117A.
~Peter

Oops! Your right, I think I meant to post ?amperes = VA/(Volts x 1.732) so 117A = 42KVA/360