How are panels sized?

[thetacon]

For example if you have a 100 amp single phase panel does it have 100 amps per phase thus having a total of 200 amps or is it 100 amps for the entire panel: and if the panel is 3 phase do you get 300 amps total or is there any correction for the 3 phase (x 1.732)?

 

[petersonra]

Each line can have a maximum of 100A. On a 120/240 system, that would indeed give you 200A of 120 L-N. On a three phas 120/208 panel, you would get a maxium of 300A 120V L-N.

But any 2 or 3 pole loads would reduce the amount of 120V current available.

 

[iwire]

I agree with Bob's explanation but want to add that we do not really 'size' panels by amps.

When deciding what size panel we need we use 'Volt-Amperes' or 'VA'.

Which is really much easer when you have both 240 volt and 120 volt loads to figure for the same panel.

A typical 100 amp single phase 240 panel has about 24,000 VA available.

A typical 200 amp single phase 240 panel has about 48,000 VA available.

A typical 200 amp three phase 208 panel has about 72,000 VA available.

You add up all your loads in VAs then you can determine the minimum size feeder and panel for that load.

 

[charlie b]

. . . if you have a 100 amp single phase panel does it have 100 amps per phase thus having a total of 200 amps. . . .

Welcome to the Forum.

Let me suggest that the sooner you get out of the habit of using the phrase ?amps per phase,? the better off you will be. That phrase has no meaning, and it can lead to invalid conclusions.

You have asked a good question, and it is one that is often asked. The answer is that you generally can?t add amps to amps and get amps. That is because an amp is not an amp. Sound confusing?

What I mean is that the value of current in a wire, expressed in units of amps, does not tell the whole story. The current in one wire will not necessarily reach its peak value at the same time that the current in the other wire reaches its peak. So you can?t just add the peak values. You have to take into account the ?phase angle,? a term that represents the difference in TIME between the peak value of current in one wire and the peak value in the other wire.

I?ll not go into the math here and now, mostly because I don?t know how much of these concepts you have already learned. But there are plenty of resources available to you.

 

[thetacon]

All of these were very helpful. My confusion arises from when installing a panel in is always refered to as say a '200 amp panel' which appears to be a bit of a misleading name because it is the feeder size not the load capacity of the panel. once you get into 3 phase it was not clear how to arrive at the total.

So let me see if I have this clear. to calculate the the VA of a panel i need to use the L-N voltage per phase. A single phase 120/240 system is sized at 240 x amps. A 3 phase 120/208 panel I would use 360 x amps. not the phase to phase voltage of 208.

is that correct ?
thanks

 

[Smart $]

So let me see if I have this clear. to calculate the the VA of a panel i need to use the L-N voltage per phase. A single phase 120/240 system is sized at 240 x amps. A 3 phase 120/208 panel I would use 360 x amps. not the phase to phase voltage of 208.

is that correct ?
thanks

Well, actually it can be figured with either L-N voltage or L-L voltage...
For example, a 120/240 panel with 100A Main:

240V ? 100A = 24000VA, or
120V ? 2 "lines" ? 100A = 24000VA​

For a 208Y/120 panel with 100A Main:

208V ? 1.732 ? 100A = 36000VA, or
120V ? 3 "lines" ? 100A = 36000VA​

However, you must remember that continuous loads subtract from your panel VA by 125% of their rated current draw. Furthermore, some panel OCPD's rated 800A or less may actually be at the next higher standard size than its feeder's ampacity... and you cannot exceed the feeder's ampacity.

 

[charlie b]

A 3 phase 120/208 panel I would use 360 x amps. not the phase to phase voltage of 208.

Actually, the value "360" comes from 208 times the square root of three. In three phase systems, that "square root of three" factor will find its way into your calculations one way or another.