panel rating

[justinaway1]

How many amps can be drawn thru a 3 phase panel, fed from a 3 pole 100 amp breaker? Customer wants to know how much capacity is available for new equipment. Amp meter shows it currently drawing 175 amps. 60/60/55 per respective phase. Would it be total of 300A?

 

[infinity]

If this is a 208Y/120 volt panel, you could have 3-120 volt, single phase loads of 100 amps each.

 

[jwjrw]

How do you figure that? How bout if you had one 120v load and one 208 load? What code #?

 

[augie47]

The most common answer would be 60 amps.
You have to look at it in more than 1 way. As infinity states, if all the loads were 120 volt then you would have the capacity of (3) 100 amp 120 volt loads.
Loads are more often looked at in kw. Your panel has the capacity of 36.1 kw.
So you would first look at your total load in kw, but then make sure, if thery are single phase loads, that you don't place more than 100 amps per phase.
I hope that makes sense.

 

[roger]

How do you figure that? How bout if you had one 120v load and one 208 load? What code #?

It basically boils down to the fact that there is 36000 watts available from the panel and they can be distributed any way as long as any one phase doesn't have to supply more than 12000 watts, this is not governed by any code, just physics.

Roger

 

[iwire]

It basically boils down to the fact that there is 36000 watts available from the panel and they can be distributed any way as long as any one phase doesn't have to supply more than 12000 watts, this is not governed by any code, just physics.

I agree we have to forget about amps and think watts.



(And I am not sucking up:grin

 

[roger]

(And I am not sucking up:grin

ROTFLMAO. :grin:

Roger

 

[jwjrw]

So single phase would have 24000w available and no phase loaded more than 12k also?

 

[roger]

So single phase would have 24000w available and no phase loaded more than 12k also?

Correct.

Roger

 

[nakulak]

aren't you guys assuming that the loads are non-continuous ?

 

[roger]

aren't you guys assuming that the loads are non-continuous ?

No, watts are watts.

Roger

 

[infinity]

I agree we have to forget about amps and think watts.

I agree.

 

[charlie b]

Amp meter shows it currently drawing 175 amps. 60/60/55 per respective phase. Would it be total of 300A?

This is not drawing 175 amps. Others have said something similar, but let me clearly state that you don't add amps to amps and get amps. Three-phase systems simply do not work that way. The amps in Phase A are not the same as the amps on Phase B, and neither is the same as the amps on Phase C, nor for that matter are the voltages that drive each of the three currents. Each phase current reaches its peak values at different times, so you can't simply add them up to get at "total."

 

[gmtt]

Assuming that loads are continuous, 80% of 100 or 80A the most you can draw per phase. You can connect a new 3-phase load of more than 7.2KW (20*120*3) plus an additional load of .6 kw in phase drawing 55A. In other words 1-ph, 3-ph combination load of total of 7.8 KW of additional load. If all load are non continuous (running no more than 3 hours continuously) then you can load up to 100% of the rating pushing to 40*120+40*120+45*120=15kw of new load.

 

[John Paul]

If this is a 208Y/120 volt panel, you could have 3-120 volt, single phase loads of 100 amps each.

Or one 3-phase 80A continuous load.

208V x 100A x 1.73 = 36kW

36kW x .8 = 28.8kW

28.8kW/208/1.734 = 80A

 

[infinity]

This is not drawing 175 amps. Others have said something similar, but let me clearly state that you don't add amps to amps and get amps. Three-phase systems simply do not work that way. The amps in Phase A are not the same as the amps on Phase B, and neither is the same as the amps on Phase C, nor for that matter are the voltages that drive each of the three currents. Each phase current reaches its peak values at different times, so you can't simply add them up to get at "total."

But would it be incorrect to say that if all of the loads are 120 volts that you have "used up" (lack of a better term) 175 amps of the 300 amp, 120 volt ampacity?