3 phase panel

[newb]

my journeyman told me that with a 100amp 3phase panel you can have 240amps, his reasoning, 100a x 3phases=300a, 300a x .80 adjustment = 240a. thats not right is it.
if i use #2 copper out of the 75degree colomn gives me 115 amps to work with and then multiply that by 80% for 4 current carrying conductors gives me a final ampacity of 92 amps for #2 copper. i can then use a 100amp breaker to service a load of 92 amps. all the loads will be single phase loads. anyone have some input

 

[kingpb]

You can have a total of 240A worth of load between all three phases, but the conductors are sized per phase. That does not mean you can have 240A on one phase, and zero on the other two. The load on each phase should not be roughly more than 80A. (80 x 3=240A)

Hope that helps.

 

[Smart $]

my journeyman told me that with a 100amp 3phase panel you can have 240amps, his reasoning, 100a x 3phases=300a, 300a x .80 adjustment = 240a. thats not right is it.
if i use #2 copper out of the 75degree colomn gives me 115 amps to work with and then multiply that by 80% for 4 current carrying conductors gives me a final ampacity of 92 amps for #2 copper. i can then use a 100amp breaker to service a load of 92 amps. all the loads will be single phase loads. anyone have some input

Actually it is a bit more complex than that. But that is a safe approach. Using an 80% adjustment factor is assuming all loads are continuous. Non-continuous loads are permitted 100%. And depending on demand factors that apply, you could actually exceed the 100A OCPD rating for connected loads per phase and still be compliant.

Don't get caught up in "rules-of-thumb" though they are great for quick estimations. The proscribed method is provided in Article 220. You first calculate your minmum branch circuit loads through Parts I and II, then you calculate minimum feeder and service loads through Part III, IV, or V. If actual exceeds the calculated minimums, you use actual. Branch circuit ocpd's must be rated at not less than 125% of continuous plus 100% of non-continuous loads. The same goes for feeder ampacity. Feeder ocpd's are determined by the feeder ampactiy, not the load it carries.

 

[steve66]

my journeyman told me that with a 100amp 3phase panel you can have 240amps, his reasoning, 100a x 3phases=300a, 300a x .80 adjustment = 240a. thats not right is it.
if i use #2 copper out of the 75degree colomn gives me 115 amps to work with and then multiply that by 80% for 4 current carrying conductors gives me a final ampacity of 92 amps for #2 copper. i can then use a 100amp breaker to service a load of 92 amps. all the loads will be single phase loads. anyone have some input

That does assume all the loads are line-neutral. (For example, 120V loads in a 208V 3 phase panel).

If the loads were line-line, it would be a completely different story.

Remember, your power out can't exceed the power in. So for a 208V, 3 phase, 100A panel, the power in is:

208 volts * 100 amps * sqrt(3) = 36,026 VA in.

For line-neutral loads, the power out is:

120 volts * 300 amps = 36,000 VA out (assuming they are all ballanced on the 3 phases).

Steve