If I have a 120/240 Panel 100 amp can I have 80 amps on each phase.

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kjroller

Senior Member
Location
Dawson Mn
Occupation
Master electrician
My question is Just to clarify what I believe I already know but in a 120/240 panel you only want to load it to 80% continuously I understand so 80 amps on a 100-amp service and each phase can carry 80 amps. Is this correct and would the same statement be true for a 100-amp 3phase 480-volt panel you could have 80 amps on each of the 3 phases?
 
kjroller said:
My question is Just to clarify what I believe I already know but in a 120/240 panel you only want to load it to 80% continuously I understand so 80 amps on a 100-amp service and each phase can carry 80 amps.
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Not for a service sized at 83% of the rating, per 310.12
 
kjroller said:
My question is Just to clarify what I believe I already know but in a 120/240 panel you only want to load it to 80% continuously I understand so 80 amps on a 100-amp service and each phase can carry 80 amps. Is this correct and would the same statement be true for a 100-amp 3phase 480-volt panel you could have 80 amps on each of the 3 phases?
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You can load a panel up to it's capacity. Highly doubtful that all the loads would be continuous.
 
Joethemechanic said:
Doesn't that only apply to residential?
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Yeah, we had a long thread on it a while back 310.12 is a old section going back to the 1950's when it was a note in the ampacity table.
It started as ampacity adjustment for residential service wire, I believe it was largely justified based on two 2 current carrying conductors.
310.12 requires the service has to be single-phase and the has to carry entire load of a one-family dwellings.

The other thing is 310.12 references the 'service rating' which is not the always same thing as calculated load.
230.79 is where 'service rating' comes from
Example D2(a) Optional Calculation for One-Family Dwelling shows how they arrive at 100A 'service rating'
with a 90A calculated load for an example.
 
I really think most services are way oversized. It was amazing how much got done with a 60amp REA service. Now people who really don't do much of anything substantial think they all need a 200 amp service

 
Joethemechanic said:
I really think most services are way oversized. It was amazing how much got done with a 60amp REA service. Now people who really don't do much of anything substantial think they all need a 200 amp service

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Well that brings back some memories. Dad's old welder is still at the 'farm'. It's been moved it to the good stuff storage area and not used for years. I never learned past rolling it out for someone else to use.
 
Back to the OP: Your underlying question is basically 'If the panel ( or breaker) says 100A, that means 100A per leg/phase'. The answer to that is a hard YES.

There may be factors that reduce the number ( continuous loading, breaker limits, etc). But the number is for each leg/phase, not some sort of total for the whole panel or breaker or circuit.
 
Joethemechanic said:
I really think most services are way oversized. It was amazing how much got done with a 60amp REA service. Now people who really don't do much of anything substantial think they all need a 200 amp service

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I agree 100% with the oversized service. Years ago I replaced at least a dozen 60 amp services that had natural gas for everything but the electric stove that was usually feed with 50 amp fuses. Most of these houses had 2 to 4 window air conditioners or central air. I always pulled out the main 60 amp fuses & the stove fuses & inspected them Think everyone were original cheapest non time delay fuses made in the 1940's.I run our central AC all summer and have natural gas for everything so only average 2 to 3 KW per hour. Of course when some some home owners had central AC installed some contractors would tell them they should replace the old service with a 200 amp service which they could install.
 
garbo said:
I agree 100% with the oversized service. Years ago I replaced at least a dozen 60 amp services that had natural gas for everything but the electric stove that was usually feed with 50 amp fuses. Most of these houses had 2 to 4 window air conditioners or central air. I always pulled out the main 60 amp fuses & the stove fuses & inspected them Think everyone were original cheapest non time delay fuses made in the 1940's.I run our central AC all summer and have natural gas for everything so only average 2 to 3 KW per hour. Of course when some some home owners had central AC installed some contractors would tell them they should replace the old service with a 200 amp service which they could install.
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When I was in my 20's and working out of my grandmother's barn she had a 100 amp service from the 1940's. There was a 50 amp 240 volt feeder going out to the barn. I ran all kind of stuff out there, 5 hp compressor, all kinds of corded power tools, lots of florescent lighting, oil burner, machine tools, refrigerator, 2 mercury vapor lamps outside and a block heater for the truck. The only thing I didn't run on electric was the welder because it was a Hobart with a 2 cylinder Wisconsin TJD engine on it.

She had a big house with an electric range and dryer and all the typical household appliances

We never blew a main fuse or a fuse in the 50 amp barn feeder. I worked out in the barn all the time, and she cooked like she was running a restaurant. I never gave even a thought to how much I could run at one time (except on branch circuits), but in all that time, and my grandfather doing pretty much all the same kind of stuff for years before I was even born that 100 amp 240 volt single phase service was enough
 
I have seen lighting panels that are only 277V lighting loads that are on 3 hours or more, but yeah not really common.
OP is asking about permissible loading of a 100-amp service that has been been determined to have only continuous loads.
If you had a 100A panel with 80 amps of 480/277V lighting (on 3 hrs of more) NEC 230.42 / 215.3 would require the service / feeder, and by extension the panel to be sized to 125% of continuous loads or 100A.
There is no dwelling unit service that would be only continuous loads, so the question of 310.12 [310.15(B)(7) in the 2017] is moot.
 
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