300 amp vs 400 amp service


Old 01-04-10, 06:43 PM
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300 amp vs 400 amp service

I have 2 150 amp service boxes in my house. We are adding a afloor and may need some additioanl service? I may just run a 100 sub panel upstairs to the addition and wire from that box vs main boxes. SO what would I need to do to increase my servie from 300 to 400 amps? Is this just upgrading to another box or is it more "juice" from power company? How would I run 100 amp sub panel? Need dual pole fuse at 50 amp each?

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Old 01-04-10, 06:46 PM
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I suggest doing a load calculation to see if you even need the additional service. Just google "load calculation"
Old 01-04-10, 07:22 PM
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I am with Tolyn this is the safest way you can deal with it.

Goggle load demand that way you will able know how much room you have left in your exsting service.

Old 01-05-10, 08:46 AM
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Will google thanks...

I guess asking the Q differently, if I were to run a 100 amp subpanel off one of my two existing 150A main panels how does that math work? Could I have 2 or 3 or 4 - 100 amp subpanels running off of a 150 amp main panel?

Or, If a 200 Amp main panel with 20 slots is full of 15A breakers the total capacity (right term?) would be 300amps
(15A x 20 slots). What if you changed them all to 20A breakers (hypothetical) you would now have 400 Amp capacity?

So Is Amp calculation more about peak load usage or total capacity?
Old 01-05-10, 08:58 AM
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The max ratings of the panels is not really relevant -- by code you could have a dozen 100A subpanels off a single 100A main. As long as the calculated load does not exceed the rating of the service it would be a legal installation. The only restriction is that a subpanel cannot exceed the rating of the supplying panel (100A main, 150A sub).

The demand load calculation is about typical use plus a safety margin. It accounts for things like the average lighting watts per square foot, cooking appliances, heating and cooling systems, laundry, hot water and other large loads like spas or power equipment. You could easily have 400A if you add up the breaker handles, but the calculated load may be under 100A.
Old 01-05-10, 01:06 PM
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Thanks for the clarification.
So then how do I know what my service really is? Even though I have 2- 150 amp main boxes, is there a chance I only have 100 amp service going to each of the boxes from POCO? Or do I maybe have 200 amp to each box and only 150A boxes were used at the time?
My fear is house is pretty old and small so I would not have thought they would have used 300A service at the time. They have not been upgraded since they were built. I would like to know if I have to upgrade the service before I start construction and I need to run gas line anyway from street.
Old 01-05-10, 06:31 PM
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Your service size would be determined by the sizes of the cables feeding the meter and the panels.
Old 01-06-10, 02:07 PM
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The NEC has an exception that allows for the sum of the service disconnect breakers to exceed the size of the service, as long as the calculated load does not exceed the service size. The size of the service is determined by the size of the service entrance conductors. So in your case you might have a 200 amp service with 2 150-amp main breaker panels attached. That's doubtful. You probably have a 300-amp service. If the service is overhead check the size of the wires/cables if you can and that will tell you the service size. Its harder to determine the wire size for an underground service.

I could let you know the calculated load if you can give me some info -

sq feet of home including basement
number of kitchen 120-volt circuits
air conditioning amps (see ac name plate)
furnace gas or electric if electric give me the size of he breakers feeding it
any other appliances or items with motors.

In any case what are you adding, if its only lighting and general receptacles and it is existing space in he home the calculated load would be no different than it was when the service was installed.
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