# max amp sub panel



## kancerr (Apr 16, 2010)

150 amp service in pg county maryland. whats the max sub panel i can install? i have a job where they have an old homeline panel that doesnt allow for tandum breakers(apparently dont make them for it). from what ive put together max sub panel i can install is 40ampere?

the customers do not want a service upgrade and just are hoping to get a sub panel. at normal operation during peak hours they are only drawing 30 amps on their 150amp service, but their 30space square d homeline box is full.


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## Bob Badger (Apr 19, 2009)

You can install as large or high amperage sub panel as you can afford.

All that matters is the total calculated load does not exceed 150 amps.


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## jwjrw (Jan 14, 2010)

You have to do a load calc to see what you have now. Is it electric heat or gas? Electric waterheater or gas? What size etc. You didn't give enough info to answer your question.


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## kancerr (Apr 16, 2010)

gas...are you sure in the state of maryland that its not limited...i was told by one of the masters at the shop that its probably only 40amp sub panel allowed for 150amp service in md.

with all lights on in the house, tvs, while they were preparing dinner doing laundry, going about normal business, they were only drawing 30ampere.


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## Dennis Alwon (May 9, 2009)

kancerr said:


> gas...are you sure in the state of maryland that its not limited...i was told by one of the masters at the shop that its probably only 40amp sub panel allowed for 150amp service in md.
> 
> with all lights on in the house, tvs, while they were preparing dinner doing laundry, going about normal business, they were only drawing 30ampere.


You really can't tell by one reading. You need to calculate the load. A panel with a 150 amp main will probably only accommodate a 125 or perhaps a 100 amp breaker max for the sub panel. This is dictated by the manufacturer.

Again do a calculation -- if you need help with that we can direct you to some online calculators for single family dwellings.


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## kancerr (Apr 16, 2010)

Dennis Alwon said:


> You really can't tell by one reading. You need to calculate the load. A panel with a 150 amp main will probably only accommodate a 125 or perhaps a 100 amp breaker max for the sub panel. This is dictated by the manufacturer.
> 
> Again do a calculation -- if you need help with that we can direct you to some online calculators for single family dwellings.


thx ill do that.


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## Bob Badger (Apr 19, 2009)

kancerr said:


> gas...are you sure in the state of maryland that its not limited...i was told by one of the masters at the shop that its probably only 40amp sub panel allowed for 150amp service in md.


If Maryland follows the NEC you could install a 400 amp rated sub panel from that 150 amp service. All the NEC cares about is the total calculated load and panel bus bars do not add anything to the calculated load.

If you want to figure out the existing load with a meter the NEC requires at least a 30 day reading on a recording meter. Or if the power company can provide the demand data you could use that.

Being just a small home it would be quickest to just do an entire load calculation for the home from scratch.


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## kancerr (Apr 16, 2010)

found calculator for it, done ty.


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