sub panel feeder size

[Copperclad]

Hello all,
I recently went to a job where the customer is having a tankless water heater installed in his residence.
The unit calls for (3) 50 amp 240 volt ckts. The maximum amperage 120 amps (at 240 volt). I would not consider this load to be continuous , or do I?
I s'pose one could take a 3hour long shower?
The customer has an older 200 amp GE load center with only 20 ckts. I found that strange, but that's what is there.
Due to lack of breaker spaces, I will be installing a sub panel right beside the existing panel.
My question is where in the code would one find how to size my sub feeders for this added 120 amp load?

Any help would be appreciated!

 

[Dennis Alwon]

Tankless water heaters are not considered continuous (art 422.13) so you could run a circuit for the 120 amp load and then sub divide it.

 

[Dennis Alwon]

Did you calculate the load for the house?

 

[suemarkp]

You need to do a load calculation (Article 220) again, first to make sure the 200A panel is large enough for this added load. Then, if you install a subpanel, you do a load calc just for what is in that subpanel to determine the size of the feeder required to serve it. I would not consider this a continuous load either (3 hours of full flow hot water would be extremely unusual).

I would not put a 125A feeder off of a 200A residential panel -- the panel may not support a breaker that large, you may not have enough bending space, and it is better to keep your heavy loads in the main panel. I'd find 6 smaller circuits to move to the subpanel and put in a smaller feeder (e.g. a 60A feeder to the subpanel). You could perhaps use a 200A subfeed lug in the main panel to feed a 200A subpanel if you really want a large subpanel.

 

[augie47]

I would second all that Dennis says/asks.
You need to take a look at the total house load as you are adding 50% of the panel capacity.
In answer to your question Art 200 would be your reference for you sub-feeder, but in this case your load is known and I agree it's non-continuous.
Your feeder conductors should be protected at their ampacity per the applicable Article (310, 338, 334, etc)

 

[Copperclad]

good advice, will do!

good advice, will do!

Thank Ya'll for you quick response.
Yes, I will do a load calc.
And yes, a great idea to move some smaller loads to the sub panel, to free up space in the main load center!
Thanks again guys , you always help me when Im stuck!

 

[cpinetree]

Ge thin breakers

Ge thin breakers

Most all 200 amp GE panels are able to use thin breakers to get to 40 circuits.

Check the label, or pull a breaker and see if it has the breaker stabs perpendicular to the regular buss bar.

Maybe just use a few thins for the 15 or 20 amp loads.

They do also make thin Double Pole 50 amp breakers, but I do not think I would want to use the thin breakers for that size load.