How do you determine "capacity" of panels

[Dolfan]

OK, I've always wanted to understand better how you determine the electical capacity of you home service panels. Obviously the "physical" capacity is do you have space for breakers to be added.

I'm considering to add a 240v 20-30A garage heater and want to think through the capacity issues.

I have a newer house with what I believe is a fairly large capacity for electrical power. From the "street" I have two 150A panels with a single 150A breaker in each. That then feeds two panels in my basement, I assume each is a 150A rated panel back to the panels outside.

So for each panel how do you determine the current load and capacity? I have tons of physical space but how to know when you are at the limit of the electrical load?

Thanks.

 

[ishiboo]

OK, I've always wanted to understand better how you determine the electical capacity of you home service panels. Obviously the "physical" capacity is do you have space for breakers to be added.

I'm considering to add a 240v 20-30A garage heater and want to think through the capacity issues.

I have a newer house with what I believe is a fairly large capacity for electrical power. From the "street" I have two 150A panels with a single 150A breaker in each. That then feeds two panels in my basement, I assume each is a 150A rated panel back to the panels outside.

So for each panel how do you determine the current load and capacity? I have tons of physical space but how to know when you are at the limit of the electrical load?

Thanks.

Never seen that around here with the two single breakers outside, at least residential. Are there main breakers on the inside panels? If not, I'd guess they are likely both 150A panels with 300A service. If there are, the main breaker is probably pre-installed for the maximum rating of the panel.

Only way to actually know this is to look at the model # of the panel/bus bar inside and look it up.

Remember that the breakers are sized for maximum load, so even if you have a 150A box with 200A of breakers in it, that does not mean there's no room for your heater... depends on how heavily loaded each circuit is. Usually only a small fraction for residential w/o all-electric heat.

 

[Charles (in GA)]

The house a couple of doors down from me has a 320 amp meter socket and dual 150 amp disconnects each feeding 150 amp panelboards. It was a small house and they didn't half fill them, and I thought it peculiar at the time that they would go to the trouble, instead of a 40 space 200 amp panel. House now has an addition to it, but I'm willing to bet it still has lots of empty panel slots. This same electrician use a 30amp dryer cord and plug as a "disconnect" on the water heater. This is a code violation, NEC 422.16(A).

Charles

 

[Dolfan]

Remember that the breakers are sized for maximum load, so even if you have a 150A box with 200A of breakers in it, that does not mean there's no room for your heater... depends on how heavily loaded each circuit is. Usually only a small fraction for residential w/o all-electric heat.

Yeah, I realize that you don't just add the breakers and see if you are over. I was just wondering is there an accepted percentage to apply for typical good practice? Something like add all breakers then take 75% as a fair safety level and use that? I'll have to do a little addition tonight and see what I've got.

I agree though I highly doubt that I don't have the needed capacity. Just don't want to push my luck.

 

[Dolfan]

The house a couple of doors down from me has a 320 amp meter socket and dual 150 amp disconnects each feeding 150 amp panelboards.
Charles

Charles, this sounds much like what I've got, and I'm in the ATL area like you so maybe it is more common, I know my entire small subdivision (30 homes) are done this way.

 

[hidollartoys]

The current "load" can be determined by using an amp clamp. The mearsuments should be taken during "peak" usage.

The other way is to do a "load calculation". NEC provides a method for this and it may or may not provide the actual "load" on the panels since this method allows for some derating and is also based in square footage. This approach is used to determine the minimum size of the service.

Another way is to evaluate your electric bill. It will provide you with a kw/hr usage. This is at best an average but points to the max usage on the service.

Unless your dwelling was wired based on bare minimum requirements, the panels will usually be chosen for the quanity of circuits available and some future expansion.

 

[Norcal]

The only way to determine is to do load calc's.

 

[Dolfan]

Wow, didn't realize how much difference there is in the amount of breaker amps in relation to the actual capacity of the panel. I did a quick count on one panel this morning and there are breakers with capacity that adds to 310A, but like I said I'm pretty sure it is a 150A panel and that it is feed from that 150 cut off outside.

Seems like a lot, but I don't get paid to do home electrical work!!!!!

 

[ishiboo]

Wow, didn't realize how much difference there is in the amount of breaker amps in relation to the actual capacity of the panel. I did a quick count on one panel this morning and there are breakers with capacity that adds to 310A, but like I said I'm pretty sure it is a 150A panel and that it is feed from that 150 cut off outside.

Seems like a lot, but I don't get paid to do home electrical work!!!!!

And many people with 310A of breakers would have a typical load of 20-30A during the day without the air conditioning on

 

[Dolfan]

And many people with 310A of breakers would have a typical load of 20-30A during the day without the air conditioning on

Agreed, In this panel is the Dryer and Oven, which those two alone are 80A as I remember, but of course they are only sporadically used.

I was just surprised how much the breakers really added up to in total, never really looked before.