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Posted By: cavo148 Panel maximum? - 01/20/04 09:40 PM
I have an existing residential 200A. main lug panel already feeding some subpanels with a 100A. DP ckt. bkr, a 30A. Dp ckt. bkr, and some other DP utilization ckts. feeding electric ranges, etc. I'd like to add some more ckts. for an addition on the home and there are several remaining unused spaces on this panel. My question is, how do I determine the maximum amount of ckts and ampacity still available without overloading this panel, per the NEC? Or, should I take readings to determine the peak usage and base the available leftover on those?

Thanks,
Andy
Posted By: russ m Re: Panel maximum? - 01/21/04 04:20 AM
The right way to do this would be to use Article 220 for Load Calculations.
You should read thru it, do your 3va per sq ft and specific appliances and motors.
Then come back with questions
Posted By: cavo148 Re: Panel maximum? - 01/21/04 12:18 PM
Hey Russ, thanks for the reply. I thought of the load calculations also, but that would give me the allowable minumum load per the NEC. I want to make sure I'm not overloading the allowable peak usage of this existing panel, located in the basement. I should mention that it's a large, older home with at least two "lighting & branch-circuit" subpanels located on an upper floor and in the garage. There is also a heated inground pool on premise. BTW, wish it was my house...without the taxes of course.

Thanks,
Andy
Posted By: George Re: Panel maximum? - 01/21/04 04:14 PM
cavo148 ---

I think that actual usage is a good way to determine if a service needs to be upgraded.

It is certainly more accurate than a one size fits all formula.
Posted By: russ m Re: Panel maximum? - 01/21/04 04:30 PM
True the code is minimum, and not always adequate, of coarse with out the calcs you don't even know what minimum is.
Maybe if you draw up a list of the loads the owner will realize minimum is not enough.
Posted By: cavo148 Re: Panel maximum? - 01/21/04 04:53 PM
It would make sense then to take the readings to find peak usage and use the NEC calculations to determine the load of the pool equip., central A/C, and the future addition. BTW, the house is in the Northeast, so I'll have to use the nameplate ratings and NEC calcs. to determine the loads for the pool and A/C equip.

Thanks,
Andy
Posted By: iwire Re: Panel maximum? - 01/22/04 12:59 AM
If you do the NEC load calculations you will not come up short on power.

I would be willing to bet that if you do the calculations you will need a larger service.

If you take real load readings you will probably not need a larger service.

This is one area the NEC is not minimum.

The utility's many times figure the load to be 50% of the NECs calculations. [Linked Image]
Posted By: Redsy Re: Panel maximum? - 01/22/04 01:37 AM
cavo,
See 220.31 for existing dwellings to determine if the code would require a larger service.

iwire,

I know what you mean regarding utility comopanies. Just look at how many homes are fed by a 50 KVA pole mounted transformer.

[This message has been edited by Redsy (edited 01-21-2004).]
Posted By: electricman2 Re: Panel maximum? - 01/22/04 02:03 AM
quote by Redsy
Quote
I know what you mean regarding utility comopanies. Just look at how many homes are fed by a 50 KVA pole mounted transformer.
We have a 25KVA transformer feeding 4 houses. [Linked Image]
Posted By: iwire Re: Panel maximum? - 01/22/04 02:11 AM
I was not just joking around. [Linked Image]

On one of the other forums a power company engineer hangs out and he said point blank that is the method they use to size their equipment.

50% of the NEC calculations along with many years of load data for different occupancy's.

If it ends up small they can replace it. [Linked Image]

You hardly ever see their equipment burning up.
Posted By: Bjarney Re: Panel maximum? - 01/22/04 05:46 AM
US Electric utilities have an insane number of standards with respect to transformers, and if those of the distribution variety fail, the likelihood of a quick, cookie-cutter replacement is good.
http://standards.ieee.org/catalog/olis/dtransformers.html
Posted By: Edward Re: Panel maximum? - 01/23/04 03:01 AM
I think Andy is trying to ask:
"If my main panel is 200Amp main and i have a 100A subfeed, 30A stove, 40Amp range, 40Amp A/C, and (10) 20amp single pole breakers and still room in the panel to pull additional circuits can i do that? Or i will max out the panel?"

I think that is what he is trying to ask.

Edward
Posted By: iwire Re: Panel maximum? - 01/23/04 10:33 AM
Yes that is what he is asking and I do not think it can be answered here with the info given.

We have no idea what the connected load of the 100 amp sub feed is, an AC unit with a 40 amp breaker does not draw 40 amps and 10 20 amp SP breakers do not draw 200 amps.

There are only two ways to answer this. [Linked Image]

1)Take real load measurements with an amp clamp. On a large job we would put in a data logger for a few days to a week.

2)Do the load calculations from the NEC.

I was just at a commercial job looking at a 400 amp panel with six 200 and two 100 amp 3 phase breakers in it (1400 amps total).

Was this panel maxed out?

I would have to say no as it has been that way for many years. [Linked Image]
Posted By: cavo148 Re: Panel maximum? - 01/23/04 12:56 PM
Actually, what Andy is asking is how would you determine if this panel or any existing residential panel is maxed-out already, per the NEC. I believe to be absolutely sure I'd have to take the current readings for actual demand and calculate (per NEC) the pool equip. and the A/C equip. loads, since it's off-season for them. This overall existing load together with calculated load of the future living space should tell the story.

For business sake, we'd want to sell an upgraded service if justified, right? But, maybe our competitor is hinting that it's not really necessary for the additional circuits. Now you have to make your case.

What's your opinion?
Andy
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