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#87092 01/20/04 05:40 PM
Joined: Jan 2004
Posts: 79
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cavo148 Offline OP
Member
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

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#87093 01/21/04 12:20 AM
Joined: Feb 2003
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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

#87094 01/21/04 08:18 AM
Joined: Jan 2004
Posts: 79
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cavo148 Offline OP
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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

#87095 01/21/04 12:14 PM
Joined: Feb 2002
Posts: 375
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Member
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.

#87096 01/21/04 12:30 PM
Joined: Feb 2003
Posts: 169
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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.

#87097 01/21/04 12:53 PM
Joined: Jan 2004
Posts: 79
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cavo148 Offline OP
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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

#87098 01/21/04 08:59 PM
Joined: Jan 2003
Posts: 4,391
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Moderator
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]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#87099 01/21/04 09:37 PM
Joined: Mar 2001
Posts: 2,056
R
Member
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).]

#87100 01/21/04 10:03 PM
Joined: Nov 2003
Posts: 269
E
Member
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]


John
#87101 01/21/04 10:11 PM
Joined: Jan 2003
Posts: 4,391
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Moderator
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.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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