Secondary panel problem


Old 09-09-04, 08:26 AM
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Secondary panel problem

This is my dilema...after doing the load calculation for a 200amp main panel I came up with 195.77083amps or 196amps. We want to run a 100amp "sub-panel" in the garage, which is attached, and the load calculation for the "sub-panel" is 69.43amps or 70amps. My question is whether or not the 200amp main panel in the house can support the secondary panel.
Also, in regards to determining conductor size for the subpanel, Table 310.15(B)(6) in the NEC says that the ampacity ratings for listed conductors are only for main power to dwellings and cannot be used to determine the feeder size to Panel B(subpanel) because this feeder does not carry the main power to the home. So basically it's saying that this table cannot be applied to the feeder to Panel B(subpanel) because that feeder carries only part of the load in the residence. So how should I go about determining the size? I'm guessing #3copper would do or should I jump to #4copper? Thanks for the replies ahead of time and I tried to be as thorough as possible with the info.
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Old 09-09-04, 08:40 AM
Join Date: Sep 2000
Location: United States
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Do the load calculation on the main panel again, also including everything that will be running off the subpanel. In effect, everything running off the subpanel is also running off the main panel.

You are correct. 310.15(B)(6) cannot be used to size your feeder. Use 310.16. The 100-amp subpanel should be fed by #3 copper (unless you are using a NM, in which case you should use #2).
Old 09-09-04, 09:55 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
We need to know the calculation procedure you followed that resulted in a calculation of 200 amps (approx).

As best you know, does your calculation conform to NEC values?

Good Luck & Enjoy the Experience!!!!
Old 09-09-04, 10:59 AM
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I'm using an Electrical Wiring Residential book that is based on the 2002 NEC and it states that there are two methods for calculating services on homes that is permitted by the NEC. Method one is outlined in Article 220, parts I and II. Method two is given in Article 220 part III. I'm in the process of recalculating it all together as Mr. Nelson stated in his post. I'm not sure that the outcome is going to be in my favor though? I'll get back to ya guys with the new numbers asap. Thanks again.
Old 09-09-04, 11:03 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
If you want us to double-check your calculations, provide us with all the inputs you're using.
Old 09-10-04, 06:49 AM
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Sorry it took a while to get back to the thread...a lot going on yesterday and today. This is what I'm using:
Combined sqft of the house and 2 story attached garage is 3514sqft.

1. [email protected] per foot=10,542va which equals 6 lighting branch circuits.

2. small appliance load
clothes washer-1
[email protected]=6000va

3. laundry branch circuit:210.11(c)(2)&220.16(B)
[email protected]=1500va

4. total general lighting, small appl, laundry=18,042va

5. NET
a. first [email protected]%=3000va
b. line 4: 18,[email protected]%=5265va

6. oven/stove=14,050va/(14kw)exceeds 12kw by 2kw/ 2kwx5%=10%increase, therefore: 8kw+0.8kw=8.8kw which eqautes to 8,800kw

7. Electric dryer=5700va

8. electric furnace, A/C, Heat pump
A/C 27x240=6480va
Electric Furnace=13,000va

9. NET computed general lighting, small appliance, laundry, ranges, ovens, cooktop units, HVAC=35,765va

10. Fastened-in-place appl.
water heater=3000va
water pump=1920
hydromassage tub=1500va
Air comp=7,200va TOTAL=16,120va

11. apply 75% demand factor(220.17) for four or more fastened-in-place:

12. total computed load=47,855va

13. 25% largest motor: 8x240x0.25=480va+47855=48355va/240v=201.3958

That's the basic jist of it. Not sure if it's all right but the amperage exceeds the rating of the main breaker...
Old 09-10-04, 11:33 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
The MINIMUM ampacity of the Service-conductors for the connected loads as listed is 160 amps with a voltage factor of 220 volts.

I suspect you may be in error with the 7200 kva value for the air-compressor motor because you list a value of 8 amps for the largest motor. If you do have a 7200 kva motor-load, then you have a 5-7 HP motor driving the air-compressor.

If you do have a motor with Full-load current value of 30 amps, the rating of the Main-breaker could exceed the ampacity rating of the Service conductors. The relevant Article is 430.63, Rating ( circuit-breaker) Motor/Lighting/Appliance loads.

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