Sub Panel rating

[sfav8r]

Greetings. I am installing three sub panels for a building that has two residential units and one commercial space. The service is 400amp. I'm trying to decide if a 125 amp panel is sufficient for each of the residential units. My calculations are listed below:

1500 Sq ft. x 3 is 4500 watts for general lighting.
2 appliance circuits = 3000 watts
1 laundry circuit 1500 watts

Total is 9000 watts

100% of first 3,000 plus 35% of the remaining 6000=
3000 + 2100 = 5,100 watts for Lighting ans small appliance load

1 Range, less than 12kv is derated to 8,000 watts.

There are 10, 1,000 watt baseboard heaters = 10,000 at 100%

Dryer 5,000
Water Heater 4,500
Dishwasher 1,440
Disposal 1440
Microwave 1440

total other loads is 13,820 @ 75% is 10,365

Total 33,465 divided by 240 is 139.43 amps.

So, I suppose 150 amp panel for each resi unit would squeak by.

Does my math look OK here?

Also, as far as calculating the entire service. Do I add the two resi units at 100% then do calcs on the commercial?

Thanks.

 

[haskindm]

Math looks OK except that yu can't take 75% of dryer. Re-read 220.53. To calculate the load for the entire building you will need to start over. You will still use the first 3000 VA @100% and then 35% of the remainder which now has more small appliance and laundry circuits as well as more square feet. You now have two ranges which will be calculated at 11KW. You will also need to add in any "house" or common loads. You will also be able to use 220.53 for more than four appliances.

 

[suemarkp]

You may want to try the optional calculation for the residential units. You'll get a nice discount on the heaters since you have more than 4 (assuming there are more than 4 thermostats), and a larger discount on most everything else (40% factor instead of 75%).

For the total service, you add up all the general lighting and small appliance loads of all the spaces (and I think you'll have to use the standard calculation for that). Then list all the fastened-in-place items. You can then apply the multiple item discounts (e.g. two ranges are 11 KW instead of 8+8, but two dryers are just summed) and sum everything up.

The standard calculation really needs to have a factor applied for multiple independent resistance heaters, but I don't see any allowed.

 

[sfav8r]

Good info. thanks!