Sizing of sub panels in multi-family dwelling and main service

spencer_electricianJuly 26, 2007

I'm still in school to become a master electrician and I work with another master electrician. A recent job possibility has both of us a little confused. It is just 2 not as common situations combined together. A house needs to be wired as four units. Two upstairs and 2 downstairs. The general wants everything to be electric, no gas at all. Each unit needing a 12kva furnace, 10kva oven, 5kva dryer, and a hot water heater, 800 sqft. general lighting, laundry circuit, and 2 small appliance. A while back when I figured up a load calc it was right around 130 amps. Now of course all of these can be deducted since there are four units but what about the sizing of the sub-panels and meters? Is it the units need a 150 amp service while the whole building needs a 400 amp service? The boss keeps saying no, the units only need 100 amps because of deductions but I see it as the unit must have its calculated load available to the sub-panel. Where the main service, deductions may take place.

If 150 amps is the answer. Does such a peice exist? A meter assembly with 5 meters (One meter for general hallways) and 150 amp breakers with a 400 amp main?

Thanks for suggestions, it helps me learn planning these things whether they're for one of his jobs or just practice. I just can't seem to find this type of situation in the books.

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The feeders and panels in the individual units need to be sized for the connected load. You do NOT size the service then divide by the number of units, less house load, to determine the feeder and panel size in the units.

A good review of the examples in Annex D should show you how this is done. When figuring the service size you are allowed to use 17KW for 4 ranges (not over 12KW each). This does NOT translate to only allowing 4.25 KW in each unit for a range, nor does it allow that since the 8KW has been used to calculate the unit feeder size that you can now use the 220.55 table column A and B to figure the service size if the range is over 8 3/4 KW.

As to the meter cans, main breakers, and service disconnect/OCPD, virtually any combination can be ordered. With rare exceptions you won't walk in to a distributer and buy exactly what you want off the shelf. Get a few service equipment catalogs and browse a bit.

Be sure your furnace includes the blower as this is often separate with electric heat, and water heaters come in a variety of sizes.

    Bookmark   July 28, 2007 at 11:08AM
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