Multi family dwelling unit calculations

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rp9ball

Member
Location
WI
If I did these calcs correctly, why is there such a discrepancy between the two load calculations.
Standard vs 220.84

In your experience which one would use.

See attachment

Thank you for your help in advance :)
 

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dema

Senior Member
Location
Indiana
Service Load

Service Load

If you read 220.84C, you have to recalculate the load per apartment without the discounts prior to using the .38 multiplier. That means you give up your discount for over 3000VA - you use 3VA/sq ft for the whole apartment. You also must have an electric stove and either air conditioner or heater in the calculation, which you do. So, this loses your 40% reduction that you got on the panelboard sizing. It also loses you any discounts you have for multiple ranges, heaters or other things.

So, for the service size, you take the undiscounted values, sum them up and multiply THAT by 38%. Then you add your discounted house panel loads. Please read the intoductory paragraph of 220.82 B and 220.84 C and see the differences.

You need a bigger service.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
If I did these calcs correctly, why is there such a discrepancy between the two load calculations.
Standard vs 220.84

In your experience which one would use.

Your standard calculation should be a little lower, and your optional calculation should be a little higher...the optional will be about 30% lower rather than the 45% lower that your calc shows.

In your standard calc, your dryer load should be 68,250va, not 105,000va. 14x2=28...28 dryers have a demand of 45,500va, which equals 22,750 per phase, which equal 68,250va 3ph equivalent.

The oven demand load should be 64,500va 3ph equivalent.

When you added the standard loads, you included the full appliance load (186,880) rather than the demand load (140,160.)

On the optional method calc, you need to use the nameplate rating of the range rather than 8kw.

Also, for your House Panel calc, you can't use the dwelling unit lighting demand.
 

dema

Senior Member
Location
Indiana
VA is VA

VA is VA

The dryers will be spread out on the three phases. So the there is no need to take a single phase VA and multiply it to something higher. Or at least not for anything but the 28th one. The first 27 would be equally distributed per phase.
 
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