NEC 220.55 - Multi-Family Calculation with Different SIze Rage & Cooktop

[cdcengineer]

Alright, good-morning all.

I am posed with a new one here this morning. I have (28) dwelling units, (14) with 8kw range & (14) with 2.766kW cooktops. The units are served with 120/208V, 1-phase, but the distribution panel which serves this bank of units is 208Y/120V, 3-phase.

220.55 allows where two or more single phase ranges are supplied by a 3-phase feeder, the total load shall be calculated on the basis of twice the number connected between any two phases.

Therefore we have a max of (5) ranges and (5) cooktops connected between any two phases. I was originally under the impression that I should calculate the ranges as:

Twice the maximum - 2 x 5 = 10, table 220.55 --- Column C - 25kW+(25kW/2) = 37,500

And the cooktops should be handled as:

Twice the maximum - 2 x 5 = 10, table 220.55 --- Column A 49% x (10 x 2.766kW) = 13553 + (13553/2) = 20,330

Total Electric Cooking = 37,500 + 20,330 = 57,830

However after reading a bit closer, I'm now considering that table 220.55, note #3 applies. Which after still considering that there are only (5) of each type of cooking appliance connected between any two phases, this would result in the following:

Cooktops - Twice the maximum - 2 x 5 = 10, table 220.55 --- Column A 49% x (10 x 2.766kW) = 13553 + (13553/2) = 20,330

Ranges - Twice the maximum - 2 x 5 = 10, table 220.55 --- Column B 34% x (10 x 8kW) = 27200 + (27200/2) = 40,800

Total Electric Cooking = 20,330 + 40,800 = 61,130

And finally there is always the possibility that this should be handled as if all the appliances are combined:

Twice the maximum - 2 x 10 = 20, table 220.55 --- Columns A & B [35% x (10 x 2.766kW)] + [28% x (10 x 8kW)] = 32081 + (32081/2) = 48,122

I have performed multi-unit calculations before, but never with different size electric cooking appliances. Can anyone provide some opinions on how this should be handled?

As always, your input is greatly appreciated.

 

[cdcengineer]

Not a single taker? C'mon fellas.

 

[cdcengineer]

Alright, after further deliberating, it is my interpretation of 220.55 that the following is how to handle this situation:

Twice the maximum # of appliances connected between any two phases - 2 x 10 = 20, table 220.55 --- Column C = 35kW, divide by 2 and multiply the result by 3, this will give us the equivalent 3-phase load = 52,500

This is also after considering Annex example D5a which shows that if these appliances were of the same load rating at 8kW, than this would result in a lower total electric cooking appliance load than I had calculated in my first attempt indicated above.

Quiet here today. Might be a good sign everyone is busy at work.

 

[mcnut]

heard a pin drop...

heard a pin drop...

Do the 14 units with the cook tops have additional cooking appliances like single or double ovens?
When there is a cooktop here in Florida it is usually paired with a another appliance.

 

[cdcengineer]

No other cooking appliance. There are main units with the ranges and then suites with cooktops only.

 

[drateenergy]

250.55

250.55

28 Units
14- 8kw Ranges
14- 2.766 Cooktops

Everything should be done separately right to the end. as you said they are in different unit types

maximum # connected between any two phase = 5 C Phase and A phase darw it out if u have to.
2 x 5=10
8000 x 10 = 80000va
column B 10 = 34%
80000 x .34 = 27200va
single phase 27200/2 = per phase 13600va
three phase 13600 x 3 = 4533 va

so on and so forth do the same with the cooktops and total up your 3 phase service Va.
hope this helps.

 

[Dennis Alwon]

CD-- annex D is your friend. Look at Example D5(a). They do the calc for 10 ranges feed from a 3 phase system.

 

[kingpb]

"Multi-Family Calculation with Different SIze Rage & Cooktop"

I would imagine multiple families living together with only one range would cause them to have different levels of rage..............:lol::lol::lol::lol::lol:

Couldn't resist!