Another Range Calc question for a 137 unit Apartment

[jay j]

Ok, I need some help, I have been looking at this for some time.

I have a 137unit apartment building and having some problems with the Range portion of the calc. Why is the NEC table 220.55 comumn's B and C so far off in my calc?

What in the world did I miss?

The ranges they are using are those small 24" ranges with a load of 8,700VA. If I use column B vs Column C the calc is so far different.

If I read it correctly I can use Column C if I choose even though the load is under 8.75KW?

See Calcs Below


Electric Ranges for a 208Y/120-V, 3-phase, 4-wire system (NEC 220.55 Column B)

Building Voltage 120/208/3
8,700VA per Range
Total Ranges=137
Max Ranges per any 2 Phases = 92
92 Ranges * 8,700 = 800,400VA
Demand Per Table, 800,400VA * .16 = 128,064VA
Per Phase Demand 128,064VA / 2 = 64,032
Equivalent 3-Phase Load = 64,032*3 = 192,096VA



Electric Ranges for a 208Y/120-V, 3-phase, 4-wire system (NEC 220.55 Column C)

Building Voltage 120/208/3
Ranges less than 12,000VA
Total Ranges=137
Max Ranges per any 2 Phases = 92
Demand per table 92 * 750VA = 69,000VA
Demand per table 69,000VA + 25,000VA = 94.400VA
Per Phase Demand 94,000VA / 2 = 47,000
Equivalent 3-Phase Load = 47,000*3 = 141,000VA


So what in the world did I do wrong? Why would using a 12,000VA range vs using a 8,700VA range make the calc that far scewed with the smaller range making for a bigger service.

One would think the larger range would make for a larger service.

If I did my calc right it would not pay to use column B unless the range load is less then about 6,300VA. So I must have a problem someware.

Thanks all

Jay

 

[Dennis Alwon]

Read the notes to the table -- Read the heading on the table and go to note #3

 

[jay j]

But does not note #3 say "it shall be permissible".

I guess I'm looking at it wrong, I did not take that as a shall. With the word permissible I was thinking either or.

 

[Bob NH]

But does not note #3 say "it shall be permissible".

I guess I'm looking at it wrong, I did not take that as a shall. With the word permissible I was thinking either or.

It is either/or for ranges up to 8.75 kW. You may use the column that results in the lower load.

Column C applies to all except that it "shall be permissible" to use Column B if that results in lower load.

About the kW rating, is that for 208 Volts or 240? Some appliances use the same range for both and have different kW ratings for the two voltages.

 

[jay j]

It is the load for 208V.

I never had to use column B before, most of the time the ranges are over 8.75KW. With this many units it bumps me to 3500amp service vs a 3000amp service depending on what way I look at it.

Thanks

Jay

 

[Dennis Alwon]

Sorry I had to step away. I agree it is an either or and you can choose the lower load. I never quite understood the Range demand table. Sometimes it will come out to where the calculated load is so small you cannot imagine that the circuit would hold esp. during Thanksgiving dinners.