load calculation

[myoung2165]

i've been an electrical contractor in dayton ohio since 1992. I started doing electrical work in 1972. i am now semi-retired and am trying to help one of my ex employees with a load calculation. i have done many calculations on single families but rarely on multi family. bruce, my ex employee did a service change on a 3 family in dayton. he fed the building with 200 amps and has a 100 amp panel in each unit. the inspector requested a load calculation. i used a combo of examples D2(b) and D4(a) from the 2008 code book. me and the the inspector are way off from each other. just wanted to get some other opinions.

here it goes
3 family with each unit having 560 sq ft, the units do not have a laundry, the range nameplate rating is 11500va, there is an electric water heater with a 2500va rating. there is one 120volt window a/c unit. there is 5600watts of heating. thats about it.

would like to see what you guys/gals come up with for the load on each unit and what size main feeders. the inspector and i have different figures with an 184amp difference. i really think hes wrong. the whole building only has 1680 sq ft. please help

 

[charlie b]

UNIT CALCULATION (OPTIONAL METHOD)
560 x 3 = 1,680.
2 SA circuits @ 1500 each = 3,000
Laundry circuit is omitted.
Range: One at 11,500
Water Heater: One at 2,500
Heating: 5,600
Disregard window A/C unit.
Subtotal 24,280.

Take first 10,000 at 100%, take remainder at 40%:
New total: 15,712.
Divide by 240, and get 65 amps.
Use a 100 amp unit feeder.

OVERALL SERVICE CALCULATION (OPTIONAL METHOD)
560 x 3 x 3 = 5,040.
6 SA circuits @ 1500 each = 9,000
Laundry circuit is omitted.
Range: Three at 11,500 = 34,500
Water Heater: Three at 2,500 = 7,500
Heating: Three at 5,600 = 16,800
Disregard window A/C unit.
Subtotal 72,840.

Take first 10,000 at 100%, take remainder at 40%:
New total: 35,136.
Divide by 240, and get 146 amps.
Use a 200 amp service.

 

[Benton]

That's a pretty cool gesture charlie b....

 

[Npstewart]

11,500 seems high for a range, I think I only use 8,000 typically.

What about the microwave?

 

[charlie b]

11,500 seems high for a range, I think I only use 8,000 typically.

I started by using the "standard method," and that method allows us to use 8kW for any range up to 12 kW. Then I remembered that the "optional method" almost always gives me a smaller answer. In that method, you use the actual nameplate value of the range.

 

[myoung2165]

thanks charlie b. thats close to what i came up with. the inspector figured 238 amps. my point was the whole building only has 1680 sq ft. any house that size could be fed with 200 amps. thanks again. mike

 

[Npstewart]

Good to know, thanks.

 

[Dennis Alwon]

What about the microwave?

What about it. It was not mentioned by the OP. It would be figured as part of the SABC , just as the blender, mixer, Cuisinart etc would be.

 

[charlie b]

The inspector figured 238 amps.

Here is what I get with the standard method. See the NOTE in the middle of this derivation, and you might have the reason for the inspector's answer. But the Optional Method gives a lower answer, and therefore we are allowed to use the Optional Method's answer.

UNIT CALCULATION (STANDARD METHOD)
560 x 3 = 1,680.
2 SA circuits @ 1500 each = 3,000
Laundry circuit is omitted.
Subtotal 4,680
Take first 3,000 at 100%, take remainder at 35%:
New subtotal 3,588

Range: One at 8,000
Water Heater: One at 2,500
Heating: 5,600
Disregard window A/C unit.
Total: 19,688.
Divide by 240, and get 82 amps.
Use a 100 amp unit feeder.

NOTE:
If all you do now is to multiply by 3, based on having 3 units, you get a service load of 246 amps. It is not necessary to do that, however, as shown below.

OVERALL SERVICE CALCULATION (STANDARD METHOD)
560 x 3 x 3 = 5,040.
6 SA circuits @ 1500 each = 9,000
Laundry circuit is omitted.
Subtotal 14,040
Take first 3,000 at 100%, take remainder at 35%:
New subtotal 6,864

Ranges: Three, each less than 12,000, Table 220.55 says to use 14,000
Water Heaters: Three at 2,500 = 7,500
Heating: Three at 5,600 = 16,800
Disregard window A/C units.
Total 45,164
Divide by 240, and get 188 amps.
Use a 200 amp service.

 

[hurk27]

What about it. It was not mentioned by the OP. It would be figured as part of the SABC , just as the blender, mixer, Cuisinart etc would be.

only if it sits on the counter top and plugs into a counter receptacle, if fixed in place, I would think you would have to include it? 220.3(B)(1)

And since most micro-mates are not rated in excess of 1? kW 220.19 can't be used for it.

 

[Dennis Alwon]

only if it sits on the counter top and plugs into a counter receptacle, if fixed in place, I would think you would have to include it? 220.3(B)(1)

And since most micro-mates are not rated in excess of 1? kW 220.19 can't be used for it.

That was my point-- the op didn't mention it so I assumed it was not fixed in place. I realized it must be included if it is fixed.

 

[LarryFine]

M, welcome to the forum!

the inspector figured 238 amps.

That's probably 'connected load', which the service will never see.