Main feeder load calculation for dwelling from 208Y/120V service

[Tom_huizinga]

In Informative Annex D, there is an example (example D5(a)) of a standard calculation for the main feeder load for a multifamily dwelling from a 208Y/120V service. In this example, the range load (29000VA) is divided by 2 (to find per-phase demand) and multiplied by 3 (to calculate equivalent 3-phase load). It's not clear to me why this is necessary as there appears to be no requirement for this step in 220.55.

Furthermore, there is no consideration of equivalent 3-phase load in the optional calculation (example D5(b)) which follows, which is a very similar scenario with a multifamily dwelling served by 208Y/120V.

I have a homework problem that requires me to do standard and optional calculations for a similar scenario, and my main load results were drastically different between the two because the optional calculation apparently doesn't require me to find equivalent 3 phase load...

So my question... is example D5(b) wrong?

 

[jumper]

I assume that you are referring to ranges, single phase, on a three phase supply.

See the last sentence of 220.55.

“Where two or more single-phase ranges are supplied by
a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the maximum number connected between any two phases”

The examples in the annex are correct.

Do you understand the reason why?

 

[Tom_huizinga]

Do you understand the reason why?

No I don't. Why is it not necessary to find the equivalent 3-phase load for the range in the optional calculation?

If D5(b) is correct, then the optional calculation is going to give a result that is drastically lower than the standard calculation every time. Right?

 

[Tom_huizinga]

Demonstration

Demonstration

For the benefit of myself and everyone else who is interested in my question, and for fear of getting lost in the abstract, here is a demonstration.

Let's consider Example D5(b)'s scenario, as D5(a)'s doesn't contain electric heat or A/C and is therefore ineligible for the optional calculation (per 220.84(A)) for main feeder load.

Here are the specs:
*Multifamily dwelling, w/ 40 units, each 840 sqft.
*No laundry machines
*Each has an 8000VA range
*Each has 4 1500VA electric heaters (rated at 240V)
*Each has a 2500VA electric water heater (rated at 240V)
*No motor loads
*Service is 208Y/120V

STANDARD CALCULATION (a la example D5(a))

1. General lighting:
840 sqft x 3VA/sqft = 2520VA
1500VA x 2 small appliance circuits = 3000VA
2520VA + 3000VA = 5520VA per unit
5520VA x 40 units = 220800VA
Application of demand factor:
3000VA @ 100% = 3000VA
117000 @ 35% = 40950VA
100800 @ 25% = 25200VA
Total general lighting load (3000 + 40950 + 25200): 69,150VA

2. Range load:
Max connected between any two phases: 14 (14 + 13 + 13 = 40 ranges)
2 x 14 = 28
Per Table 220.55 column C, demand is 15000VA + 28000va = 43000VA
*Per-phase demand = 43000VA / 2 = 21500VA
*Equivalent 3-phase load: 21500VA x 3 = 64,500VA

3. Fixed appliance load:
Water heater (240V): 2500VA x 40 units = 100,000VA
(side-note: I'm not sure if we're supposed to find equivalent 3-phase load here too. If we do, then the difference between the standard and optional calculations will be even more radically different than they already are...)

4. Heating & A/C loads:
Electric heat (240V): 1500VA x 4 x 40 units = 240,000VA
(same here... why do we not need equivalent 3-phase load??? But I digress...)

5. Total service load:
69,150VA + 64,500VA + 100,000VA + 240,000VA = 473,650VA
Convert to amps:
473,650 / (208V*sqrt(3))=1315A

(Ignore the neutral load for now...)

OPTIONAL CALCULATION (Example D5(b))

1. Non-heating loads:
Lighting (with no demand factor applied): 220,800VA
Range (nameplate rating x quantity): 8000 x 40 = 320,000VA
Fixed appliance (water heater) 2500VA x 40 = 100,000VA

2. Heating loads:
Electric heaters: 1500VA x 4 x 40 = 240,000VA

3. Total load:
220,800 + 320,000 + 100,000 + 240,000 = 880,800VA
Application of demand factor (Table 220.84):
880,800 x 28% = 246,624VA
Convert to amps: 246,624VA / (208*sqrt(3)) = 685A

Obviously something is wrong.

 

[Tom_huizinga]

Anyone?

Anyone?

Anyone willing to help me out here? Where'd I go wrong?

 

[jumper]

No I don't. Why is it not necessary to find the equivalent 3-phase load for the range in the optional calculation?

If D5(b) is correct, then the optional calculation is going to give a result that is drastically lower than the standard calculation every time. Right?

Yes.

 

[jumper]

For the benefit of myself and everyone else who is interested in my question, and for fear of getting lost in the abstract, here is a demonstration.

Let's consider Example D5(b)'s scenario, as D5(a)'s doesn't contain electric heat or A/C and is therefore ineligible for the optional calculation (per 220.84(A)) for main feeder load.

Here are the specs:
*Multifamily dwelling, w/ 40 units, each 840 sqft.
*No laundry machines
*Each has an 8000VA range
*Each has 4 1500VA electric heaters (rated at 240V)
*Each has a 2500VA electric water heater (rated at 240V)
*No motor loads
*Service is 208Y/120V

STANDARD CALCULATION (a la example D5(a))

1. General lighting:
840 sqft x 3VA/sqft = 2520VA
1500VA x 2 small appliance circuits = 3000VA
2520VA + 3000VA = 5520VA per unit
5520VA x 40 units = 220800VA
Application of demand factor:
3000VA @ 100% = 3000VA
117000 @ 35% = 40950VA
100800 @ 25% = 25200VA
Total general lighting load (3000 + 40950 + 25200): 69,150VA

2. Range load:
Max connected between any two phases: 14 (14 + 13 + 13 = 40 ranges)
2 x 14 = 28
Per Table 220.55 column C, demand is 15000VA + 28000va = 43000VA
*Per-phase demand = 43000VA / 2 = 21500VA
*Equivalent 3-phase load: 21500VA x 3 = 64,500VA

3. Fixed appliance load:
Water heater (240V): 2500VA x 40 units = 100,000VA
(side-note: I'm not sure if we're supposed to find equivalent 3-phase load here too. If we do, then the difference between the standard and optional calculations will be even more radically different than they already are...)

4. Heating & A/C loads:
Electric heat (240V): 1500VA x 4 x 40 units = 240,000VA
(same here... why do we not need equivalent 3-phase load??? But I digress...)

5. Total service load:
69,150VA + 64,500VA + 100,000VA + 240,000VA = 473,650VA
Convert to amps:
473,650 / (208V*sqrt(3))=1315A

(Ignore the neutral load for now...)

OPTIONAL CALCULATION (Example D5(b))

1. Non-heating loads:
Lighting (with no demand factor applied): 220,800VA
Range (nameplate rating x quantity): 8000 x 40 = 320,000VA
Fixed appliance (water heater) 2500VA x 40 = 100,000VA

2. Heating loads:
Electric heaters: 1500VA x 4 x 40 = 240,000VA

3. Total load:
220,800 + 320,000 + 100,000 + 240,000 = 880,800VA
Application of demand factor (Table 220.84):
880,800 x 28% = 246,624VA
Convert to amps: 246,624VA / (208*sqrt(3)) = 685A

Obviously something is wrong.

I am too tired to run your numbers, but the optional method is often 25-40% less than a standard calculation.

 

[Geon_W]

The key here is to note that 220.55 only applies to the standard calculations Part 3, It does not apply to the optional calculations which is in Part 4. See also 220. 40 and 220.80 to understand the scope of each part.

This should shed some more light.