RESIDENTIAL LOAD CALCULATION

[LUIS FERNANDEZ]

I'm doing a residential job which has 2 electrical panels minimun because is a large house. Can you apply demand to each panel separetely using Table 220.82 and reapply demand to the service feeder or if you apply demand only to the service feeder, then how do you calculate each panel size as far as the general lighng and receptacles, let's say that you have 6 general lighting in one panel and 8 on the other panel, what load you show for each branch circuit. the code does not have any example of this situation and it happens a lot in big residential units. Please advice.

 

[jtester]

I am not sure I would use 220.82 for subpanels. It is appropriate for service to the entire dwelling, but not to subpanels within a dwelling.

That being said, you can't double diversify by using multipliers from any of the load calc tables twice. For instance, you can use 220.42 on each subpanel, but you can't then add those two loads added together at a main, and then apply 220.42 again.

Jim T

 

[bob]

280 is listed optional feeder and service load caculations. It seems to me that it would apply in caculations for a panel. The diversity factor would be higher than when applied to a service.

 

[suemarkp]

For my 400A service, I did the following:

One calculation for the service conductors that looked at the entire house.
One calculation for 200A panel #1 done with the equipment and square footage it serves.
Another like the previous for the second 200A panel.

What ends up happening is if your service calculates to 280A, you most likely won't be able to serve it with a 200A and 100A panel because you have to start each panel from the beginning and pay the penalty for the initial load that has no diverity factor applied to it (the 10 KW base in the optional calculation or the first 3 KW in the standard calcualtion plus the 75% factor for more than 4 fixed appliances may not be reached in a given panel) .

I was even having some difficulty spreading out my 325A calculated service to my two 200A panels. I was able to do it, but you have to pick your loads carefully. One interesting thing that happened is the kitchen range calculation. I have two kitchens in the house each with a 12KW range. For the entire service, this part calculates to 11 KW. If both range circuits are in the same panel, they count as 11 KW in that one panel. But if one is in each panel, each panel will be assessed at 8 KW. This effectively means your panels need to accomodate 5 KW more amps than what the service calculates to.

 

[LarryFine]

Mark, I just scanned 230, and nothing gave me the impression that the quantity of panels in one service is relevant when computing load. Please elaborate.

 

[tshea]

I just did a 320Amp service--2-200Amp panels, 1 is served by a service rated ATS. Most of the load is on the non-generator panel, about 110 calculated Amps. The generator panel has about 45 calculated Amps.
The house is 2700 sqft, A/C, Hot tub, gas appliances (dryer, stove top, furnace 1 & 2), 3 baths, mucho kitchen.

Your situation will require carefully planning which circuits go where.

 

[suemarkp]

Mark, I just scanned 230, and nothing gave me the impression that the quantity of panels in one service is relevant when computing load. Please elaborate.

For the service, that is true - you calculate things based on the calculation rules and number of panels isnt' an issue. The service has the entire dwelling as the conected load.

But if you split this service into two sets of service conductors feeding two panels, as is typical with 400A services, you probably don't want to run 400 KCMIL to a 200A panel. You can reduce your wire to serve the connected load, which is a panel. Determining the connected load requires doing a load calculation again based on what is connect to that panel. Although the code doesn't say it well, 220.10 implies to me that you calculate per panel only the portions of the total service circuits that are served by a given feeder or service wire set. Article 230.42 refers you to article 220.

 

[tbonse]

Re: Residential Load Calculation

Re: Residential Load Calculation

Luis, after combining what is said in 220.40 and 220.82, you have two options.

1) calculate the demand factor for each subpanel independently and use the sum to determine the feeder required for the entire house, but the feeder to each of the panels should be based on the demand factor of the respective panel, but do not apply 220.82 again.

2) provide a feeder to each sub panel at its maximum capacity and calculate the demand factor for the dwelling as a whole to calculate the feeder required for the service entrance from your utility company

 

[wmgeorge]

Large Service for Residential

Large Service for Residential

I'm doing a residential job which has 2 electrical panels minimun because is a large house. Can you apply demand to each panel separetely using Table 220.82 and reapply demand to the service feeder or if you apply demand only to the service feeder, then how do you calculate each panel size as far as the general lighng and receptacles, let's say that you have 6 general lighting in one panel and 8 on the other panel, what load you show for each branch circuit. the code does not have any example of this situation and it happens a lot in big residential units. Please advice.

My thought for the day. I was visiting with my cousin who lives in Germany over the Holiday weekend, and since he is a EE and his son who his 25 or so has a PHD in EE we had some interesting talks. Anyway... his electricial service to the house is 400 / 230 volt 50 cycle 3 phase. I would think on these large residential jobs, the power companies here (USA) should consider the same... three phase, at no extra charge. I'd think they'd love to see those 3 phase lines balanced out, instead of... say 400 amps on single phase!!