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Discussion Starter · #1 ·
My problem is a 24 unit co-op building that was renovated in the late 1970s. It has three phase, 4 wire, electrical service conductors that feed a 400 Ampere 3 pole fused switch which supplies the apartments and the public areas. I am trying to work backwards from the 400 amp electrical switch to figure out approximately what connected load per apartment is allowed in view of the 400 amp service. Here are my ballpark calculations.
Convert the 400A demand load back to kVA by multiplying by 360 (208 x 1.73) = 144kVA. From that I subtract demand from common areas (lighting, boiler burner, pumps and fans) 4kVA, leaving 140kVA as calculated load for the 24 apartment units. Assuming a demand factor around 0.4, then the connected load allowed per apartment works out to (140/0.4/24) 14.6kVA. Eighty percent (safety factor) of that is 11.7kVA. Subtracting a window air conditioner (1000W) that runs continuously during the summer leaves 10.7kVA. Am I correct in concluding that, within the limits of the fuses and wire size in each branch circuit, the 400A service makes 10.7kVA available for connected load at the 120V receptacles in each apartment? (The apartments each have 100A panels fed by 3-wire #8 AWG cable - a neutral and two “hot”).
Appreciate comments pointing out any errors I may be making, or whether this looks about right. Thanks.
 

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other than subtracting your house load, I don't like your caclulations.

I think you should stop there and go review the allowance and demand factors for multi units. Also, where in article 220 do you know of an 80% safety factor ?

try doing a load calc for the average apartment, use the demand factors for appliances, use the demand for multifamily, and see if you can come close to your calculation for the watts available.

(JMSO - I honestly don't know how to go backwards from the available kva to the amount for each apartment because it makes my head spin)
 
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Discussion Starter · #5 ·
Thanks for the comments.

Pretty sure the 3 wire cable is #8 AWG – it looks about right and that’s what’s in the engineering report describing the system. But I went down into the basement and poked around a bit more. Next to the bank of meters is the main apartment circuit breaker panel and there’s a 40A circuit breaker in each of the hot legs for each apartment. That’s about right for #8 and it tells me that the max power available to each apt is (40x208) 8.3kVA with 208V appliances, (40x120) 4.8kVA with 120V appliances, or something in between with a mixture of appliances. Despite the 100A panel in each apartment with 4x15A and 2x20A branch circuits, the main breaker will pop (at least I hope it will!) when the draw reaches 40A. It also answers part of my load question: 24 apartments each drawing 40A would total 960A, plus 40A for the public areas, makes 1000A, but as the main switch is 400A it looks like a demand factor of (400/1000) 0.4 must have been used.
The 80% was a mistaken attempt to apply the circuit breaker safety factor for continuous loads to the apartment feeder. Bad idea.
 

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Thanks for the comments.

Pretty sure the 3 wire cable is #8 AWG – it looks about right and that’s what’s in the engineering report describing the system. But I went down into the basement and poked around a bit more. Next to the bank of meters is the main apartment circuit breaker panel and there’s a 40A circuit breaker in each of the hot legs for each apartment. That’s about right for #8 and it tells me that the max power available to each apt is (40x208) 8.3kVA with 208V appliances, (40x120) 4.8kVA with 120V appliances, or something in between with a mixture of appliances. Despite the 100A panel in each apartment with 4x15A and 2x20A branch circuits, the main breaker will pop (at least I hope it will!) when the draw reaches 40A. It also answers part of my load question: 24 apartments each drawing 40A would total 960A, plus 40A for the public areas, makes 1000A, but as the main switch is 400A it looks like a demand factor of (400/1000) 0.4 must have been used.
The 80% was a mistaken attempt to apply the circuit breaker safety factor for continuous loads to the apartment feeder. Bad idea.
Stop adding stuff up, and do A REAL NEC LOAD CALC
 

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Discussion Starter · #11 ·
Stop adding stuff up, and do A REAL NEC LOAD CALC
I don’t see how doing a new load calculation addresses the problem. We’re not rewiring the building or swapping out the 400A service (yet). What I want to do is make an informed judgment about how many more units can install washer/dryers (or other appliances that were not anticipated in the 1970 load calculations) to their apartments before we reach the limits of the 400A service. Some? Yes. 24? No.
We haven’t had circuit breakers popping, and the addition of a washer/dryer in an individual apartment won’t exceed the 40A limit on the individual feeder (gas ranges, central heat and hot water). What I’m wondering about is the upstream picture -- between the meter bank distribution panel and the 400A switch.
 

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I don’t see how doing a new load calculation addresses the problem. We’re not rewiring the building or swapping out the 400A service (yet). What I want to do is make an informed judgment about how many more units can install washer/dryers (or other appliances that were not anticipated in the 1970 load calculations) to their apartments before we reach the limits of the 400A service. Some? Yes. 24? No.
We haven’t had circuit breakers popping, and the addition of a washer/dryer in an individual apartment won’t exceed the 40A limit on the individual feeder (gas ranges, central heat and hot water). What I’m wondering about is the upstream picture -- between the meter bank distribution panel and the 400A switch.
If you don't see the significance of am NEC load Calc, we might as well shut this thread down.
Are there not provisions in the NEC to allow additional loads to be added to an existing service or feeder?

I think that is what " puzzled " is politely asking.

I agree that doing a new demand calculation is needed, but does the NEC not allow other methods to be used regarding existing installations?

The CEC allows a slightly different method:

8-106 (8)

Where additional loads are to be added to an existing service or feeder, the augmented load shall be permitted to be calculated by adding the sum of additional loads, with demand factors permitted by this Code to the maximum demand load of the existing installation as measured over the most recent 12-month period, but the new calculated load shall be subject to Rule 8-104(4) and (5).

Is there similar wording in the NEC?

Borgi
 

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Discussion Starter · #15 ·
Borgi -- Yes there is a section in the NEC, 220.83, for adding to existing load and after the first 8kVA it allows you to add load at 40%. Thanks.

BKMichael65 -- Given the #8 wire size of the feeders that run to the individual units I guess we didn't get that option! City code may have been different in 1970s.

Anyway, to return to the original problem which was to determine what additional load the building wiring could support as individuals added dishwashers and electric dryers to their apartments, I made up a load calculation spreadsheet which allowed me to incrementally add dishwashers and/or dryers to the standard allowances for lighting and small appliance circuits and to watch what happened to the load. It turns out that they don't have as much of an effect as I feared because with 6 dryers or more dryer load is calculated at 75% or less so it takes a lot to reach the 400A service limit. What's more of a problem is the 40A limit to each apartment -- when your dryer is pulling 25-30A it won't take much more to pop the circuit breaker.
 
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