I am currently working on a multi family condominium complex with 118 units. These units range in size from 728 sq feet up to 1418 sq feet. I have done a load calculation for each individual unit based on NEC Article 220. My calculated load per the code for the 728 sq ft condo is 144 amps. By my understanding a service of 150 amps is sufficient. I am working with an engineer who feels that we need to multiply the 144 amp load by 125% to equal 180 amps or a 200 amp service. He is basing this on 230.42 (A) (1). My understanding is that this would be only for continuous loads and that none of the residential loads qualify as continuous loads.
Dwelling Unit Service Calculations
- Thread starter newkirk
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Re: Dwelling Unit Service Calculations
You should be using 220 to caculate the individual load per apartment. Look at the def of continuous loads. Check 220.4A. Your largest motor will likley be the A/C unit. If so apply the 125% the the A/C load and add to the other caculated load. You do not apply the 125% to the entire load.
You should be using 220 to caculate the individual load per apartment. Look at the def of continuous loads. Check 220.4A. Your largest motor will likley be the A/C unit. If so apply the 125% the the A/C load and add to the other caculated load. You do not apply the 125% to the entire load.
growler
Senior Member
- Location
- Atlanta,GA
Re: Dwelling Unit Service Calculations
Newkirk, are these units totally electric, with electric water heater, range, heat and dryer. This is for my own information because I have never had a space of 728 sq ft calculate out that high. Could you supply a few of the numbers , like heat pump size.
Newkirk, are these units totally electric, with electric water heater, range, heat and dryer. This is for my own information because I have never had a space of 728 sq ft calculate out that high. Could you supply a few of the numbers , like heat pump size.
Re: Dwelling Unit Service Calculations
The article 220 calculation will give you the MINIMUM size required for safety. Read article 90 regarding the purpose of the code. There are lots of good reasons to provide more than the MINIMUM, such as adequacy, convenience, and future expansion. A 25% factor is reasonable, but has nothing to do with continuous loads. It is between you, your wallet, and your conscience.
The article 220 calculation will give you the MINIMUM size required for safety. Read article 90 regarding the purpose of the code. There are lots of good reasons to provide more than the MINIMUM, such as adequacy, convenience, and future expansion. A 25% factor is reasonable, but has nothing to do with continuous loads. It is between you, your wallet, and your conscience.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Dwelling Unit Service Calculations
I agree with haskindm. I also agree with the engineer's recommendation (i.e., to go with 200 amp panels). But I disagree with the engineer's basis for that recommendation. The engineer is misreading 230, and has misunderstood the function of 220.
Not having seen your calculation, I cannot tell you whether you did it right, or whether you missed something. But presuming you have the right calculation results, you can certainly use a 150 amp service. I did a quick calculation on the possible changes from the smallest unit to the largest unit, and my guess is that the largest unit might also get by with a 150 amp service.
That said, I believe it would be a better design decision to use a higher rated service. My basis is simply to allow for future expansion, and to accommodate those tenants who will use larger ranges, or water heaters, or whatever.
I agree with haskindm. I also agree with the engineer's recommendation (i.e., to go with 200 amp panels). But I disagree with the engineer's basis for that recommendation. The engineer is misreading 230, and has misunderstood the function of 220.
Not having seen your calculation, I cannot tell you whether you did it right, or whether you missed something. But presuming you have the right calculation results, you can certainly use a 150 amp service. I did a quick calculation on the possible changes from the smallest unit to the largest unit, and my guess is that the largest unit might also get by with a 150 amp service.
That said, I believe it would be a better design decision to use a higher rated service. My basis is simply to allow for future expansion, and to accommodate those tenants who will use larger ranges, or water heaters, or whatever.
Re: Dwelling Unit Service Calculations
All
Thanks for the great response. To clarify, yes this is an all electric application and furthermore the services are 208V 1phase which has pushed the ampacity above what we are accustom to seeing with 240V 1phase dwelling service. I have taken all this into account with my calculations. The project is a nine story 118 unit condominium building. Of the 118 units 62 units calculate below 150A, 24 are below 175A and 32 are below 200A. As this project has budget issues I have recommended 62 units at 150A and the remaining 56 units at 200A. The largest unit I have recommended for 150A service is 973 sq feet. These are small units and given my experience the actual load on these services will be much less. These are one bead room one bathroom high rise condos without the ability to add square footage or utility building common with single family dwellings. The meter banks for this project are all located in the basement or first floor so the savings is fairly significant if we can avoid dragging 200A feeders all the way up the building. My calculations are as follows:
Service Load Calc A1
Number of Units 15
Total Sq Foot 728
Total Sq Ft x 3va 2184
Small Appliance 3000
Laundry Load 1500
Total 6684
Demand Adj 3000
Next 117 kVa @35% 1289.4
Adj Demand 4289.4
Dryer 5kva min 5000
Cooking 8kw min 8000
AC/Heat 5130
Fixed Appl
Water Heater 6000
Dishwasher 1100
Disposal 1700
Exh Fan 250
Microwave 1000
Other
10050
75% Demand 4 or more 7537.5
Largest Moter 25% 500
Total VA 30456.9
Amps at 208 1ph 146.4274
Comments Please.
Thanks
All
Thanks for the great response. To clarify, yes this is an all electric application and furthermore the services are 208V 1phase which has pushed the ampacity above what we are accustom to seeing with 240V 1phase dwelling service. I have taken all this into account with my calculations. The project is a nine story 118 unit condominium building. Of the 118 units 62 units calculate below 150A, 24 are below 175A and 32 are below 200A. As this project has budget issues I have recommended 62 units at 150A and the remaining 56 units at 200A. The largest unit I have recommended for 150A service is 973 sq feet. These are small units and given my experience the actual load on these services will be much less. These are one bead room one bathroom high rise condos without the ability to add square footage or utility building common with single family dwellings. The meter banks for this project are all located in the basement or first floor so the savings is fairly significant if we can avoid dragging 200A feeders all the way up the building. My calculations are as follows:
Service Load Calc A1
Number of Units 15
Total Sq Foot 728
Total Sq Ft x 3va 2184
Small Appliance 3000
Laundry Load 1500
Total 6684
Demand Adj 3000
Next 117 kVa @35% 1289.4
Adj Demand 4289.4
Dryer 5kva min 5000
Cooking 8kw min 8000
AC/Heat 5130
Fixed Appl
Water Heater 6000
Dishwasher 1100
Disposal 1700
Exh Fan 250
Microwave 1000
Other
10050
75% Demand 4 or more 7537.5
Largest Moter 25% 500
Total VA 30456.9
Amps at 208 1ph 146.4274
Comments Please.
Thanks
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Dwelling Unit Service Calculations
In every calculation I have done, the optional method came up with a smaller result.
In every calculation I have done, the optional method came up with a smaller result.
growler
Senior Member
- Location
- Atlanta,GA
Re: Dwelling Unit Service Calculations
Newkirk, I have one more question. Are you sure of the size on the electric heat. That seems a little low. The last electric furnace I installed in a small apartment was about 40 Amps and with a heat pump you have to figure for the backup heat strips about 30 Amps. for a small apartment ( only the HVAC contractor knows for sure ). You are doing a lot of these so I wouldn't leave anything to chance.
Newkirk, I have one more question. Are you sure of the size on the electric heat. That seems a little low. The last electric furnace I installed in a small apartment was about 40 Amps and with a heat pump you have to figure for the backup heat strips about 30 Amps. for a small apartment ( only the HVAC contractor knows for sure ). You are doing a lot of these so I wouldn't leave anything to chance.
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