ting Duplexes

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wmeek

Senior Member
Location
Texas
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Electrician
I have an existing duplex with 100A Mains in each, I want to rebuild service to 125A Mains with service entrance conductors of 250MCM per 310-16 to a gutter and then tap with #2 per 310-15. to two seperate meter cans for each unit. What I believe to be in each unit is your typical Disposal,Range,Dryer,Dishwasher,vent hood, dryer, water heater and central a/c at 25A and heat of 10KW. each unit is 1100 square feet each. When I do a load calculation on each unit I come up with 143A. If thats the case how did someone come up with a 100A for the original service. These units were built back in 1971. I do not know what the codes were then for calculating a Service. Does anyone have any answers to this. I was assuming that if it had been doing fine on 100A, 125A would be suffient . If any has any suggestion they would be greatly appreciated.
Thanks
 
Existing Duplexes

Existing Duplexes

I am not adding any loads to existing service. I am just upgrading the 100A Services to 125A services. I will be installing one mast to a 6"x6" gutter with #250 MCM PER 310-16 for total connected load of the two units, and #2 per Table 310-15 to each meter can and to each panel. I have done a load calculation per Article 220 for each unit and come up with 143A (L1&L2) and 65A ( Neutral )for each unit. My Question is the old service was a 100A Service to each unit, which falls short of the calculation I have come up with. I feel that I really need to upgrade to a 150A Service to each unit. Am I doing something wrong here. Again these units were originally built back in 1971. I was not an electrician back then but maybe the process for doing a calculation was majorly different. Any more ideas or suggestions.

Thank You
 
Yes I read 220-87 But i guess i am not understanding it. But I did do the Standard Method of 143A and Optional Method of 119.8A. But still, both are over the 100A originally installed. The way I read 220-87 it is saying to take readings on maximum demand of the current loads to determine if any new loads can be added. But I am not adding and was concerned that the original 100A was not sufficient.
 
220.87.1 (2005) says you can use existing actual demand data in lieu of calculations to determine your load (unless I am reading it wrong). Isn't this the situation you now have ? Isn't the existing load want you want to use to calculate your load so you can use a slightly larger service ?
 
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Thats exactly what I am trying to do. I was just trying to figure out how it was done in the original application, and how they came up with 100A Service originally. All wiring in Duplex is original as to when it was put in back in 1971.
It just seems strange that now you would have a 150A Service per Standard Method or a 125A for the optional method. But yet it has been working with no apparent problems.
 
wmeek said:
Thats exactly what I am trying to do. I was just trying to figure out how it was done in the original application, and how they came up with 100A Service originally. All wiring in Duplex is original as to when it was put in back in 1971.
It just seems strange that now you would have a 150A Service per Standard Method or a 125A for the optional method. But yet it has been working with no apparent problems.
Click to expand...
Why don't you put up your calculations and let us see what they look like.
 
Using a spread sheet I got from my local AHJ I come up with 100 amp service for each unit. If your heat load is larger that the a/c load you only count the larger of the two. If you want, PM me your e/mail and I will send the spread sheet to you.
 
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I am curious...does the existing main to these units Trip?
If not, why are you increasing the size of the service?

"What I believe to be in each unit is your typical Disposal,Range,Dryer,Dishwasher,vent hood, dryer, water heater and central a/c at 25A and heat of 10KW"


Seems like you are guessing at the loads???


With an existing 1100SF space, it would seem to me that you can get an "exact" load for the space.
 
I am not sure if they did or did not trip. As these units are vacant. The existing service are in bad shape. Owner wants to put in a new service. Right now you have a 2P50 which I assume to be for a 10KW heat for 1100Sq. Ft unit, 2P50 (range receptacle), 2P40 (Waterheater),2P40(A/C unit) 2P30(Dryer) 3-1P20,2-1P15. I did not go into units to actually see what is there. But if you do a load calculation assuming that there is 10KW Heat which is about right for the square footage, A/C unit( Which were not as effient as todays units,dishwasher,disposal,range hood,freestanding range, waterheater, dryer with no gas serving these units that a100A service would not be enough to handle these loads. But after looking and calculating I feel like I need to install 125A Service by way of Optional Calculation.
 
I am going with A 125A Service to each unit with a 250MCM (L1,L2) and 3/0 ( Neutral)per 310-16 for service entrance conductors to a 6x6 gutter , then #2 Thhn per 310-15 for meter cans and panels to each unit. I am under the impression that I cannot use 310-15 for the service entrance conductors. Is that correct. I would think that if you can use 310-15 for each unit you could also use it for the service entrance conductors.
 
wmeek, You only need to use the larger load of the Heating & Air Conditioning

in your load calc., as they can not run at the same time.
 
wmeek said:
Thats exactly what I am trying to do. I was just trying to figure out how it was done in the original application, and how they came up with 100A Service originally. All wiring in Duplex is original as to when it was put in back in 1971.
It just seems strange that now you would have a 150A Service per Standard Method or a 125A for the optional method. But yet it has been working with no apparent problems.
Click to expand...

I'm guessing the A/C system is not original. 25A was not a breaker size in '71. Probably did not have A/C or 10 wk heat in '71.
 
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