Residential Service Calculation

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kda3310

kda3310

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Dry Prong, Louisiana
:? Where do I start to learn how to calculate how large of a meter can and panel I need for a residential service. I wired homes but would like to know how to calculate the service and panel sizes.
 
OK, thanks. In table 220.12 I will be looking for "Dwelling Unit" for a residential home, right? So if I have a 3,000 square foot home my general lighting load should be x 3 volt-amperes per square foot. (3000 x 3 = 9KVA) Am I on the right track so far?
 
infinity said:
If you have Microsoft Excel you can scroll down and download this free calculator to check your answe:
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That helps a lot. Thanks. However I need a little more help. I am doing this for a friend and he is trying to save as much money as he can. His feed will come up to his house under ground. A 50 foot run. He is responsible for the cost for that run so he wants to use aluminium. We have calculated he needs a 350 amp service. The residential calculator says we will need a 500 kcm aluminium feeder. How do I brake that down to run it parallel.
 
kda3310 said:
That helps a lot. Thanks. However I need a little more help. I am doing this for a friend and he is trying to save as much money as he can. His feed will come up to his house under ground. A 50 foot run. He is responsible for the cost for that run so he wants to use aluminium. We have calculated he needs a 350 amp service. The residential calculator says we will need a 500 kcm aluminium feeder. How do I brake that down to run it parallel.
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Divide the 350 amps by two or three sets of conductors then find the appropriate conductor size.
 
kda3310 said:
That helps a lot. Thanks. However I need a little more help. I am doing this for a friend and he is trying to save as much money as he can. His feed will come up to his house under ground. A 50 foot run. He is responsible for the cost for that run so he wants to use aluminium. We have calculated he needs a 350 amp service. The residential calculator says we will need a 500 kcm aluminium feeder. How do I brake that down to run it parallel.
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You hinted that this house may be about 3000 Sq Ft. Even without details, experience tells me that 350 amps is probably about twice the size it should be.
 
texie said:
You hinted that this house may be about 3000 Sq Ft. Even without details, experience tells me that 350 amps is probably about twice the size it should be.
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The thing that is killing me is the two heat pumps. 2400 sq feet down stairs using a 4 ton heat pump with 2 x 7.5 kva heat strips and 600 sq feet up stairs using a 1 ton heat pump with 2 x 7.5 kva heat strips. He has not bought the heat pump yet but from the best I can tell all the heat pumps I have looked up on line have a 15 kw back up heater in them. Does that sound right to y'all. He will have a 6.8 kw double oven and a 7.8 kw cook top. Other than that everything else is basic.
 
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Are you doing the Standard or Optional calculation? With two heat pumps, I'd go the optional (I do the Optional anyway with residential as it almost always seems to give a lower value than the standard and its still way higher than how the power company will size their wires). The are factors that can be applied to the heating elements (220.82(C)(3)) so they don't go in at 100% nor at the Continuous factor of 125%. The compressors go in at 100% of nameplate, and I'm assuming they mean MCA in this case. I'm not sure what to use for the resistance heat -- the nameplate KW or the nameplate MCA value before you get to apply the 65% factor.

15KVA of resistance heat in the 1 ton heat pump sounds way off -- it can't move enough air to keep that unit from overheating. 15 KVA in a 4 ton makes perfect sense. As a rule of thumb, the resistance heat of a heat pump should match its BTU output or be a bit higher. 4 ton = 48000 BTU. 48000 BTU / 3.418 = 14KW. 1 ton = 12000 BTU = 3.5KW strip heat.

Off the top of my head, I'd think this would almost fit in a 200A service. May need to go larger, but 350A sounds too large.
 
suemarkp said:
Are you doing the Standard or Optional calculation? With two heat pumps, I'd go the optional
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This is my first time doing a service calculation. I was doing standard because I did not know there was an optional. Recalculating under optional using the figures you gave me for the heat pump puts me with in range of a 200 amp service. Thanks.

Why is there two ways to calculate a dwelling unit and why would I use standard over optional?
 
Does any one know why there is two ways to calculate residential. I am trying to find out why I would use the standard calculation over the optional calculation. It seems that optional works out better and cheaper for me so for what reason would anyone use the standard calculation.
 
I don't know why. All I can do is speculate. Notice the huge difference in wire sizes between what the power company runs to a house and what the NEC says you have to run to a house. I think power company experience shows that true demand low is much less than the NEC calculates. The Optional almost always gives me a lower number, especially when you have a bunch of stuff like shop tools, welders, swimming pools, etc. Even electric heat since there are some major demand factors in the Optional which are not in the standard calc.

So the reduced demand of the Optional gets a more realistic number, but still not as small as what the power company thinks we need.
 
That's what I'm seeing too. I was kinda wondering if anyone out there has used the standard calculation and why they may have used it. From the response it looks as if no one on this site has. Thanks anyways just want to know didn't need to know.
 
I'm currently studying for an exam. Here's some notes I took on the difference between the standard and optional methods:

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(If anyone sees any flaws, let me know. :p)

IMO, the optional method is easier to calculate, so that's why I would guess most people use it.

As far as the difference in values between the two? They shouldn't be so different that you can reduce your service size... unless your loads fall in the 'sweet spot' between service sizes.
 
Sure it does, just look at the equations. For the Optional, everything except heat is reduced to 40% of nameplate (once you're over 10KW). For the standard, you get a 35% factor only on the square footage VA and the small appliance circuits, a 75% factor on on fixed appliances if you reach 4 or more, there's a factor for ranges but it is larger than 40%, no demand factor on the clothes dryer or heating. Heating goes in with various demand factors for Optional.

The further past 15 KVA you go, the more the Optional calculation helps. The two calcs are probably the same if you have a 60A Service. For something near 100A, the Optional will most likely be a bit lower. When you get calculations coming to 150, 200, or more amps, you'll probably get a significant savings using the Optional (at least 1 size in service if not more). My house calculated out to 399A with Standard and 315A with Optional. I had no growth left with the Standard and crossing the 400A threshold gets REAL expensive.

The difference would have been greater if I just had more stuff and not 2 kitchens. The Standard calculation gives a nice discount for 2 ranges and all the small appliance circuits in the second laundry and kitchen. But you get hammered with the 2 dryers. I'm considering adding a second water heater which would have pushed me over with the Standard.
 
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