1. Originally Posted by suemarkp
Are you doing the Standard or Optional calculation? With two heat pumps, I'd go the optional
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?

2. 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.

3. Senior Member
Join Date
Sep 2004
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1,204
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.

4. 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.

5. I'm currently studying for an exam. Here's some notes I took on the difference between the standard and optional methods:

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

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.

6. Senior Member
Join Date
Sep 2004
Posts
1,204
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.