# Resi-Contractors: How do you calculate loads?



## Frasbee (Apr 7, 2008)

Specifically for a one family dwelling?

I'm curious because I have a pretty simple form in front of me from school, but I would guess you guys have your own means to figure this stuff out.

Is there a basic formula you use?


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## davis9 (Mar 21, 2009)

Code book. After a while you can just "know" if it needs bigger. Anything over 3500 sq ft I'll put some quick numbers together to check if it's over 200A. If it's close to that then I'll dig in a bit.

Tom


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## FCR1988 (Jul 10, 2011)

I don't know if the Illustrated Guide to the NEC is on your books list but they have a pretty easy form to follow (maybe thats the one you have). I've never done one on a job, but (i'm assuming here) when I go out on my own I'll use the two they provide.


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## backstay (Feb 3, 2011)

....


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## McClary’s Electrical (Feb 21, 2009)

I run the whole house off a 15 amp circuit, then make them pay to fix it later...


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## Frasbee (Apr 7, 2008)

So everyone pretty much uses the same formula. Kinda nice there's a standard there.


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## amptech (Sep 21, 2007)

There are a few downloadable load calculators on the web. I used to do it manually using the NEC but now I just use a calculator program. There is a free one at MH, for instance. My REMC had one on their website along with an appliance calculator.


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## Frasbee (Apr 7, 2008)

Why the use of the term "volt-amps" instead of watts? I see that all over the place.


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## walkerj (May 13, 2007)

Frasbee said:


> Why the use of the term "volt-amps" instead of watts? I see that all over the place.


Volt amps = v*i

Watts = (v*i)*pf


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## Meadow (Jan 14, 2011)

Frasbee said:


> Why the use of the term "volt-amps" instead of watts? I see that all over the place.


Power factor. Anything thats not resistive usually has a powerfactor other than 1.


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## Dennis Alwon (May 9, 2009)

When I need a load calculation done I use MH's . Go here and scroll down to free stuff.


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## RGH (Sep 12, 2011)

*Rgh*

easy guildline 1000sq ft 100amp /2000 sq ft 150 amp/ 2500+ sq ft 200 amp this is the way it usualy boils down


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## RGH (Sep 12, 2011)

simple: 1000 sq ft 100 amp/2000 sq ft 150 amp/ 2500 sq ft 200 amp 99 times out of a 100 this is what I have done


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## ptcrtn (Mar 14, 2011)

I use this one
http://www.electricalknowledge.com/SFDLoadCalc.asp


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## SEREMan2000 (Aug 29, 2011)

i use the tom henry calculations book...he has a whole collection of informative books and guides to help out


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## Magnettica (Jan 23, 2007)

mcclary's electrical said:


> I run the whole house off a 15 amp circuit, then make them pay to fix it later...


Why didn't I think of that?!

Sent from my iPad using ET Forum


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## Chris1971 (Dec 27, 2010)

Dennis Alwon said:


> When I need a load calculation done I use MH's . Go here and scroll down to free stuff.



I used the free calculation a few weeks ago. Works well.:thumbsup:


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## Island Electric (May 9, 2011)

I will start you out with the first step.

*LIGHTING LOADS:*
Example 2 story home. a perfect square shape measures 62'-0" X 51'-0"

find square footage.= 3162 X 2 stories = 6324 sq. ft.
now go to 220-12 (NEC) find DWELLING UNIT = 3 VA per sq. ft.

6324x3=18,972 VA

*SMALL APPLIANCE: 210-11 (C) & 220-52 (A)*

1500VA FOR GIGGLES LETS USE 3 CIRCUITS INSTEAD OF 2.
1500 X 3= 4500VA

*LAUNDRY 220-52 (B)*

LETS USE 1 LAUNDRY CIRCUIT
1500VA X 1= 1500VA

NOW LETS ADD THEM UP.
18972 + 4500 + 1500 = TOTAL CONNECTED LOAD IS 24972 VA

TAKE A LOOK AT 220-42 DWELLING UNITS

THE FIRST 3000 VA IS COMPUTED AT 100%
3000VA BACK TO THIS IN A MINUTE

FROM 3001 TO 120000 YOU CAN USE A 35% DEMAND FACTOR.

LOOK HERE 24972 - 3000 = 21972

21972 X .35 = 7690

NOW TAKE THE 3000 + 7690 = 10690 VA FOR LIGHTING, SMALL APPLIANCE, AND LAUNDRY FOR A 6324 SQ. FT. HOME. THIS IS STEP #1

Does this make sense to you? :thumbsup:


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## Cletis (Aug 20, 2010)

*nope*



RGH said:


> easy guildline 1000sq ft 100amp /2000 sq ft 150 amp/ 2500+ sq ft 200 amp this is the way it usualy boils down


Wrong ! Bad Formula


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## RIVETER (Sep 26, 2009)

Frasbee said:


> Specifically for a one family dwelling?
> 
> I'm curious because I have a pretty simple form in front of me from school, but I would guess you guys have your own means to figure this stuff out.
> 
> Is there a basic formula you use?


In the back of the code book.


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## RGH (Sep 12, 2011)

Cletis said:


> Wrong ! Bad Formula


This is not my formula, but like a i said a guide line for resi work in typical sq ft va requirements. Of coarse all factors should be considered. In n.y. most homes are gas on gas so requirments are lower than other areas over my 30yrs I mostly install 150 amp panels 30/40 % free space. Commercial work of coarse, way more complicated. Allways ask the ho do your decetive work!!! and plus it up:thumbup:


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