House/Panel Load Calculation

[romoman]

I built a 24x36 detached garage that is about 50 feet from my house. Unfortunately for power, it's also about 300' from the transformer that feeds my house and about the same from the other one up the road. The electric company said to have a feed to the garage I'd have to put in a pad and transformer (we have underground utilities) and that would run around $25k. I have 200 amp service to the house and would like to branch off that, but I was told I need to do a load calc to see if I have reserve to do that. I've not been real successful in finding how to do it through Google and was wondering if someone can direct me to a resource so I can do that?

 

[theoldwizard1]

You are making a mountain out of a mole hill !

Are you going to have a wood working shop or a metal shop or both in the garage ? General "rule of thumb", even with both of the above as long as there is only one person working there ate a time (one major tool running), 240V @ 60A is plenty. You should still have enough power for electric heat or A/C.

Run it from your house !

 

[Stuart in MN]

Here's an article from a respected source on how to do a residential load calculation: https://www.ecmag.com/section/codes...lations-estimating-elements-electrical-system As mentioned, chances are you can just run a branch circuit from the house and be okay but it's always a good idea to check the numbers first.

 

[Sokoloff]

Unless you're going to be smelting aluminum in the garage, you have enough power in a 200A supply to the house to pull a feeder to the garage.

 

[dcg9381]

25k? Your PoCo is smoking.
I agree with the others - for a 800 sqft garage, 60A is more than enough power. Run your 60A branch off your 200A main and you're doing to be fine.

If they need a load calculation, there are lots of places to do this. Huge differences in them - don't use the ones put together by the generator companies..

You could also "clamp" your 200A main with a tool that reads peak amps. Dunno how big your house is, but 60A is about the same as adding a hot tub to a residence and they probably wouldn't even blink if you said "hot tub".

I've got 90A to a 2400 sqft space, 2 x 2 ton heaters, hot tub, refrigerator, full kitchen, lots of lights... RV electrical hookups. You'll be fine.

 

[gsmith22]

just posted this in another thread on here yesterday. Its the most detailed NEC compliant spreadsheet check I have found:

https://www.engineersedge.com/excel...ential_electrical_load_requirements_14672.htm

You may need to read through some of the NEC sections to understand what the input should be but the code sections are listed in each heading.

NEC can be viewed for free at nfpa.org

 

[romoman]

Thanks for the input everyone. You're probably right about how much power I need out there. I was apprensive b/c I would like to heat and cool with a minisplit, but it should be fine. The inspector is by the book, so he is the one that suggested the load calc.

$25k for the tie in is because we have underground utiliities. They would have to put in a pad, transformer, cut into the line, and then run to my garage. Off the house makes the most sense and then no second meter fees.

 

[romoman]

just posted this in another thread on here yesterday. Its the most detailed NEC compliant spreadsheet check I have found:

https://www.engineersedge.com/excel...ential_electrical_load_requirements_14672.htm

You may need to read through some of the NEC sections to understand what the input should be but the code sections are listed in each heading.

NEC can be viewed for free at nfpa.org

That's for paid members only, but maybe it's worth joining (if possible).

 

[Aceman]

https://www.mikeholt.com/technical-calculations-formulas.php

 

[romoman]

https://www.mikeholt.com/technical-calculations-formulas.php

Thanks! Looking at that quickly, even when I add a few AC units to the house, I'll be around 65 amps, so I have plenty for the garage.

 

[gsmith22]

sorry about that link. the mike holt one was the one I was thinking of (thanks Aceman).
Generally speaking, if you have gas applicances (water heater, range, oven, dryer, furnace), there is no way that 200Amps won't be plenty of power to do what you want. All electric house is a totally different story. Could easily have 150amps spoken for on an all electric house. the load calc spreadsheet will help to figure out where you stand

 

[romoman]

sorry about that link. the mike holt one was the one I was thinking of (thanks Aceman).
Generally speaking, if you have gas applicances (water heater, range, oven, dryer, furnace), there is no way that 200Amps won't be plenty of power to do what you want. All electric house is a totally different story. Could easily have 150amps spoken for on an all electric house. the load calc spreadsheet will help to figure out where you stand

No worries on the link - I appreciate everyone's help!

We have an electric dryer and stove, but with those on the spreadsheet, it looks like I only use about 47 amps most of the time. If I add central A/C, it goes to about 60 amps, so still plenty for the garage to be added and stay below 50%.

 

[dcg9381]

The big one to watch out for is the "instant on" electric hot water heater - they can draw substantial power, over 100 amps for "whole house" systems.

Sounds like you're set, Romoman.

 

[sberry]

If you are missing any one of the big 3 100A is fine. Dryer, stove and water heater are the 3. I know places have a ton of stuff on 100. 200 is easy, very difficult for a common home to load it up, chance are if you are raising g he'll in the garage not applying so much load to the rest of the house. Even most air compression cycles are 2 or 3 minutes tops.

 

[romoman]

We have an electric range and dryer, but the water heater is off the boiler (SuperStor), so I should be good. There is a good chance if I'm in the garage the wife and kids could be in the house using plenty of power, but I should be good.

 

[PNWguy]

Awesome link to the spreadsheet - thanks for sharing!

BTW, what is a "small appliance" at the top? All the big kitchen stuff is listed below. Is it a reference to toasters, blenders and such? I have the usual number of those items, but don't use them all at once.

 

[westy33]

There are a number of Smart Phone apps that work good for calculations. Most are free with Android or a small fee with Iphone. I use "Electrical Calculations" by Ettore Gallina on my Droid.

 

[wyliesdiesels]

If you are missing any one of the big 3 100A is fine. Dryer, stove and water heater are the 3. I know places have a ton of stuff on 100. 200 is easy, very difficult for a common home to load it up, chance are if you are raising g he'll in the garage not applying so much load to the rest of the house. Even most air compression cycles are 2 or 3 minutes tops.

I dont know that a dryer would be considered a large load. Theyre typically around 20a and the elements cycle on and off throughout the dry cycle. A water heater would be a large load though and a range can **** some serious juice especially with all the burners and oven on.

 

[Dan O.]

Hi I'm new here and reviving an old thread because my questions are closely related to the subject matter already covered here.

Load calculations: I'm a bit concerned with the number of discrepancies in methods depending on the source; it's almost as if there is no single, universally accepted "proper" way to do this and that can't be right.

To be clear, here is a list of issues I've run into:

1) For a simple load calculation (main panel only, no subs) some methods claim that outdoor fixtures/receptacles are included in the basic load calculation (square footage of living space X 3 watts) while others call for fixtures/receptacles in outdoor or detached locations to be accounted for in a separate calculation; what's the correct approach here?

2) The addition of one or more sub-panels seems to complicate matters especially if one doesn't have a definitive answer to the question above. The idea that the basic lighting/receptacle calculation (for the main) accounts for outdoor/detached kinda stops making any kind of sense once you need to do a load calculation for a sub-panel to power a detached garage. In this case, don't you simply account for each light fixture, outlet and fixed appliance and/or motor load?

3) Safety adjustment for the calculation of sub-panel loads: I've seen claims that NEC code (no code # cited) calls for the gross load to be multiplied by 1.25 when dealing with a sub-panel, I've also seen no mention at all of this seemingly important factor in other sub-panel load calculators. This bothers me most of all because saying an arbitrary factor of 1.25 should account for voltage drop in every sub-panel scenario (whether it be 24 inches or 24 feet from the main) seems fishy; is this adjustment legit or not?

4) Finally, can my total maximum load rating be anything short of my service amperage rating or should it be limited to 80% of that rating (as in an 80 amp limitation for 100 amp service)? I've seen load calculators that mention this 80% limitation and others that do not. Also, how might this then apply to sub-panel loads? All sources I've found say a sub-panel feeder breaker need only be sufficient for the calculated load (28 amps requirement = 30 amp feeder breaker), no mention of any 80% limitation for subs.

Sorry for the long post but where load calculations are concerned I'm just thoroughly confused at this point. If anybody can shed some light here and perhaps even cite some NEC code sections it would be much appreciated.

 

[Jim greengo]

What's the square footage of the house?

 

[Dan O.]

As it happens, I am attempting to do a load calc for a 1,200 sq ft house w/100 amp service and an existing 60 amp sub-panel serving the kitchen, bath and a few other dedicated circuits. The main panel is in a location and is of a design that won't easily permit the addition of circuits I'll need to adequately serve the garage; so I'd like to add a 30 amp sub-panel for that purpose.

My list of questions has less to do with my specific situation and more with my confusion over why almost no two load calculation methods seem to be exactly alike. I guess I'm looking for somebody to go through my list and tell me what's what and perhaps direct me towards a reliable source of info.

 

[Dan O.]

Nobody? Can't say I blame you.

At this point I'm thinking about just contacting my city's building/planning dept. to see if they have an established method for load calculations as part of the permitting process, they must, right?

I wish I could post some links so I could better illustrate the kind of descrepancies I'm talking about (can't include links until my 5th post). I did manage to find a couple other threads on the forums here that help out somewhat.

I'm probably making this way more complicated than it needs to be but I really don't need to go into the permit office with a stack of plans only to have them pick them apart 100 different ways; would rather get things (mostly) right on the first try.

 

[wyliesdiesels]

Mile holt has a good load calc sheet on his site

 

[AntonLargiader]

1) For a simple load calculation (main panel only, no subs) some methods claim that outdoor fixtures/receptacles are included in the basic load calculation (square footage of living space X 3 watts) while others call for fixtures/receptacles in outdoor or detached locations to be accounted for in a separate calculation; what's the correct approach here?

2) The addition of one or more sub-panels seems to complicate matters especially if one doesn't have a definitive answer to the question above. ...

Bummer you haven't gotten answers; these are good questions. But there is a lot of info buried in past threads about load calcs on this site.

Until you get better answers: 1) when I did my load calc for a project I was doing I didn't do anything special about the outdoor receps and no one mentioned anything. I think they are just rolled into the square footage load. And 2) there is derating applied to the whole-house feeder (based on the assumption of mixed load) that does not apply to any subpanels because there is no equivalent assumption of mixed load for them. So the sub feeders may have to be heavier, but that doesn't mean the main feeder has to be heavier because the main doesn't care how the load is distributed (subs or not) downstream.

 

[mike93lx]

My last house had an electric stove, electric dryer a fridge and a freezer. Heat was oil. Plus three sump pumps and a/c's in the summer

The house spent the first 50 years of its life on 60a service, never an issue. I went all the way up to 100a when I replaced the panel.

People consistently overestimate the amount of power needed.

 

[nadogail]

IMHO, you should treat it like you were adding a cooktop, for which you can forget about a load calc.

 

[Dan O.]

Thanks guys for the responses. I'm finding delayed responses are far more helpful than those that try to answer questions I never asked, which is what I'm getting on another forum.

@ wyliediesels:

I'm familiar with the Mike Holt calculator, this seems to be the "gold standard" that I see replicated to one extent or another most frequently, thanks for the suggestion.

@ mkie93lx:

The house I grew up in had 60 amp service the entire time I lived there. It also had 4 circuits for the entire house and garage. We didn't have sh*t in the way of appliances; my mom was afraid of microwaves, actually preferred the laundromat vs having the machines and the closest we got to a/c was a swamp cooler (was probably pulling just as many amps). Cracks me up to what you were running on 60 amps, how much of that stuff were you able to run at once? I'm sure if one conciously staggers loads/has enough circuits then living w/60amp service is not as bad as we made it.

I'm gonna distill my list of questions to this:

The 1,200 sq ft house w/100 amp service as mentioned currently has a 60 amp sub feeding the majority of house circuits (small appl, laundry and misc dedicated). The main panel houses only 3 circuits (house lighting, bedroom/living room receptacles and one dedicated window a/c receptacle). I would like to:

1. Confirm that the house subpanel is configured correctly.

2. Figure out how to configure a subpanel for the garage that would support 2 20 amp receptical circuits, 1 for interior/ext lighting and possibly one dedicated circuit for window a/c or other similar appliance.

3. With regard to existing and proposed subpanel loads, confirm that existing main panel circuits can remain as is.

I can use the Mike Holt calculator (or similar) to determine whether or not my total calculated load will be fine with my 100 amp service; if everything fit in one panel I'd be done. I need help with figuring out the subpanel loads and circuit distribution; examples would go a long way.

 

[mike93lx]

No staggering anything, we just lived in the house without a thought, ever, of what was running at once.

I guess if we were running a load of laundry on high in the dryer, while baking a cake and cooking something, while the fridges kicked on and I started the table saw, it might have been a problem. But it never was

 

[Dan O.]

With the appliances you mention I can only imagine that at least some of them must've been on dedicated circuits. At our house it was like spinning a roulette wheel w/only four slots. I remember as a young teenager, trying to run a smallish air compressor that would flip breakers no matter where I plugged it in.

 

[mike93lx]

With the appliances you mention I can only imagine that at least some of them must've been on dedicated circuits. At our house it was like spinning a roulette wheel w/only four slots. I remember as a young teenager, trying to run a smallish air compressor that would flip breakers no matter where I plugged it in.

Sure, but that isn't the point. I was highlighting the overall service to the house is a lot smaller than most would think was needed.

Before I upgraded, I think it was a 6 circuit fuse panel, with a 4 circuit fused sub and a 2 circuit circuit breaker sub. I moved it all to a 24 space panel and filled it switching key stuff to dedicated circuits and getting rid of all the doubled up stuff

 

[Dan O.]

Sure, but that isn't the point. I was highlighting the overall service to the house is a lot smaller than most would think was needed.

Before I upgraded, I think it was a 6 circuit fuse panel, with a 4 circuit fused sub and a 2 circuit circuit breaker sub. I moved it all to a 24 space panel and filled it switching key stuff to dedicated circuits and getting rid of all the doubled up stuff

Believe me, your point wasn't missed. Without having gone through the calculations I'm fairly certain 100 amps is going to be more than adequate for what I'd like to add. I really wish I could get some answers to those questions in my original post from back on 6/5, just to satisfy my curiousity if nothing else.

 

[mike93lx]

Believe me, your point wasn't missed. Without having gone through the calculations I'm fairly certain 100 amps is going to be more than adequate for what I'd like to add. I really wish I could get some answers to those questions in my original post from back on 6/5, just to satisfy my curiousity if nothing else.

Sorry, can't help with them

 

[ericm]

I'm trying to do a load calc for my situation to see if my service is enough. I currently have a 2400 sq ft house, and a 2400 sq ft barn without power. I'm planning on powering the barn and building a 1600 sq ft shop.

I'm using the Mike Holt spreadsheet. I think I figured out the house inputs. Lighting goes by sq ft so that's easy. The barn's only getting lighting and one 20a circuit for outlets, so that's easy to add. It's the shop equipment where I'm lost. I can account for compressor and lift motors in the pool section under intermittent motors and general 120v outlets as small appliance circuits but where do I account for a welder?

Also, a dumb question: I have a main panel next to the meter with a 25a for the well pump and a 125a for the house. I plan to add breakers for the barn and shop. Can those breakers add up to more than the service (which I think is 200a)? If so, what protects the service from being overloaded?

Also if anyone knows how PG&E wants this calculated I'd love to hear it. I found the green book but can't find the right part for load calculations.

 

[mike93lx]

I'm trying to do a load calc for my situation to see if my service is enough. I currently have a 2400 sq ft house, and a 2400 sq ft barn without power. I'm planning on powering the barn and building a 1600 sq ft shop.

I'm using the Mike Holt spreadsheet. I think I figured out the house inputs. Lighting goes by sq ft so that's easy. The barn's only getting lighting and one 20a circuit for outlets, so that's easy to add. It's the shop equipment where I'm lost. I can account for compressor and lift motors in the pool section under intermittent motors and general 120v outlets as small appliance circuits but where do I account for a welder?

Also, a dumb question: I have a main panel next to the meter with a 25a for the well pump and a 125a for the house. I plan to add breakers for the barn and shop. Can those breakers add up to more than the service (which I think is 200a)? If so, what protects the service from being overloaded?

Also if anyone knows how PG&E wants this calculated I'd love to hear it. I found the green book but can't find the right part for load calculations.

You will probably do better if you start your own thread.

 

[Dan O.]

I'm trying to do a load calc for my situation to see if my service is enough...

Here's hoping that not bringing a mess of bizarre questions to the party (like I did) will help get you some good answers. If you do start your own thread I look forward to any help you get. Sorry that I'm in no position to offer any advice but I would like to pick your brain on a couple points, might help us both out:

Regarding the outlets in your barn/shop space, am I reading correctly in that you accounted for them in the form of additional small appliance branch circuit(s)? Is that how you're supposed to do it (honestly don't know) or is this just a matter of making a square peg work with round hole that is the form? At least one source I referenced indicates outdoor/detached fixture/receptacle loads get itemized; I'll say no more on that source because it'll only make you as confused as I am.

On your main panel configuration is that 25a breaker serving a dedicated circuit for the pump and the 125a your main breaker for the house or is the 125a breaker serving a subpanel for the house? So currently there is no 200a main breaker in this panel?

 

[mike93lx]

You can use a lumber calculator to make such calculations with relative ease

How in the world does a lumber calculator help with sizing an electrical panel? Are you lost?

 

[PCustoms]

How in the world does a lumber calculator help with sizing an electrical panel? Are you lost?

I reported the spammer

 

[dcg9381]

I can't cite NEC code around load calculations. What I can say here, is Keep It Simple. You can get a ballpark load calculation from generac (used for sizing generators). I wouldn't do *anything* special with sub-panel loads, I'd treat those loads like they are the same as being on the main (yes, there are some minor losses).

100A is a lot of power for a 1-2 man shop that isn't commercial. I'm omitting shops that convert to 3-phase internally, or things like that. Watch your heating/AC loads, calculate the big items (water heaters, electric ovens, electric stoves, etc) - I think you'll be surprised with where you end up at.

 

[wyliesdiesels]

How in the world does a lumber calculator help with sizing an electrical panel? Are you lost?

I wouldnt respond to spammers. according to the forum rules, you could get booted.

 

[mike93lx]

I wouldnt respond to spammers. according to the forum rules, you could get booted.

Yeah, I looked at his post history and he didn't look like one, but eh, whatever.