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Residential load calculations and barns #216366 12/02/15 12:17 AM
Joined: Mar 2004
Posts: 776
BigB Offline OP
Doing a residential load calc on a one family dwelling with a horse barn but I do not see anything in 220.12 that requires me to allow for a lighting load on the barn. Does this mean I do not have to figure it in at all unless it has fastened in place or permanently connected utilization equipment? The barn is 900 sq feet, has a 125 amp sub panel and just lights and receptacles.

2017 / 2014 NEC & Related Books and Study Guides
Re: Residential load calculations and barns [Re: BigB] #216367 12/02/15 12:39 AM
Joined: Mar 2004
Posts: 776
BigB Offline OP
Also they are adding a 1300 square foot garage for diddling around in and collecting a few cars. It looks like all I have to include in the load calcs is the HVAC and the door openers? No lighting or do I just use the dwelling unit 3VA? Do I include a welder and compressor in the calcs if they are cord and plug connected? How can I know the loads for plug and cord connected equipment if I am only providing a 50 amp and a 30 amp receptacle?

I don't normally do load calcs but I am helping the architect out on this one.

Re: Residential load calculations and barns [Re: BigB] #216369 12/02/15 01:28 AM
Joined: Jul 2004
Posts: 9,558
gfretwell Offline
If you are just using the residential calcs, I doubt there is any requirement that you include the utility buildings if there is no fixed in place equipment. (HVAC etc) The general lighting load by square foot exempt them.
That is still not a good "design".
If you look at 220.103 there is some guidance for "farm loads" and if the garage is going to be a shop, you should try to determine what kind of loads are likely to be there.
The trick is trying to take into account "diversity".
It might be sufficient to just put in any HVAC load and add in the 3va per sq/ft to give them enough, assuming no extraordinary loads.

Greg Fretwell
Re: Residential load calculations and barns [Re: gfretwell] #216393 12/03/15 11:27 PM
Joined: Mar 2004
Posts: 776
BigB Offline OP
I went ahead and drew it up with 3va/sq ft for lighting and included the HVAC, welder and air compressor.Part of me didn't want to include the lighting and plug & cord equipment since I am not required to, to see if the building department would challenge it. But WTH we still got almost 200 amps to spare with all I included.

Re: Residential load calculations and barns [Re: BigB] #216396 12/04/15 11:29 AM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Barns, and other outbuildings, are not REQUIRED to be included in the load calculation.

Of course, we know better. We know that, if they can, they'll run power out there. Hello, 'man cave.' Hello, 'home business.' "Just a few receptacles?" Sure, we've heard that before!

All you can do is to treat the entire barn as an "appliance" and make your best guess for the feeder. If you're smart, you'll 'accidentally' run over-sized conduit and feeders.

I'd assume a 60-amp load, and feed the panel accordingly. By the time they plug in the space heaters, etc ... they'll use it.

Re: Residential load calculations and barns [Re: BigB] #216397 12/04/15 11:37 AM
Joined: May 2005
Posts: 984
ghost307 Offline
Reno's hit the nail squarely on the head.

Keep in mind that the Code is concerned with preventing shocks and fires; it's not a design guide.

Using the Code as such will not guarantee that there will be adequate power...only that there is a minimized risk of shock and electrical fires.

Re: Residential load calculations and barns [Re: BigB] #216399 12/04/15 02:01 PM
Joined: Jul 2004
Posts: 9,558
gfretwell Offline
The real trick is figuring out how to compute "diversity". How many of those tools will be used at the same time.
Perhaps the KVA rating should be based on the number of workers, not the nameplates of all of the "operator required" equipment.
It is really hard for a "one man band" to keep 40a of equipment running. My 230a stick welder (the biggest load I own) says it is 48a but I have been running it on a 30a dryer plug for 40 years and never tripped a breaker.

Greg Fretwell
Re: Residential load calculations and barns [Re: BigB] #216400 12/04/15 03:13 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
OK ... let's back up a bit ...

How did I 'guess' 60 amps? What does '60 amps' mean?

First off, many homes, even today, require less than 60 amps of 240V service. This is evidenced by the large number of 60-amp services existing.

If it's enough for the house, it ought to be plenty for the barn. Remember, that's 60 amps of 240 volts, or (if you prefer) 120 amps of 120 volt electricity.

Now, let's look at that barn ...

From experience, I can tell you that two consumer space heaters will be needed to heat a small, free-standing garage. Each of those heaters will "use up" a 15-amp circuit. In summer, the air conditioner will use less.
That's 30 amps.

Let's assume a microwave. Good-bye another 15-amp circuit.
30+15 = 45 amps.

Even two guys, using 'big' power tools, will be able to use the same convenience circuit. You can see this on every job site.
45+15 = 60

Just for giggles, let's throw in another 15 amps for a table saw (or welder). I use only 15 amps, as you can't work a table saw and a power tool at the same time.
60+15 = 75

Maybe toss in another 15 amps to cover the lights, the fridge, whatever.
75+15 = 90

We're still well within the capacity of a 60-amp feeder.

That "125-amp" panel is only a 125-amp panel if it's fed by a 125-amp breaker. More correctly, 125-amps is the largest breaker you can use. Feeding it with a 60-amp breaker is just fine.

As far as "diversity" is concerned ....

THAT matters only for the calculation of the house service. In that case, you fire everything up, measure what's actually used, then multiply by 80%. Now you have a figure for the service calculation.

Try to do it any other way, and you wind up counting things twice. The same situation arises with any other sub-panel. You don't "add up breakers" or "add all the loads." The sub-panel is "the load" ... it just doesn't have a nameplate.

If you're building new, using the 3-watts/square foot, etc., is a good way to guess. The more information you have, the better. The trouble is, the real world is not that precise.

"Services" come in 30-amp and 60-amp. There's a lot of room between those two. A 30-amp feeder would very likely not be enough, unless all you had was one receptacle and one light. A 60-amp feed, however, proves itself daily, in powering millions of homes.

I suppose you could consider ratings like 40 0r 50 amps, but why? All you save is -maybe- one wire size. Since I'm a real fan of over-size pipe, what's the point?

The moral is: Don't be overly precise. Unless you're Harry Potter, there is no train platform 7-3/4. laugh


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