When do we need load calc for a residence?

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JoeNorm

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Are load calculations required for residences? Certainly not around my parts, but I am wondering if we're just not doing things right.

thanks
 
Required or recommended? New construction highly advisable even, in my opinion required; remodel, recalculation would be needed to 1. Determine capacity to add additional loads, 2. Other previously added loads may have exceeded capacity for adding more loads. Have seen many times where HO wants to make an addition or other new loads and even though panel has spaces available loads on the 100A panel already exceed panel capacity with 150A of load potential, "only" reason main not tripping loads have not been concurrently active, even so the potential under current "design" overload panel and any additional load would only serve to exasperate the overload potential to the point of actual overload.
I have just made it a practice, not sure of an NEC requirement, will look to see if it is in there, perhaps someone is aware of a code section making it a requirement.
 
I agree it to be a good practice. But I am wondering if it's ever required by L&I? Maybe inspectors are allowed to make judgment calls if they see red flags?
 
When my daughter had a 1900 sqft home built in 2013, the quote from the GC included a 320 amp service. Home has gas furnace, range and hot water. I questioned him on it and his reply was that all his homes get a 320.
So I did a load calc and she saved some money with a 200. The guy had never heard of a “load calc”. He just tells his EC to put in a 320 and that’s what happens. IDK if the utility cares or not?? I don’t recall the actual calculation result, but I suspect a 100 would have been close to covering it.

Bottom line - it’s probably a good idea to do one even if not required. And based on my experience, it’s never required here.
 
retirede said:
When my daughter had a 1900 sqft home built in 2013, the quote from the GC included a 320 amp service. Home has gas furnace, range and hot water. I questioned him on it and his reply was that all his homes get a 320.
So I did a load calc and she saved some money with a 200. The guy had never heard of a “load calc”. He just tells his EC to put in a 320 and that’s what happens. IDK if the utility cares or not?? I don’t recall the actual calculation result, but I suspect a 100 would have been close to covering it.

Bottom line - it’s probably a good idea to do one even if not required. And based on my experience, it’s never required here.
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The power company probably put a transformer in that was under sized for the actual calculated load anyway.
 
JoeNorm said:
I agree it to be a good practice. But I am wondering if it's ever required by L&I? Maybe inspectors are allowed to make judgment calls if they see red flags?
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I never shown an inspector a load calculation in my life. That said if I had a really obvious undersized service/feeder the topic likely to come up.
 
retirede said:
When my daughter had a 1900 sqft home built in 2013, the quote from the GC included a 320 amp service. Home has gas furnace, range and hot water. I questioned him on it and his reply was that all his homes get a 320.
So I did a load calc and she saved some money with a 200. The guy had never heard of a “load calc”. He just tells his EC to put in a 320 and that’s what happens. IDK if the utility cares or not?? I don’t recall the actual calculation result, but I suspect a 100 would have been close to covering it.

Bottom line - it’s probably a good idea to do one even if not required. And based on my experience, it’s never required here.
Click to expand...
Good save. That is kind of random though... a 320A service?? and for a 1900sqft house?? I guess I'll need to run down to Homedepot and pick some of them new SquareD 320A main service panels.
 
retirede said:
When my daughter had a 1900 sqft home built in 2013, the quote from the GC included a 320 amp service. Home has gas furnace, range and hot water. I questioned him on it and his reply was that all his homes get a 320.
So I did a load calc and she saved some money with a 200. The guy had never heard of a “load calc”. He just tells his EC to put in a 320 and that’s what happens. IDK if the utility cares or not?? I don’t recall the actual calculation result, but I suspect a 100 would have been close to covering it.

Bottom line - it’s probably a good idea to do one even if not required. And based on my experience, it’s never required here.
Click to expand...
Well it is common and even required in some places to install 200 amp minimum even though the demand may never be close. Even on limited load single circuit applications you may find some POCO's demand 200 amp supply to the metering equipment, where you otherwise could satisify NEC with 30 or 60 amp service conductors.
 
Some areas are requiring it when you add car chargers to the system otherwise I have not done load calcs for a residence.
 
Rock86 said:
Good save. That is kind of random though... a 320A service?? and for a 1900sqft house?? I guess I'll need to run down to Homedepot and pick some of them new SquareD 320A main service panels.
Click to expand...

Not random at all, just what a 400 amp residential meter base is rated for! Here on Dominion we call them 400 amp services, but that Durham meter base they hand you is rated for 320 amps.;) Still wire two 200 amp service panels to the meter base.

Up in SMECO territory (Southern Maryland Co-Op), a co-worker wanted a "400 amp service", and SMECO said, nope, 350 amp is maximum we supply. Use one 200 amp panel and a 150 amp panel! That was back about 2006.

ALL new residences on Dominion and the surrounding Co-Ops require a Load Letter. So while the inspector never sees it, you do not get power without one!
 
Frank DuVal said:
Not random at all, just what a 400 amp residential meter base is rated for! Here on Dominion we call them 400 amp services, but that Durham meter base they hand you is rated for 320 amps.;) Still wire two 200 amp service panels to the meter base.

Up in SMECO territory (Southern Maryland Co-Op), a co-worker wanted a "400 amp service", and SMECO said, nope, 350 amp is maximum we supply. Use one 200 amp panel and a 150 amp panel! That was back about 2006.

ALL new residences on Dominion and the surrounding Co-Ops require a Load Letter. So while the inspector never sees it, you do not get power without one!
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Oh no... I am well aware of how the meters work... but for the GC to say "Hey... Mr. Electrician... throw in a 320A service!"... 320 isn't a round number to me hahaha. 150, 200, 300, 400... those are numbers I like to work with.
 
320a is merely the continuous rating of a 400a residential service. They have full-400a-rated meter bases, too; typically bolt-in.
 
Rock86 said:
Good save. That is kind of random though... a 320A service?? and for a 1900sqft house?? I guess I'll need to run down to Homedepot and pick some of them new SquareD 320A main service panels.
Click to expand...
320A is the actual rating of the largest in-line socket you can get. And it is suitable for a 400A service if the main is not 100% rated. The socket is UL listed for 25% temporary overload capacity.
 
I always include my service load calculations on whatever residential permit drawings I am sealing. It proves to the owner that the service is not over-designed and the plan reviewer or inspector that it is not under-designed.

You can serve a pretty big house at 100A, even with air conditioning, if all the appliances are gas. But 200A is the new minimum single-family service size in the Real Estate Sales world, even if everything is gas that can be. It's a "mine is bigger than yours" status thing.
 
JoeStillman said:
I always include my service load calculations on whatever residential permit drawings I am sealing. It proves to the owner that the service is not over-designed and the plan reviewer or inspector that it is not under-designed.

You can serve a pretty big house at 100A, even with air conditioning, if all the appliances are gas. But 200A is the new minimum single-family service size in the Real Estate Sales world, even if everything is gas that can be. It's a "mine is bigger than yours" status thing.
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But even you kind of said it, 200 amps at a dwelling has become somewhat automatic even if 100 is sufficient for what is there for loads.

Reality is many dwellings that have load calculation over 100 amps still would never trip a 100 amp main. If you have electric heat as the primary heating system you probably need at least minimum of 125 or 150 amp main in many cases though.
 
retirede said:
When my daughter had a 1900 sqft home built in 2013, the quote from the GC included a 320 amp service. Home has gas furnace, range and hot water. I questioned him on it and his reply was that all his homes get a 320.
So I did a load calc and she saved some money with a 200. The guy had never heard of a “load calc”. He just tells his EC to put in a 320 and that’s what happens. IDK if the utility cares or not?? I don’t recall the actual calculation result, but I suspect a 100 would have been close to covering it.

Bottom line - it’s probably a good idea to do one even if not required. And based on my experience, it’s never required here.
Click to expand...
320 amps is stupidly oversized. I did load a calculation on a 2040 sq ft house recently and it was 108 mostly because it had two heating systems with AC (duct issues).
 
Strictly speaking, the NEC requires that all services and feeders be rated for the calculated load. So any AHJ could ask you for one, and how do you know it's compliant if you haven't done it? There's no exception for residential vs. commercial (although the optional calc methods for resi are more lenient).

Reality of enforcement is as many have described already in this thread. My two cents... Many if not most residential installations are way oversized per the NEC because a little extra in material is less than the opportunity cost of spending your time doing load calcs, let alone coming back because someone tripped their main breaker. I've done quite a few load calcs to justify downsizing main breakers for solar installs to comply with article 705. I've yet to encounter a single family home that exceeded 200A, and many are less than 100A.
 
jaggedben said:
Strictly speaking, the NEC requires that all services and feeders be rated for the calculated load. So any AHJ could ask you for one, and how do you know it's compliant if you haven't done it? There's no exception for residential vs. commercial (although the optional calc methods for resi are more lenient).

Reality of enforcement is as many have described already in this thread. My two cents... Many if not most residential installations are way oversized per the NEC because a little extra in material is less than the opportunity cost of spending your time doing load calcs, let alone coming back because someone tripped their main breaker. I've done quite a few load calcs to justify downsizing main breakers for solar installs to comply with article 705. I've yet to encounter a single family home that exceeded 200A, and many are less than 100A.
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You live in the land of high electric rates.

Come to places that can have as little as 5 cents per kWH during heating season and you run into electric heat being reasonable to use cost wise, especially a few years back when gas prices were much higher than now.

I did a new home 10-15 years ago that had 60 kW of permantly installed heating units, 5 4500 watt water heaters, two laundry rooms with electric dryers, two baths with in floor electric heat (don't recall watts on those anymore).

Take away all those loads and go with gas heating as much as possible, load calc may still have gone over 100 amps, but maybe not by much. Take away the AC units and I bet it is less than 100 amps.
 
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