Electrical Load Calculation

I don’t care about proper message board etiquette such as posting links to other threads. You can continue to pick up my slack though.

Thank you for that.

I found it interesting that Juan had to start another thread just to accept the opinion already offered in the other thread, without waiting for benefit of further input from others.

Will do

Dang. I guess I made another message board mistake. Thanks for keeping me honest Jeff.

There is a good talk about Load Calculations in the book talked about in this thread :slight_smile:


Normally, as Jeff has stated, it is done at the design phase before construction is begun. This is typical of both new construction and renovation projects.

Thanks Will.

Sometimes it’s good to start another thread just to get your question separated from all the useless and unrelated-to-the-topic posts.(Like my post in this thread for example) :smiley:

That is what I was thinking. Perhaps we just don’t know how to use the message board properly.

I have a load calculation form in excel, if you wan tit.
Just e mail me@ ev@magnuminspections.com

You do this on a home inspection?

I only did load calculations when in the back up generator industry. Why would a Home Inspector ever need to do a load calculation anyway, except for the obvious.

I inspected a 1500 sq ft home not long ago, where the old fuse box had obviously been replaced by a breaker box. It contained 5 total circuits. No calculation necessary…

That doesn’t sound like a design or rehab/reno event to me. Why the load calc when installing a generator back-up system?

to properly size the back up power needed. Some wanted full electrical capability, some only partial. Most telephone offices were only back up for the batteries that operated their equipment, and a few lights. Others wanted entire building, un-interrupted, and many homeowner customers were the same.

One thing was always consistent. The partial people ALWAYS called back wanting to add more load to the generator…

No, just offering the form.

Thanks for the info.

You’re quite welcome Jeff. Here’s some more totally useless info you would never use for a home inspection: Given a home that is 200A service, people would think they need a generator that is capable of producing 200 amps. AxV=W, so that is about 48KW. I have never, ever measured a home that was drawing more than about 80-90 amps, even when everything was running at once (The heat and air conditioner never run at the same time).

About a 20KW is all anyone would ever need, unless they have an enormous house with more than one service, pool heaters, etc.

I ran my whole house (without air conditioner) for 6 days straight on a 6KW portable gasoline generator.

That is correct Jeff, but to just add to it, this also applies to Commercial not just residential.

I think when I mentioned Load Calculations in the other thread maybe the term got confusing to some. It really is nothing a typical home inspector should or will really ever need to use especially if they are not fully qualified and aware of exactly what it is, means and when it is applicable.

Although I do think between this and the other original thread some have learned a little bit about it and as I always say “Knowledge is your best tool” so really this has been a good healthy discussion as a little more knowledge can never hurt.


I have been involved in the contracting business for many years and have done just about all there is to do in new construction.

It doesn’t matter if it is residential or commercial when I do the bid I also do a load calculation of the service in order to establish the size of the service disconnect and the service entrance conductors even if an engineer has done a calculation. I will also do this calculation when doing renovation work in order to give the customer what they are paying for.

When called to evaluate an electrical system by a homeowner or realtor I also do a load calculation to ensure that the service is up to par. It is very rare that I find a service that is too small for the building being served. In most cases the ones that are too small will be a fuse panel where the appliances were changed from gas to electric or an old oil furnace was changed out for an electric.

A 2000 square foot home with a three ton heat pump and a 10kw air handler will only need a 150 amp service unless it has more than the average home such as but not limited to pool, hot tub, work shop with heavy equipment such as large air compressors and the like. Use gas appliances such as heat, water heater, and cooking appliances and this home would only require a 100 amp service.

My home has 1980 square feet all electric with two heat pumps and also supplies two out buildings one 2400 square feet brood mare barn and the other a 960 square foot work shop for my farm equipment that has air compressor, welder, drill press, grinders and the like and the calculated load is just 227 amps.

I agree that it is good for everyone to know how to do a service calculation when doing a home inspection but I don’t think it is necessary in every case. What we need to realize is that when doing a load calculation we don’t add everything that a homeowner might plug into a receptacle we only use 3 watts per square foot. It doesn’t matter if the home owner is going to plug in a blow dryer, big screen TV or even portable electric heaters we still figure the load at 3 watts per square foot.

This will equate to an 1800 square foot home having 3- 15 amp circuits for the lights and receptacles 2- 20 amp for the kitchen and dining room 1- 20 amp for the laundry and 1- 20 amp for the bathrooms. This is code compliant no matter what they buy and plug into those receptacles.

As far as the other thread is concerned even if the service is too small there is not much can be done except to spend the monies to upgrade the service and bring the two wire circuits up to par with a complete rewire. The question is which one will pay for this high dollar fix, the buyer or the seller or will it remain as it is. It is obvious that the seller is not going to spring for the fix or it would have already been done so I suppose if the buyer wants it done it will be on their dime.

What we have to keep in mind is that if it was compliant at the time of installation it is compliant today no matter what we think people is going to plug into those receptacles be it two wire or not.

Thanks for that informative post Mike