100 Amp vs 200 Amp

Hi Guys,

When do you recommend upgrading from a 100 amp circuit breaker panel to a 200 amp ?
I just inspected a 1970 home with 1930 sq ft, someone just put in a new panel, but only a 100 amp. Why not just put in a 200 amp ?
Any general guidelines for amperage vs sq ft or other rules of thumb ?


Darrell Freshour
Darrells Home Inspections:roll:

three watts per square foot plus other connected loads.

first 10kw at 100% and the rest at 40% plus heat and or AC

To expand more on mikes reference…

1.) They can use a Standard Method or Optional Method…Mike explains he optional method (220.82)which in most all cases is a less sized service than the standard method.

The good thing about electricity is diversification.......

Basically as an HI you need to understand that depending on whats in the dwelling will determine if the 100A service is large enough…if their is considerable gas appliances and so on 100A service may be fine.

If you see a 100A panel that has multiple double taps an just looks “SPACE CHALLENGED” inside then quite possibly it could stand a review…but again this is something an electrician is going to have to do.

On a side note…It quite possibly was a 100A straight change over from an older fuse 100A…happens alot because they dont have to increase the SE conductors size…and in many cases the 100A will serve he dwelling fine.

So many varibles can come into play on this…nothing can take the place of the actual calculations that a licensed electrician SHOULD be doing…

I would never recommend an upgrade unless you are a licensed electrician. I simply say that the amperage is 100 and that is considered the minimum necessary by today’s modern homes and advise them to talk to a licensed electrician about the limitations of this type of service.

100 amp may be the futures 60 amp but as long as it is common at this time there is no reason to stir the pot during inspection.

Could you be talked into moving North East a couple of hundred miles?

You sound like my kind of HI.

Just did a call where the HI stated that the service needed an upgrade to a 200 amp service on a house that had a 150 amp service and the range, water heater and heat was gas. The house was 1550 square feet and the only appliances installed were a dishwasher, microwave and a dryer.

My recommendation was to leave everything alone. The calculated load came to 82 amps to the best of my recollection.

If I am in doubt, I perform a rough load calculation myself (the Codecheck Electrical has a great example page that will walk you through how to do it). I do not share the results of the calculation with the client, but if my load calc is close to or over the service capacity of the home, then I recommend that they consult with an electrician to determine if their service has adequate capacity. In a home that has gas heat, gas range, and gas hot water, 100 ams is almost always OK unless the home is huge.

Mike, I am surprised that it was that high with gas heat, hot water, and range.

Paul may confirm this but many 100 amp services (at 240V= 24,000 Watts) never get above 50-60 amps draw. So why think the home needs a 200 amp service if it has not been added to or had some major electrical appliance additions, like an electric kiln for pottery. Here we have electrically heated homes of 1500-1600 sq ft meeting code at 100 amps.

Better to size the entrance properly than to guess that it needs more power. I find many electricians way oversize the entrance here especially if there is some electric heat in the home, automatically go to 200 amps!! Probably 125 or 150 amps would have been adequate.

This is a waste of materials that may never get used…no wonder copper is up 300%+ and oil is $88 per barrel…conservation is a word that many do not have in their vocabulary. On the Fine Home Building forum, they call the large poorly built homes with all the fixins on oversized lots …McMansions!!!

That sounds quite high!! What else eelctrical was in the house?

Yes, I have been in homes with a 320/400 service and had (2) ovens, (4) AC Units, (2) Sub-Zero fridges, and all lights on ( 125 Recess Lights ) and so on and everything you could think of plugged in and running…and only read about 185 A on the service…

Basically…it boils down to diversification of loads…and thats a wonderful thing about electricity…The NEC is always going to be a cautious standard…meaning things are always a bit oversized…take the difference between a standard calculation and an optional method calculation…can be a HUGE difference…and both quite legal.

To add another side to this discussion: some of the older (and newer) 100 amp panels don’t have a whole lot of room in them for all the necessary circuits required today (new and remodeled homes). AFCI’s take up a lot of space, lots of dedicated circuits etc. Sometimes having the bigger panel (given that costs aren’t that different for the panels themselves) and all that extra room is worth the larger service regardless of the calculated load of the home.

1550 x 3 = 4650
2 small appliance and laundry = 4500
Dryer = 5000
Microwave and dishwasher (estimate) 2400

Total of all the above = 16550

First 10000 at 100% = 10000

6550 at 40% = 2620

Total = 12620

Air conditioner at 7200

Total = 19820

19820 divided by 240 volts = 82.85

This 82.85 amp load is not what the service will draw all the time it is only the load that was calculated to size the service with.


Electrically speaking...nothing in the NEC says you have to size in regards for the future...in fact their is a myth that you must leave 10% of OCPD space in a panel for future installation...when in fact you can fill a panel to 100%.

It is important to take the dwelling at what it is at the time of inspection, if the house is finished and the panel was updated to the 100A and it runs all existing appliances and so on.......you can't force to future needs....as the buyer is buying what the see....not what they want....to see

I agree with you in that you need to make the statement that the panel may not be fine if the client plans on making changes and additions to the dwelling in the future...but as of right now unless you do a calculaion and qualified to do so.....the service may be fine even at 100A.....remember 100A is the minimum size required.

As for AFCI and so on......they make AFCI Classified listed breakers that will fit most any brand panel now ( excluding FPE and Zinsco...)...so if their is a breaker for a bedroom in the panel now......making it an AFCI breaker is not a big problem for a client if they wish to update that for their safety...

I agree with your premise...and I understand ( remember I am a Home Inspector also ).....but the 100A panel may be fine...heck I see plenty of 200A panels that leave no room for expanding....in regards to breakers but it does not need a 400A service upgrade because of it......so it is very important to understand diversification in electrical services.

Is this required by codes or insurance companies?

I did a very small house (550-600 sq ft over a crawl space, no laundry) inspection for a retiring single lady a few years back which had a 10-15 years old 240v, 60 amp entrance. Did the standard calcs by the local codes and it was OK at 60 amps but I warned the lady that the insurance company would want to see a 100 amp service. 3-4 months later, got a call from her saying she finally bought the house but it had been upgraded to 100 amps by the vendor even though the local electrical inspector said it was still alright. The insurance companies have too much power!!!

Exactly did a prestine 750 sq.ft. home Gas stove gas fire place gas dryer Gas water heater.60 amp sq D home was perfect. But The insurance siad it must be a 100 amp service .

. Cookie

I would like some help on this one.

100 Amp service in a 1880 / 2300 square foot home with an original carriage shed/barn. Barn had about 5 lights and some outlets.

The house had a 220 line in the basement for a dryer, electric hot water, electric range, new kitchen with trash compactor, disposal, exhaust vents ETC. what made me suspicious was the entry cable for the 100 amp was 4 gauge and there was a 2 gauge cable running up the chimney case to the third floor. Up there was the sub panel you see in the picture which had another 220 line for a second dryer, ( the one in the basement is not being used ), electric heat for the third floor and all the outlets and lights.

Your thought are appreciated, Thanks.

Might not be much help as I’m going by our Canadian code: The #4 must have been from an older 70 amp entrance and the panel was upgraded/replaced.

Depending on the amount of electric heat, 100 amps might be alright but it appears you don’t have that due to the entry cable size.

Brian, that was my thought as I suspect the original service was 60 amp fuses.
There is a fuse box, 15 amp that still controls the emergency shut off for the boiler.

The seller also claimed that one of the largest electrical contractors in the city owned the house before him and did all of the electrical upgrades, so in his mind everything is OK.

I then said if that is true why aren’t there any GFCIs next to the bathroom sinks… Room went silent!!!

Ahh. That’s why it is 82 amps. I figured low 50’s without the AC.