Thread: 100 amp main, two wire system with three window AC units

Good Morning!

House built in 1944, one owner, no upgrades except breaker panel which had 100 amp breaker two wire system servicing the lights, refrig, hot water heater, furnace and stove in addition to three 120 window AC units on the lights circuit. The question is whether the three AC units running will overload the system and cause a breaker to trip out? They added the three prong receptacle without ground to be able to plug the AC units in!

What comments would you make other than the obvious?

Thanks,

Jim Murphy

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Originally Posted by JIM MURPHY

The question is whether the three AC units running will overload the system and cause a breaker to trip out?

Possibly, however, the occupants would know the answer to that question, so I would make a comment in the report recommending the client directly ask the sellers that question.

That way, if the sellers say 'No.' and the buyers have a problem, it becomes potential fraud on the sellers part and they can cover the cost to re-wire to a 120 volt / 240 volt service and panel.

I would also state that the service needs to be changed to a 120 volt / 240 volt service and panel REGARDLESS of the answer the seller gives to the above question.

Now, if the client "forgets" to ask the seller and they have problems, *THEY and only THEY* are to blame - you stated the service and panel need to be changed and THEY IGNORED that statement. Had THEY not ignored your statement THEY probably could have gotten some or all of the cost covered before closing and use the sellers money for that work.

Thanks Jerry! The occupant was a 90 year old woman who has since been moved to a retirement home. Not sure we will get a good answer back. However, the buyers also want to add a clothes dryer and have new appliances to go into the kitchen so they are going to have to upgrade to a larger amperage and a new panel since it was limited to 125 amps.

If they were 110 AC units, wouldn't it still be too much on the system?

If they are putting in a new panel, they need to change the service to a 120 volt / 240 volt service and not just leave the old 120 volt service.

Also, with the older lady, she probably never used more than one a/c at a time - whichever room she was in, and as old people are usually colder (due to less blood circulation) she probably did not use the a/c as often, or set it to run as long (cool as much) as your client will.

Kind of like houses on septic systems where I would tell my clients (young family moving in) with a seller (older couple moving out) who had absolutely "no problems" with the septic system - yeah, they did not push it to its limits like your family will, if you don't pump it out and have it properly inspected before closing you probably won't make it a week or a month before it backs up on you.

In your case, your client will likely turn all the a/c on along with everything else, and then wonder why the main keeps tripping.

During an HI, I would turn them on just to see if they are working.

As to the question, you can't give a yes/no without testing and taking on the responsibilty for a wrong answer. Thus refer it to the current owner or say "Do the age of the units and size of the power system I can not determine their load and suggest a professional electrician determinethe is a panel upgrade is required."

Regardless 200AMP service is standard for residential housing. Unless this was a log cabin in the woods with only a couple table lamps, lol.

Originally Posted by Ken Lyons

Regardless 200AMP service is standard for residential housing.

It is?

Since when?

Originally Posted by Jerry Peck

It is?
Since when?

Guess I should rephrase.

200AMP is Common for residential in my area. And strongly suggested.

Local code requires it for single family housing (detached and attached).
The only sub 200 service that is permitted from our local electric companies are 40 amp for traffic lights, alarm system and other low power devices on easements and 100 amp for bill boards. Some residential houses are going to 400 amp (6k+ square feet living) and 3 phase commercial of course has even larger i.e. 660 amp as required.

My house, 1945, used to have 45 amp service till about 40 years ago when they went and swapped every ones meter base for 100/150/200 amp. Still have the silloette of the old round meter base.

Just in the last 5 years all the major electric companies, in FL, replaced everyones meter with a RF-digital and required meter base upgrades for any old ones they found.
-- Electricians made a fortune up dating all these old bases and adding disconnects. (so it would be a 100 amp disconnect/breaker, on a 200 amp service drop)


100 amp service may be fine, provided you don't have any large appliances, heaven forbid you run an electric range while your use your central air or heat. (50 compressor, 30 blower, 50 range, 20 for task lighting)

What size are the AC units ?You must determine the loading before you can make a proper decision on the service. If they are only 10000 BTU they only draw 10 amps each no more than a dryer (3x10a=30a) so the service is ok. Think about all the people that have hot tubs (50a) and pools (15a) all running on 100 a service and no problems. If the system is overloaded the Main fuse or breaker will protect it.I am a certified Master Electrician for 35 years.....

I too agree the system should be upgraded to at least 100a 120/240 v minimum. I did not see at first it was only a 2 wire system.

Here is a picture of the panel. Judge for yourself!

Originally Posted by Ken Lyons

Guess I should rephrase.

200AMP is Common for residential in my area. And strongly suggested.

Local code requires it for single family housing (detached and attached).

Your local area is not allowed to make those requirements as you are in Florida and Florida is under one code ... the Florida Building Codes.

You need to check your local AHJ as I think someone fed you a line.

The only sub 200 service that is permitted from our local electric companies are 40 amp for traffic lights, alarm system and other low power devices on easements and 100 amp for bill boards. Some residential houses are going to 400 amp (6k+ square feet living) and 3 phase commercial of course has even larger i.e. 660 amp as required.

The electric companies do not trump code, if you are getting your information from your electric companies you need to go to a better source - your local AHJ. The power company requirements stop at the end of their service lines, they cannot control anything past that, and even the line from the transformer to the house, which belongs to the power company, must meet code.

100 amp service may be fine, provided you don't have any large appliances, heaven forbid you run an electric range while your use your central air or heat. (50 compressor, 30 blower, 50 range, 20 for task lighting)

100 amp is fine for many things, even large appliances such as electric ranges and central air and heat, if you don't believe it to be so, look at all the 100 amp condos out there with electric ranges, electric water heaters, electric air conditioners and electric strip heating.

Ken, you need to double check your sources and make sure you understand what they told you, and then not believe sources which are giving you misinformation.

A normal 1,500 sf house will be just fine on 125 amps and even less. Would it be nice to have all homes 150 amps and above, sure, but only 100 amps is required as the minimum and the load calculations during the electrical design and plan review establish and approve the service requirements.

Originally Posted by JIM MURPHY

Here is a picture of the panel. Judge for yourself!

Jim,

Well, at least that is a 120 volt / 240 volt service, which helps a lot.

There are three conductors in that service: 2-hot phase conductors going to a back fed main (which is not properly anchored down as required for back fed breakers) and the neutral/ground which goes to the neutral bar.

Size I can't tell from the photo, but at least it is 120/240.

I'd do the same strategy Ken mentioned. Turn on all three, and go about the rest of the inspection. A couple of hours won't tell you everything, but you should know if they are going to start tripping breakers before that long. If there's an electric over and clothes dryer, turn them on too while you are in there.

This needs serious attention no doubt as I can see now. Replace it . Replace it with 100A 120/240 V 32 circuits minimum and new incoming feeders sized to 100 A and proper grounding as I can imagine this is poor too.If you hire a Electrcal Contractor from the area they will know all the local electrical codes and make sure you get a Certificate of Electrical Inspection from the contractor upon completion as this assures you that the project is performed to all local electrical codes and safe. It will then meet todays standards and you will be good to go. .......Joe

Jim Murphy, thanks for the picture attach to your thread.

Jerry P. cover it well, well enough, that I can't add thing.

But if I was a buyer, I would be asking for a service up grade. 120/240
at 150 amps. But 100 amps is standard for a house that size in my
State. Just a foot note: A.C. UNITS that run at 240 volts I believe
are better units. But that just my opinion/

Is the 150 amp. over kill? Ever take a look at what a plasma T.V uses.

I would just use a comment like this to cover yourself when breakers do start tripping wildly:

When a service panel rating is 100 amps or less, an upgrade may be required if higher demand is required. This system was standard at the time of construction, and unless otherwise noted, appears to be serviceable. Some power management may need to be observed when major appliances are electric such as range/ovens, dryers, hot water heaters, and air conditioners.

Well said Micheal S.

I was curious about the TV usage mentioned, so I looked it up. Huge variations between different brands of TVs. The highest usage draw was over 5 amps. There are other sets that are just as large that only pull 1.5 amps. Plasmas pull on average a lot more, but not all. I'm still in the stone age with a 27" Trinitron, but his chart is worth looking at if you are buying a new TV. The estimated annual costs range from a low of $21 to over $120.

The chart: 150 HDTVs' power consumption compared - CNET Reviews

The good, fair and poor are relative to the number of Tvs in the set, with a third in each. That's why some plasmas rated differently with similar numbers to some of the LCD machines.

The only way to figure out what size service is required is to do an electric load calculation in accordance with the local code that is adopted or that is required by law (such as Florida).

Originally Posted by ben jacks

The panel is a MLO, 125A, 120/240V rated panel with a 60Amp 120/240V 3 wire, 1 ph Service Entrance hookup.

There should be a one-armed bandit somewhere between the meter and this Main lug only panel.

Not necessary as that panel has a main in it and is the service equipment. As I stated in a previous post, that back fed main breaker needs to be properly anchored down at the other end.

If no main service disconnect, then it is advisable to change out this system.

And it does have a main service disconnect in that panel.

First the panel needs a grounding/bonding system installed as the incoming 3-wire lacks equipment grounding of branch circuits.

No, the neutral serves as the grounding point.

Second, there must be a minimum code of the following circuits to match the described data given in the original post;

You are applying new wiring requirements to an older home, those requirements were not there in 1944 when the house was built and there is no requirement to update the home.

*IF* the house was being re-wired, then the new (current) code requirements would apply.