100 Amp service questioned by home inspector

[artpi]

Hello,
I'm selling my home and the buyer hired a 'home inspector' (who is not an electrician) to verify condition. The buyer's home inspector noted that "the 100 amp panel exceeds the number of circuits and piggyback for a 100 amp panel"? This appears to be a standard service panel with 24 installed circuits and no added subpanels. If I add up each breaker circuit, the total is approx. 415amps. Does that represent a problem? Interestingly, when I purchased this house 6 years ago, my home inspector noted that the electrical service had 24 circuits and was just fine! All the homes in the neighborhood are 100 amp svc., and I've never had any problems with power. Do I need to hire an electrican to verify this? Or can I calculate myself if a problem exists? Any help would be appreciated as I need to respond to the buyer within a few days!
Thank you..

Art P.

 

[charlie b]

Re: 100 Amp service questioned by home inspector

Adding up the ratings on the 24 breakers gives you a number that has no meaning whatsoever! Ignore it completely. What matters is your total load. If the 24 loads actually draw only 1 amp each, then you total load is 24 amps, not 415.

There is a real question as to whether a 100 amp service panel is adequate for the home. But that is not a code-compliance issue. The code only cares that the installation is safe. The code does not care whether you have enough power to run all of the electrical equipment you have chosen to buy. If you plug in too many things, and keep getting those annoying circuit breaker trips (I know you said that you?ve had no problems, I?m just making a comparison), the code will be happy. That is because the breaker has performed its safety function, and prevented the wires from over-heating. If you don?t like the frequent nuisance trips, then you may choose to upgrade your installation to, say, a 200 amp service. That?s your choice. But understand that it is not within the power of this ?Home Inspector? to command you to do it. He (or she) will have no code authority behind such a command.

If you want to get an idea of whether a 100 amp service is enough for your home, then the NEC has a process you can use to calculate you load requirements. It?s a bit tricky, but here is a broad brush on the subject:
1. Multiply the living space (no garage, no unfinished basement) by 3.
2. Add 1500 (to cover the required laundry circuits)
3. Multiply the result by 35%.
4. Add 3000 (to cover the required kitchen small appliance circuits)
5. Add a minimum of 8000 (to cover the electric range)
6. Add the ratings (in watts or in VA) of any fixed electrical items (including water heater, dishwasher, disposal, trash compactor, etc. Do not include receptacles used by lamps, microwave ovens or other items, as they are included in item #1).
7. Add the larger of your air conditioner and your home heating system.
8. Divide the result by 240.
9. If your answer is close to, or over 100, then it may be a good idea to call a professional electrician for advice and perhaps an upgrade.
10. If your answer is below 90, then there is no question of needing a larger panel.

 

[tom baker]

Re: 100 Amp service questioned by home inspector

"Home Inspector" OK sure. Tell him this: The wires are protected at there ampacity on the service side by the main breaker. Downstream of it each circuit has an individual circuit breaker, to protect its own circuit.

Its highly unlikely that the service is overloaded. You probably have gas heat, gas water heater and dryer. There is very little electrical load.

 

[bennie]

Re: 100 Amp service questioned by home inspector

Contact the serving utility, they can tell what the demand is by the bill. There is a formula to use by dividing the KWH by a time/quantity constant.

 

[charlie]

Re: 100 Amp service questioned by home inspector

Bennie, I'm sorry to tell you that we can only estimate the demand of several customers on a single transformer. We use a formula of the total of four months energy usage in the summer (two different ranges) or four months energy usage in the winter to arrive at an estimated loading on our transformers. It is an educated guess based on test areas of demand vs energy usage to arrive at the formulas.

We take the answers with a grain of salt and use them as the estimates that they are.

Sorry, Charlie (I love signing my name that way)

 

[jro]

Re: 100 Amp service questioned by home inspector

You also have to look at how many circuit breakers the panel can take, Ex. 100 amp 8-16, 12-20, 16-20 etc. just because you can add piggyback breakers in a panel, this does not mean you can do it, each panel has different ratings, also if it is a MLO panel you are limited as to the number of disconnects NEC 2002 ART.230.71(A) Maximum Number of Disconnects.

 

[bennie]

Re: 100 Amp service questioned by home inspector

I know the only accurate method to determine peak demand is with a recording demand meter.

I can read the demand on a disc type watthour meter. Turn on the usual appliances and calculate the demand by the disc rotation.

My friend at PGE tells me he can calculate the peak demand by using constants times the total, based on many factors of usage modeling. He uses these to calculate the service drop in kilowatt feet.

 

[charlie]

Re: 100 Amp service questioned by home inspector

Sorry Bennie, I thought you were talking about using the information from the electric bill. I wrote the following for our engineers:

If you want the total instantaneous demand while in the field, this method will work. This method will give you the instantaneous demand for only that point in time and will not give any unbalance. In other words, if a customer has three-fourths of his load on one leg and one-fourth on the other, you would not be able to tell that.

This method will work on any meter, including a primary meter.

1. Look at the meter for a Kh (secondary watt-hour constant) number, this is the meter constant and indicates the energy used by the load for each revolution of the disk. If you are looking at a CT cabinet, someone will have marked a multiplier and is used to increase the Kh value for the CT ratio.

2. Count the revolutions of the disk for one minute, if you want a little more accuracy, use a little more time.

3. Multiply the number of revolutions in one minute by the Kh, and then by 60. Divide the resultant by 1000.

Example:

You are at a service that has a meter with a Kh of 3.6 and time the turns at 68 per minute.
(68)(3.6)(60) / 1000 = 14.688 kW

If this is a single phase service, the load in amperes is
14.688 / 0.24 = 61.2 amperes

This is just an instant in time and is of no use for any kind of load study. However, it may prove useful if you want a quick approximation of the load while you are in the field.
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Larger commercial and industrial services have demand meters and we can get the peak demand from those readings. I don not know of any electric utilities that have that information for residential customers.