Existing dwelling load calculations

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frank112

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I have a Question about load calculations for an existing one family dwelling. A friend is selling his house, and the buyers home inspector claims the service is to small for the connected load. I am a commercial electrician by trade, and just a field man, so load calculations are new to me.
The house is 1734 sq. feet. It has no small appliance circuits other than existing branch circuits. It is an older House. It has a 12.5 KW range, an AC with min circ ampacity of 20.1 min amp. (all I could get). A 30 amp dryer cicuit that is not being used, a gas dryer is installed.
No other specific loads
I did some calculations using 220.83(A) of the 2005 code (2002 book is at work} and come up with well under 100 amps. The house has a 100 amp fuse panel with 2 small sub panels (other issues I think I can fix) Do these calculations sound right
Thanks in advance
 
Re: Existing dwelling load calculations

I wonder about other loads such as the water heater, dish washer, the laundry, disposal, microwave, pumps of any kind, and so on. There is probably more load than you are calculating. In many cases, the load for the heater is greater than that of the A/C, so that number may have also been a little low.

It is very possible that the service is large enough for a 1700 sqft home with gas appliances. I would get a more accurate calculation of possible load and use 220.31(B) of the 2002 to see if the number is still under 100-amp.
 
Re: Existing dwelling load calculations

How far over 100 amps is he? Remove that unused dryer circuit receptacle (or tell them that's what you'll do to correct the "defect") and you won't have to count it. That will remove 21A from the calculation right there.

If the water heater is electric, he may still have a problem.
 
Re: Existing dwelling load calculations

I agree with bryan. You can't make any arguement if you don't do the whole calculation.

Mike Holt has or had I think an excel program available on line that made the process pretty easy. Maybe somebody could help me on that.

I don't know anything about the "home inspector" you're dealing with, he may be perfectly credible. You can usually tell by how specific the report is.

I've seen some building reports that were so hokey there would literally be a disclaimer a the bottom of each inspected subject explaining how it's not possible to actually assertain the information in the report. Of cousre I'm paraphrasing. The things are kind of like form letters where some of the blanks are filled in.
 
Re: Existing dwelling load calculations

Thank you for your help, as far as I know, the heat, water heater, and dryer are all gas appliances. No sump pump, but I'm sure there is a refrigerator and microwave. In total there is one 100 amp main pullout fuse, one 60 amp pullout for the range, a 30 amp fusable disc. for the AC, two 30 amp fuses for the unused dryer circuit, and I think 10 15 amp fuses, four in the main panel and six on a six fuse block sub panel, for branch circuits,most of which, someone added a plug or two per circuit. I'll get more accurate information tomorrow. Thanks again
 
Re: Existing dwelling load calculations

Here a home with a fuse panel can`t get insurance when sold.Guess different in other places.
 
Re: Existing dwelling load calculations

frank, be careful of those home inspectors! Especially the ones that do it all....elec,construction, plumbing, etc. You may not have the right to question the inspector, but the seller( your friend) does! Get your friend to ask for the calculations from the inspector as well as the entire inspection report! Potential buyer may want the house brought up to existing code, panel upgrade to breakers, minimum of 2 appliance circuits in kitchen, GFCI's, etc! Potential buyer may have an electric dryer and does not want to go out and buy a gas dryer! Did not intend to down home inspectors, but have dealt with all kinds, most are very reliable! The stories I could tell!
 
Re: Existing dwelling load calculations

Originally posted by frank112: It has no small appliance circuits other than existing branch circuits.
Click to expand...
Maybe true, maybe false, but in any case, irrelevant. The load calculation has to account for two SA circuits.

Do these calculations sound right?
Click to expand...
If your answer was ?well under 100 amps,? then I would not agree. Here is my first cut at a load calculation, based on the limited information you provided:

1734 sq ft. times 3 w/sq. ft = 5202 W.
Two SA Circuits @ 1500 W each = 3000 W.
One Laundry Circuit @ 1500 W = 1500 W
Subtotal = 9702.

Take first 3000 W at 100% = 3000 W
Take remaining 6702 W at 35% = 2346 W.
New subtotal = 5346 W.

Now add 8000 plus 5% of 8000 for the range = 8400 (Table 220.19, column C, Note 1)
Now add 5000 for the dryer.
Now add 4824 for the A/C unit (20.1 times 240).
The refrigerator and microwave count as part of the 3W/sq. ft, so don?t add extra for them.

Final total is 23,570 W.
Divide by 240 to get 98.2 amps.

I conclude that (1) A 100 amp service meets the minimum required by code, (2) The home inspector has no authority to require an upgrade, and (3) If I were the buyer, I would insist on an upgrade to 200 amps, with the cost to be negotiated as part of the sale.
 
Re: Existing dwelling load calculations

Charlie,
I was basing my calculations on 220.31(A)

1734sq. ft. at 3kva =5202
2 small app. =3000
laundry =1500
12.5 kw range =8400 per note one
dryer =5000
ac (20.1 min. cir), =4824
________
total 27,926
-8000 at 100%
______________________
19,926 at 40% = 7,970.4
+ 8,000
____________
15,970.4

15,970 total kva
____________________
240
=66.54 or 67amps
Like I said, I'm new at Load calculations, but would really like to learn about this. Does 220.31(A) apply?
 
Re: Existing dwelling load calculations

Originally posted by frank112: Charlie, I was basing my calculations on 220.31(A). Does 220.31(A) apply?
Click to expand...
Interesting question. You are not building a new house, nor are you adding load to an existing house. So I suppose that 220 does not apply at all. But I think that if you wish to prove that an existing service is adequate, then you should treat it as though it were a new construction.

Once I had found an acceptable answer (i.e., under 100 amps) using the normal method, I did not think about using the optional method. In your case, you could use 220.30; I think that 220.31 is not the right article.

Please note, however, that the optional methods both require you to use the nameplate rating of all appliances. You don?t get to use the Table 220.19 demand factors for the range. Here is my revision to your calculation:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">1734sq. ft. at 3va = 5202</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">2 small appliance = 3000</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">laundry = 1500</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">12.5 kw range = 12,500</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">dryer = 5000</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">ac (20.1 min. circuit amps) = 4824</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Subtotal = 32,026</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Count the first 10,000 at 100%</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Count the remaining 22,026 at 40% = 8,810</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Total is 18,810 VA</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Divide by 240 to get 78.3 amps.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">This is, as you said earlier, well under 100.
QED

[ October 19, 2004, 07:22 PM: Message edited by: charlie b ]
 
Re: Existing dwelling load calculations

I was just wondering why a dryer load [5000 VA] is included in the calculation. He specifically noted "...dryer are all gas appliances".
~Peter
 
Re: Existing dwelling load calculations

Peter, There is a circuit installed for the dryer so it still has to be counted even thought its not currently used.
 
Re: Existing dwelling load calculations

Curt is right. The purpose of the service load calculation is to make sure that there is enough power to handle all the loads that might be connected, while accounting for the likelihood that not all items would be running at the same time. The receptacle is there, so it must be counted.
 
Re: Existing dwelling load calculations

Frank,
In Memphis,Tn The city and county code allows us to fiqure existing load as follows:The first 6 120 volt circuits count 10 amps each,the next 6 count as 7 and then all remaining count as five.Range or oven is 35 amps,Dryers are 20,220V. Window A.C. is 10 and central a.c. is nameplate as usual. This is an example for range,dryer and eight circuits.
35 35
20 20
10 10
10 10
10 10
7 7
-------
87 amps total per phase
Thought you might be interested.
 
Re: Existing dwelling load calculations

Watching you all calculate these loads is great.
Thats my weakest link.Is there a study guide you would recommendwith examples and code references to help learn this?
 
Re: Existing dwelling load calculations

You might check out the home page of Mike Holt?s web site. He offers a number of very good reference books.
 
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