Load Calculations

Reply

  #1  
Old 08-07-07, 05:20 AM
Member
Thread Starter
Join Date: Aug 2007
Posts: 6
Load Calculations

Is there a formula to calculate how many branch circuits are allowable in a service panel? For example, a circuit serving 5 100-watt ceiling lamps would only draw about 5 amps, although it would be wired to a 15-amp breaker. TIA for your response.
 
Sponsored Links
  #2  
Old 08-07-07, 05:52 AM
Member
Join Date: Mar 2006
Posts: 142
A 15 amp circuit on a 120 volt system will support up to 15x120=1,800 watts. Maximize usage at 80% to be safe, 1,800x0.80=1,440 watts. That means a 15 amp circuit could serve up to 14 100 watt light bulbs, for example. However, there are special requirements for certain circuits -- check with a pro if you're not sure.
 
  #3  
Old 08-07-07, 01:13 PM
Member
Join Date: Sep 2000
Location: United States
Posts: 18,497
I believe that you are asking about a "demand load calculation". Google that term. It will tell you how to do the computation. It's not trivial, and you'll need to collect a lot of information to do it properly.

What you should know is that the sum of the breakers sizes in a panel is completely meaningless. Everything in your house is never running at the same time, and the demand load calculation takes this into account. The calculation is primarily based on the square footage of your house and the size and number of your major electrical appliances.
 
  #4  
Old 08-07-07, 01:18 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,617
The number of allowable circuits in a panel will be on the label within the panel. With that said the Code limit is no more than 42 circuits unless limited by manufacturers instructions.
 
  #5  
Old 08-09-07, 05:16 PM
Member
Join Date: Apr 2006
Location: New Orleans, LA
Posts: 121
You'll need to know the square footage of your home. Multiply your square footage x's 3VA. Figure in your small appliance branch circuits with 1500VA per circuit (minimum of 2 by code). Figure in your laundry branch circuit (for washing machine) of 1500VA. Add your VA's from your general lighting and power, small appliance circuits and laundry. First 3000VA is rated at 100%, after the first 3000VA then take that amount and multiply it by demand factor of 35% then add the 3000VA back to get your total for your general power and lighting. Here's an example for a 2000 sq. ft house.

2000 sq. ft. X 3VA= 6000VA
small appliance branch circuits (min. of 2x1500VA)= 3000VA
laundry circuit= 1500VA
Total VA= 10,500VA

10,500VA - 3000VA(@100%)=7500VA
7500VA X 35% demand factor=2625VA
First 3000VA + 2625VA d.f.= 5625VA total general power and lighting.

If you want to figure out your general lighting and power without s.a.b.c. and laundry then do this for the same 2000 sq.ft house.

2000 sq.ft x 3va = 6000va
6000va / 120volts=50 amps
50amps/ number of 15 circuits=3.33 or 3 circuits.
or if you're doing 20 amp circuits
50amps/number 20 amp circuits= 2.5 or 2 circuits.
 
  #6  
Old 08-10-07, 12:28 PM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
The correct factor in the 3 watts per sq=ft , and small-appliance load calculations should be 240 volts.
 
  #7  
Old 08-11-07, 09:14 AM
Member
Thread Starter
Join Date: Aug 2007
Posts: 6
Many thanks

I appreciate all the prompt and useful responses to my post. This is a great forum and, as a DIY'er, I will be following all the threads to help me understand both the code and the practical applications.
 
  #8  
Old 08-15-07, 04:10 PM
Member
Join Date: Mar 2006
Posts: 142
I'm also trying to do a load calculation on my house. I downloaded some of the worksheets to do the computations but am getting hung up on the appliance va values. The worksheets say to use the name plate rating, but I can't seem to find those. The name plate on my wall oven, for example, has the following printed on it:

General Electric Louisville KY
LISTED UL 37IG Household use
U.L. File No. KW
E37202 120/240 V 120/208 V PN PLT 135-1

The serial number is stamped on the name plate. 3.4 and 2.6 are stamped above the 120/240 V and 120/208 V, respectively.

Does the 3.4 refer to kW rating? Is this the value I should use? I have the same question for the other appliances. I have a 2,300 square foot house with a wall oven, dishwasher, refrigerator, microwave, garbage disposal, 2 small appliance circuits & clothes washer. The dryer, water heater, cooktop and central heater are gas. No jacuzzi's, A/C or other big appliances. I'm trying to determine if 100A service is o.k.
 

Last edited by walks; 08-15-07 at 04:23 PM. Reason: more info
  #9  
Old 08-16-07, 09:41 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,530
> The serial number is stamped on the name plate. 3.4 and 2.6 are stamped
> above the 120/240 V and 120/208 V, respectively.

> Does the 3.4 refer to kW rating? Is this the value I should use?

Yes. The oven draws 3.4kW if connected to a 240V power system (residential building) and 2.6kW if connected to a 208V power system (commercial building). For a heat-based appliance, like an oven, volt-amps (VA) are equal to watts (W); so, the oven draws a maximum of 3,400 VA.

> The dryer, water heater, cooktop and central heater are gas. No jacuzzi's,
> A/C or other big appliances. I'm trying to determine if 100A service is o.k.

Given this info, it's very likely that your 100A service is just fine.
 
  #10  
Old 08-16-07, 10:19 AM
Member
Join Date: Mar 2006
Posts: 142
Thank you. I completed the load calculations and came up with less than 70 amps, so yes the 100 amp service is fine.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes