Replies

1. 
   MikeCallahan | Apr 23, 2003 05:55am | #1

   I think you really want to know how many watts per circuit. 1000 watts + or - 500 for a circuit of 15 amps would be about right from my limited electrical knowledge. How many outlets per circuit would depend on the prospective use.

   We may be slow, But we're expensive.
2. 
   ThomPeterson | Apr 23, 2003 02:40pm | #2

   The NEC dosen't specify a maximum quantity. You asked for a rule-of-thumb and a good one would be 10 outlets for 15A and 13 for 20A.

1. 
   brownbagg | Apr 23, 2003 03:20pm | #3

   120 volts x 15 amps = 1800 watts x 70 % = 1260 watts

   maxium watts on a 15 amps is 1260

1. 
   alecs | Apr 23, 2003 04:34pm | #4

   FYI:

   Your calculation of watts = volts x amps is good for DC, but AC only when it's purely resisitive load.  Otherwise you need to add power factor. 

   A.C. Watts = Volts RMS * Amps RMS * Sqrt (number of phases) * Power Factor.

   In single phase, you can leave out the sqrt term, because the square root of 1 = 1.

   Power factor is probably pretty high (0.9 to 1.0??) for residential compared to the industrial stuff I usually deal with, but if there are a lot of electronics, or any motors or other inductive loads, you should probably count on a power factor of less than unity, which would mean that you need more amps to get the same watts.   Or to say it another way, you will trip your 15A breaker with less connected watts. 

   Let's see, residential inductive loads that I can think of: fridges, power tools, kitchen appliances like mixers, garbage disposals, fans, dishwashers, washing machines, dryers (the motor anyway), fluorescent lighting, air conditioners, computers, stereos, etc.

   All of these would tend to drive power factor down.  Electric heat (along with stoves, toasters, dryers) and incandescent light bulbs are probably the only unity power factor loads in a house.

   I did some power monitoring in my house a while ago, and the avg power factor was 0.81 but got as low as 0.56 at times!!

1. 
   IronHelix | Apr 24, 2003 02:45pm | #5

   Whew!!!!   That left me in the dust!  

   So with that FYI, what is the answer to the man's question?

   Lost in an Electrical Wonderland.

   ..................Iron Helix

1. 
   alecs | Apr 24, 2003 03:15pm | #6

   15 amps * 80% current limit * 120V * 0.8 pf = 1152 W

3. 
   BillHartmann | Apr 24, 2003 05:16pm | #7

   There is no NEC limit on the number of recptacles for residential use (but there is for commercial).

   However, a number of local codes do have limits so check on them.

   After you take care of the require circuits for kitchens, bathrooms, and washer what kind of loads do you have left.

   Table lamps - 50 watts, floor lamps - 150. Big screen TV - 500? TV - 150. Clock radio - 25. Home theather ?? Boom Box - 100. Computer and monitor -350, more if lazer printer. Portable vaccum cleaner -700.

   Now all of those devices are not being used at the same time. And all of the recptacles won't be used either.

   The NEC calls out 180 watts per recptacle. While that is not required residential use, it is still a good working number. So that does give you the 10 recptacle on a 15 amp circuit and 13 on a 20 amp circuit.

   But if you are going to have a high powered home theater run a separate circuit(s) for that.

   Also for the garage and basement have at least one 20 amp circuit each with nothing but garage and basement recptacles on it. And if you know that are you going to have a shop in one of those areas then, of course, more circuits.

1. 
   Francorosso | Apr 24, 2003 07:09pm | #8

   Thanks for the info, just what I was looking for.

   Frank

2. 
   NannyGee | Apr 24, 2003 11:49pm | #9

   I trust your (& others') numbers, but it makes me wonder; if a 15 AMP circuit can only handle 1100-1300 watts and 15 AMP circuits are standard for 90% of outlets in 90% of new construction....

   How is it that you can go to any hardware store and buy a $30, 1500 watt space heater and plug it in anywhere (supposedly)?

1. 
   BillHartmann | Apr 25, 2003 01:02am | #10

   I don't have any actual numbers but 20 amp circuits for recptacles are considered as "standard" in any quality construction and is local code in some places.

   But in most cases the actual recptacles are 15 amps configuration, but rated as 20 amps for feed thru. And if you have a quality recptacle, not the 39 cent ones, the contacts inside are the same for the 20 amp recp as for the 15 amp ones.

   All that said 15 amp circuit is 1800 watts. For CONTINOUS use you are suppose to only load it to 80%, that is 1440. Continous is over 3 hours.

2. 
   alecs | Apr 26, 2003 04:43am | #11

   a space heater will have nearly unity power factor (not quite 1.0 because of the fan.)  So then you are looking at 1440 W continuous if you assume 80% of the ampacity for allowable current.  Also, the space heater is thermostatically controlled and will not likely run at 1500W for very long.