View Full Version : How many watts can a 15 amp circuit support?

May 2, 2005, 05:19 PM
Recently had my 35 year old Federal Pacific service entrance replaced. As I was labelling the circuits, I found that the 15 amp furnace fan circuit also has a bunch of lights on it. If I add them all up plus the 12 amps for the fan (that's what's on the fan label and I assume it's the max load at startup), I come up with about 2200 watts.

I know that's way more than the 1800 available from a 15 amp circuit but they are never all on at the same time and I've never blown the breaker. It would be unusual for more than about 300 watts worth of lights to be one at the same time.

Should I move some things off that circuit anyway? Should the furnace fan have its own dedicated circuit?

May 2, 2005, 07:47 PM
Likely it is wired up with 12 gauge wire, and you could use a 20 amp circuit breaker. Still, that would be overloaded. Furnaces should have a circuit of their own supplying the power for the fan and controls. I think the best thing is to run a new 20 amp, 12 gauge wire circuit for the lights. 2200 watts is about 20 amps, not leaving the required margin. Some of the lights still need to be moved to a lighter loaded circuit. I hope your new box has plenty of room for extra circuits. Check your other circuits. Some of the others may be over loaded too. In addition to the furnace, other large appliances such as refrigerators and freezers should be on their own circuit. Certain things that code allowed in the past may be required to be updated to current codes when you do other work. This could mean upgrading to GFI in locations such as bathrooms, outside, basement, and near the kitchen sink. No big deal. Just add a GFI at the first outlet and use the feed through contacts to protect the rest of the circuit. GFI outlets are very reasonable now, 20 years ago, I think they were $40 each.

Note, tkrussel seems to know code better than I do.

May 3, 2005, 01:55 AM
Code only allows any circuit to be loaded 80 % of its rated load. As you noted most devices will not be on all the time, however, there only needs to be one time for all to be on to exceed the allowed ampacity of th circuit, and to be sure there is absolutely no fire hazard, the 80% rule should be followed.

A 15 amp circuit can carry only a total of 1440 watts,which is 80% of the 1800 watts found by mulitplying the volts times amps,15 x 120 x 80%= 1440.

A 20 amp circuit can be loaded 1920 watts, 120 x 20 x 80%=1920 watts.

If the panel was recently replaced I am sure there is a 15 amp breaker because the installer found a #14 wire, which can only use a 15 breaker,and a 20 amp breaker cannot be connected to it.

The furnace blower nameplate of 12 amps does not include startup, or "Locked Rotor Amps". Motors need to be protected by a breaker 125% of the load, 12 x 125%= 15. This can use # 14 wire and a 15 amp breaker, and should be alone on the circuit.

Also having the lights in the basement on a different circuit will come in handy should the furncae breaker ever trip or needs to be shut off, so you can have light to work on the furnace.

As Labman mentioned about refrigerators and freezers, having them on their own circuit is recommended, so if something on the circuit trips the breaker, you will not lose all the food it the refrigeration, but code does not require this, since these units do not draw much current, usually 3-4 amps .

You msut be absolutely sure what size wire you have before installing a larger breaker on it.Either read the markings on the wire insulation, and if you can't see the markings, get one of the wire strippers that have wire gauges on it to measure the wire.

And replacing the FPE panel was the best thing you ever did, I can tell you horror stories about FPE.

Hope this helps

May 3, 2005, 05:20 AM
All the other large load devices (A/C, range, oven, etc.) are on their own circuits so I'm OK there. The original house wiring apparently wasn't done too well since there are a couple of 20 amp circuits with very little connected to them, then the furnace fan 15 amp circuit with a potential 2000+ watt load. I'll probably just call the electrician who installed the panel to make the changes.

Btw, I just became aware of the FPE issues a year or so ago when I had a new A/C unit put in. The HVAC guy suggested I get it replaced and the electrician strongly advised it also. There are plenty of empty spots in the panel for new circuits.

May 7, 2005, 06:27 AM
The 20 amp circuits with little on them are probably there for a purpose. They may have been installed for the previous ower for specific appliances.If these circuits are in the kitchen, there should be no more than two outlets per circuit, and they are referred to as " appliance circuits" by code.

Not that you implied anything to make me think this, I caution you about adding any other outlets on the 20 amp circuits unless you know exactly what they are for ,of course, depending on where the circuits are located.

At the time your house was wired, dining rooms were required to have 20 amp circuits. This practice is no longer required by recent code editions.

Laundry areas are to have one 20 amp circuit, along with bathrooms, and of course kitchens. If the 20 amp circuits you have are not in these areas, they may have been for certain appliances the former occupant had, refrigerators, freezer, window air conditioners. If none of these examples apply, then you can add to the circuit, and take advantage of them.

As good rule of thumb is to have no more than 10 outlets on a 20 amp circuit. However, this rule does not apply to residential installations, only to all other installations other than residential, commercial such as stores, offices, etc, and to industriial such as factories, ware houses, etc.

Even thou does not apply to homes, I do agree that no more than 10 outlets on a 20 amp circuit is a good rule.

I enjoy arguing with people that think it must apply to homes, since the code does not require this, I try to be clear and informative. Funny how people make stuff up.

For example, code does not REQUIRE refrigerators to be on their own separate circuit. Having a reefer on its own circuit makes tremenous common sense, every home I have wired has the reefer on its own circuit.

The code lists requirements for minimums and maximums. Wiring better than the code requires is recommended, makes common sense, allows for peace of mind, and is done by every good electrician. But when someone says only a couple of outlets are needed in a certain are, the code will dictate if this is allowed. Used and understood properly the code can be your friend.

I realize you did not ask for this info, just thought I would respond to help with clarification and information to you and others who could benefit from it.

Thank you for listening.

Aug 22, 2005, 09:57 PM
I totally agree with Labman and Tkrussell about putting the major stuff such as the fridge on individual circuits in addition to the required 2*20 amp countertop circuits... if only everyone did that lol.

Oct 25, 2007, 12:16 PM
Tkrussel knows his material. I just wanted to add that for any given circuit stranded wire is always better because it presents with 30% more surface area than solid wire. Since currents flows along the surface of the conductor it boils down to greater efficiency and lower electrical bills since the current has an "easier" time getting to where it's going to be used. If you really want to go nuts you can even upsize the wire. None of our jobs have 14 ga. Wire. We start with a mininum of 12 ga. STRANDED. The initial costs are indeed significantly greater, but take the long view. The return on your investment will continue for years after you've recouped the initial extra cost.

Oct 25, 2007, 02:08 PM
THHN- I've heard and read about this stranded versus solid wire and it appears to make sense on the surface-no pun intended- but how does the current traveling "easier" reduce the bills? If you have a circuit drawing 5 amps @ 300 watts that's what it draws no matter the type of wire. The real advantage to stranded is it is easier to pull.

Oct 25, 2007, 04:20 PM
The phenomenon that THHN is referring to is called Skin Effect, and is not desirable as it adds to the reactance and impedance of an AC circuit. Increasing the impedance of a circuit does not save energy.

It is also negligible with low voltage such as 120/240 and 277/480, and conductors larger than 300 MCM.

Skin effect is battled by utility and transmission systems by using cables with either nonconductive cores, or even cables with hollow centers, so that the remaining metal is used more effectively.

Additional information can be found here:
Skin effect - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Skin_effect#Introduction)

Oct 25, 2007, 05:58 PM
Tk-that's kind of what I thought. One of my car magazines had an article on auto wiring and cited the same theory in solid vs. strand- though autos use strand for the flex and movement incurred. As for the cost, haven't used stranded in a while but I thought it was cheaper than solid... hmmmmm

Oct 26, 2007, 08:17 AM
On Stranded wire, with AC Voltage, I believe there are less Eddy Currents, around each strand, and has a canceling effect, oppose to a Solid conductor, 1 large field?
Stranded cost more, Is still what I use?