Watts/Amps from outlet?

Charc

Well-Known Member
So correct me if I'm wrong here. I believe I read on here that you can draw a maximum of 15 amps / around 1000 watts from a standard Edison outlet. My question is simple (and sorry for so many questions guys! I sense some of you would rather I stop asking so many.) when you mention "per outlet" is that for the singular outlet, or for the entire plate (standard 2 outlet plate). I'm not sure if I'm being clear, I'll try and rephrase if you don't get what I'm trying to ask.

gafftaper

Senior Team
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The circuit breakers are typically 15 (or sometimes 20 amps). This means the total draw of EVERY outlet on that circuit can not add up to more than that 15 amps at once. So outlet #1 Can draw 10 amps, outlet #2 o can draw 3 amps, but outlet #3 only has 2 amps left to use or the breaker will blow.

This can be a real problem in old houses (like mine). Back in the day people didn't have as many household electrical appliances. I had a problem at Christmas a few years back where if the Christmas tree, Stereo, and a few lights were on, the breaker would blow when the fridge kicked in. I have since added a few new circuits.

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Its actually per circuit. Most standard house outlets are rated for 15 amps per complete outlet, ie only 15 amps can move in from the wires connected to the physical hardware. There are 20 amp rated outlets, they are the ones with one blade at a 90 deg angle. Theoretically, the breaker that supples power to the circuit is rated at the maximum allowable amperage of the cable and the hardware at the end of the cable. Sometimes this is not done in residential situations where they will put in a 20 amp breaker and cable to support 20 amps but put in 15 amp outlets saying that the 20 amps is spread over X # of outlets. From what I can recall this is against current code (but its how my current apt is wired...). When it comes to how a building is wired, very rarely do they put in 15 amp outlets, most industrial/school/what not situations are wired for 20 amp circuits. Go to the breaker box and see what the breakers are rated at, thats the maximum amount of power you can pull from that circuit. Also remember, that the power of one circuit can be spread over multiple outlets. There are cool little tools that can trace circuits to the breaker box and to other outlets, or you can do what most people do, the guess and check method. i.e. load up a circuit till it blows, then load up another cicurit till it blows. Does it work, yes, is it the best solution, no.

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The circuit breakers are typically 15 (or sometimes 20 amps). This means the total draw of EVERY outlet on that circuit can not add up to more than that 15 amps at once. So outlet #1 Can draw 10 amps, outlet #2 o can draw 3 amps, but outlet #3 only has 2 amps left to use or the breaker will blow.
This can be a real problem in old houses (like mine). Back in the day people didn't have as many household electrical appliances. I had a problem at Christmas a few years back where if the Christmas tree, Stereo, and a few lights were on, the breaker would blow when the fridge kicked in. I have since added a few new circuits.

My apt is like that, if i have my PC, my TV, my DVR going, and my AC on, it blows. Its no fun to have to draft on a hot day.

zac850

Well-Known Member
The way it generally works is that you have 15 amps running out of one beaker for a normal household edison outlet.

From this point on, all it matters is how that circuit is wired. Some houses all the outlets in one room are on one circuit breaker. Other places, especially kitchens and other places where they expect large loads to be plugged in, every outlet is a different circuit breaker.

In one of my old theater spaces we had a breaker wired to several wall outlets. This was specially wired up for us to use some shoebox dimmers. In this case, each outlet was wired to a different circuit, in some cases there were two circuits in each outlet box.

There are really only two ways to find out. First would be to dig up the electrical plans of the building. Second would be to go and flip on and off the breakers, and see which outlets stop working. (make sure not to flip off anything important)

Foxinabox10

Active Member
Although it is necessary to have the blade turned 90 degrees to pull 20 amps on one cable, most places do not have the blade turned when the breaker is 20 amps. In general, most commercial buildings and schools run only 20 amp circuits.

BillESC

Well-Known Member
To clear up one misstatement, a 15 amp outlet (or circuit) will supply 1800w of power. The formula is Amps X Volts = Wattage.

15 X 120 = 1800

6ftstudios

Member
I've toured for a number of years with different organizations (often performing in churches/schools). We'd travel with all our own sound, lighting, and spotlight. For each set-up we'd need 5-8 different circuts for our set-up. It can take a LONG time to just guess.

I def. recommend getting a circuit tracker. Spend 30-60 bucks and you're good to go. of course, you'll have to test the breaker to double check that the tracker was right, but most of them are fairly accurate.

Van

CBMod
CB Mods
To clear up one misstatement, a 15 amp outlet (or circuit) will supply 1800w of power. The formula is Amps X Volts = Wattage.

15 X 120 = 1800

15 x 110 = 1650

15 x 115 = 1725

As supply voltages can vary according to a number of different factors.
Always check your voltage prior to guessing how many watts you can load a breaker with. Always use a 10% rule, that is, load a breaker only to within 10% of it's maximum allowable. Sometimes breakers get "weak" and will shut down below thier rated amprerage.

Oldman

Member
If you want to go a step further, the National Electrical Code allows one to load a circuit up to 80% of the circuit rating for a permanently connected load. Although what we do is never really permanent, the reliability of our operation is certainly enhanced by following the 80% rule.
The configuration of the outlet may or may not have any relationship to the circuit rating. If I connect a duplex NEMA 5-15 receptacle (the normal edison receptacle) to a 20 amp circuit it is acceptable. If I am going to connect a single receptacle (not duplex) as the only receptacle on the circuit, and it is a 20 Amp circuit, then the NEMA 5-20 recptacle (the receptacle with the blade turned at right angles) is required. One may always use a higher rated device with a lower rated circuit, but the opposite is not true if there is only one device on the circuit. If you are going to be dealing with a variety of different circuit ratings then you should get a copy of a NEMA ratings chart to be sure you are using the correct plugs and receptacles since there is a different device for every different circuit configuration.
Oldman

Charc

Well-Known Member
15 x 110 = 1650
15 x 115 = 1725
As supply voltages can vary according to a number of different factors.
Always check your voltage prior to guessing how many watts you can load a breaker with. Always use a 10% rule, that is, load a breaker only to within 10% of it's maximum allowable. Sometimes breakers get "weak" and will shut down below thier rated amprerage.

Yea, using this information I'll recommend a maximum of 1500 watts

SHARYNF

Well-Known Member
This whole area can be really confusing. There is sort of a residential look the other way code situation where multpile standard 15 amp outlets can all be connected to a single 20 amp breaker. Thinking is that in a home you rarely use the full 20 amps on a single connection, so the flexibility and probably infact a cost savings. The the wiring from the breaker to the outlets should be 12 awg wire. One other problem with the tester is it only really works if you can find every outlet on the connection. Sure it will tell you if the outlet is connected to the breaker, and that is great information, but usually what gets you is the odd ball extended outlet somewhere that is also on the branch, the device plugged into it is turned off at the moment, and the receptcal is off in some really weird place. You get all set up think you are good to go and find out that some coffee pot or hair dryer or exhaust fan or something is on the same branch.

this is why in pro setups you just about always try to get your own distro, get it connected into the panel, and then you know exactly what is on the system from the panel to you. I have found that bringing along a Square d breaker and a Siemens breaker also helps, since you usually can get a local electrician to do the interconnect, if you have your own breaker then you don't need to have a disconnect etc.

While ohms law works in many cases, typically with a lamp on a dimmer it does not quite work the same way, so, if the voltage is lower, it is not like the dimmer/lamp draws more amps, in fact I think you will find that in a ac system, what you get is a lamp that does not go to full brightness, so the amps draw will remain the same, it is not like the dimmer will attempt to draw more amps in an effort to reach the same wattage. It is the difference between resistive vs inductive/reactive load. This for example is why most dimmers do not want you to run inductive/reactive loads and why transformers on dimmers are not a good idea. You do not want the device in an attempt to produce an output to keep increasing the amps to make up for the reduced volts.

http://www.ibiblio.org/obp/electricCircuits/AC/AC_11.html
Anyway just some thoughts
Sharyn

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ship

Senior Team Emeritus
or you can do what most people do, the guess and check method. i.e. load up a circuit till it blows, then load up another cicurit till it blows. Does it work, yes, is it the best solution, no.

That's a dangerous "most people do" solution. What if it is overloaded but the breaker doesn't trip by way of being old or warn out in telling you if it can "handle" the load? What if it's only say 14ga wire or has a loose connection somewhere in the system? Often max. loading at or past 127% of the load on the circuit breaker will take a bit of time to get warm enough to blow a breaker. Question at times is what melts down before the breaker in a other than overload or short is melting before that breaker trips. This given the breaker trips at all.

Best to do a circuit tracer as recommended and draw up a map with where each outlet is powered up from in a circuit breaker panel. Than also have an electrician inspect the system for proper wiring - at times those what wire places do odd things, this and lamp sockets or lighting loads are also on receptacle loading - only an electrician or someone really experienced can tell for shure what the maximum loading of a outlet should be.

Do up the map by way of circuit tracers which don't work all that well (at times there will be circuits that could be one of a few you detect.) or a map by way of plug in light which goes off when you find the proper circuit. Than once the map to the extent you can draw up is done including all circuits and lighting, have a electrician and or supervisor TD/ME or maintinence person have a look at how it's all really wired and inspect/verify. Than have them determine what's safe by way of outlet for what kind of loading and under what conditions. At that point also, it might be possible that if you need a dedicated circuit, it would be feasible to just run you a dedicated circuit for your specific need.

Foxinabox10

Active Member
Odds are that if you talk to the facilities department at any location that they will have a map-out of what outlets are on what circuits.