How do you tell how much juice a circuit can handle?

[balzac.thegreat]

This seems like a dumb question, but I rent at the house I'm at now so I'm not fully aware of the house's limitations. I've set up shop in what little space I have in the garage. The owner walled off half of the space so she could store stuff in that half. The result is that I have 1 usable outlet in the wall. There is another outlet in the ceiling for the garage door opener which I only use for an additional overhead light.

My question is, how do I tell how much juice I have before I trip the circuit? I've ran my table saw (21829) and my little 2 gallon shop vac at the same time and nothing happened. But I always unplug my compressor (this HF model) before I use the saw because I have a suspicion that this is too much for the circuit to handle. How can I quantify this so I know how much head room I have?...would like to get a DC at some point as well so this is part of the reason I'm asking. (I do have a multimeter if that helps me get my answer)

[LarryG]

Easy: look at the breaker (or fuse, in an older panel) and see what its rating is. Then tally up the maximum amperage of everything*** that's powered by that circuit, using the info on the labels that will be somewhere on those items. If the total amperage of everything that will be running simultaneously exceeds the rating of the breaker (fuse), at some point it's going to trip (blow). Note that this means it's okay to have, say, a total of 60A worth of stuff plugged into a 20A circuit as long as no more than 20A worth is running at any given time.

As a general rule, you don't want to exceed 80% of the breaker's (fuse's) rating for continuous loads. Intermittent loads up to 100% are okay.

(***If you're unsure what all these are, and/or don't know which breaker or fuse controls which circuit, trip the breaker or unscrew the fuse and note what goes off and/or won't turn on.)

[LCHIEN]

yes, as Lrry says, find out what branch of the circuit breaker panel your equipment is on (the brute force way is to put a light on the outlet and turn off the breakers until the light goes out, then leave that one breaker off and plug in the light ot nearby outlets and find out which outlets are on that breaker). Then look at the number etched on the end of the breaker handle.

One work is that equipment do not always pull the amperage listed on the nameplate.
For example the BT3000 saw probably says 15 Amps but it you plug it on and turn it on you will see about 5-6 amps while it runs. Only when you load it down and mkae it do some work will the amperage approach the maximum.
Different appliances will work differently. Some like vacs and DCs will use the nameplate amps continuoulsy while running. others like saws and power tools are very suject to applied loading. Lights, draw full power depending on the bulbs installed, while compressors, air conditioners, heaters and refrigerators run intermittently drawing max amps when on but may be on and off due to thermostatic or pressure controls. They may also switch on unpredictably causing situations where they trip the breaker when more than one comes on and other equipment is in use.

[JimD]

Looking at the breaker is the best way but the size of the wire and the outlets also will tell you if it is a 15A or a 20A circuit (assuming its 110V). If you have some #14 and #12 wire laying around to compare to (or you can buy a foot at HD or Lowes) and you pull the outlet out far enough to see the wires, it is fairly easy to see the difference. Most outlets also have molded into them the amp rating. #14 wire is for a 15A circuit and #12 is for a 20A. You can use bigger than you need to but that is pretty rare.

Jim

[rnelson0]

(the brute force way is to put a light on the outlet and turn off the breakers until the light goes out, then leave that one breaker off and plug in the light ot nearby outlets and find out which outlets are on that breaker).

Use a radio. You can't see a light from two floors down, but you can hear the radio if you crank it!

In general a 15A will work for most of your tools as long as you don't turn them all on at once. I did 20A circuits so that I could run a few things at once if I ever have the need. The other thing is that motors generate a huge load when they start up but then settle down. Even a brief moment of 16A will trip your 15A breaker, even if it drops to 5A in the next millisecond.

Check for manuals online. If you're looking at bulbs, you take the Wattage and divide by the Voltage to get the Amps. Normally, this means W divide by 120, so a 120W bulb is 1A of power. Those in the UK need not apply

[balzac.thegreat]

Well...it was easy to tell.

All the circuits are labeled 20. Except for 2; one of which is labeled 40 and another of which is labeled 100.

[LCHIEN]

Use a radio. You can't see a light from two floors down, but you can hear the radio if you crank it!

.... Even a brief moment of 16A will trip your 15A breaker, even if it drops to 5A in the next millisecond.

...

Good idea about the radio.

Completely wrong about tripping a 15A breaker in a "Brief moment"

The following is a typical breaker performance specification for a general purpose breaker from Square D.
At the end are charts (starting page 29) showing trip time range vs. normalized current.

http://ecatalog.squared.com/techlib/displaydocument.cfm?id=0730CT9801&action=view

16A would be 1.066 of nominal current rating.
If you look up 1.066 on the horizontal axis (normalized current load)
and go straight up to the shaded trip point curve, you'll see that
this overload will trip the breaker no sooner than 150 seconds and perhaps much, much longer. Its only a 6% overload. In fact the curves tell you that their 15A breaker is really a 15 to 18.5A breaker, e.g.
no less than 15, or nominally 16.75 +/-10% ( everything made has tolerances). In this case, it may actually be able to pass 18.5 Amps indefinately.

Breakers are designed to allow small overloads for long periods of time and modest overloads for short periods of time but trip Quickly on large overloads. But even large overloads (10 times over) still can take up to 16.6 milliseconds, certainly not guaranteed in one millisecond, if you look at the curve.

I go into this is greater detail in this post

http://www.bt3central.com/showthread.php?t=26187&highlight=breaker+curves