Electrical Q: What's the biggest load I can put on one house circuit?

Factual Questions
1

I’ll soon have a 21" monitor (used, but still… yay!) and I’m starting to wonder if I might not be reaching the load limit for my so-called “computer room” here. My machines aren’t fast, but I presume they use the same amount of juice anyhow, and there seem to be… five. Or six. Up to seven depending on my work. Only 3 monitors so far, soon to be 4, as well as associated scanners, speakers, printers, etc. And the TV.
The TV is probably 150 watts, the new(ish) monitor specs say 160 watts, the power supplies average 275W each, but I’m guessing they don’t draw that all the time. About now is when my electrical knowledge runs out. To me this looks like 2500 watts. Which I think is alot, since the 1500W electric/oil heater upstairs dims the lights when it’s on.

Now, I’m given to understand that volts * amps = watts, giving 120V * 15A = 1800W max - which I appear to be already over right now. So assuming I have only one 15 amp breaker serving only this room, should I be worried? And if so, what can I do about it?

2

I’d make sure everything was plugged into powerbars with their own breakers, for starters.

The computer’s power supply doesn’t need nearly all of it’s available 275 watts in normal operation. Even the monitors and television don’t burn a steady 160/150 watts unless you have their brightness and/or volume cranked up to maximum all the time.

I’ve got 3x20" monitors, computer with 300W power supply, television and a 500 halogen lamp typically running at the same time in my office, and when summer gets here, I’ll no doubt be running the ceiling fan or air conditioner. I haven’t had a problem with tripping breakers, but then again everything is plugged into various outlets and my house’s wiring is pretty messed up anyway, so it’s doubtful I’m overstressing any one circuit.

Are you sure all the outlets in your “computer room” are on a single circuit? If you avoid bringing in heavier appliances like blenders, space heaters and air conditioners you should be fine.

3

Some things in your favor:

UPS’s will detect a voltage drop and supliment with battery power, this will ‘soften’ the start up requirement of a computer and monitor. Then again some modern USP’s have voltage regulation devices which will draw even more current to make up for a low voltage condition (which very well might happen during high current loads)

IIRC A circuit breaker can tolerate some overload conditions to allow a high starting current. Perhaps a 15 amp breaker could supply 17 amps for 4 seconds before tripping, but trip almost instantly if a huge load (short) was applied.

If it is a problem you can look for an outlet on another breaker, even run an extention cord from another room or plug somnething in another room on that circuit to another outlet (on a different circuit).

If all else fails you may be able to replace the 15 amp breaker with 20 amp. Well you can do it physically, but you would have to find out is it safe and allowed to do. IIRC 12 gauge copper (10 gauge for Al) wire can safely handel a 20 amp circuit. If this is the case you should be able to up that circuit breaker to 20 amps. Also just to point out that the chain of outlets should also be supplied with 12 gauge wire too. IF you have 14 gauge copper (or 12 AL) then the highest you could go would be 15 amps*****
*******IANA Electrician, I have told you the ‘rules’ as I understand them 2nd and 3rd hand, If you take my advice you MUST verify what I said with someone who knows the code. I’m sure someone on this board might be able to give you any info you need to actually do this.

4

First of all, don’t assume anything about the electrical service to the room. There could be two or more breakers assigned to the room or, on the other hand, only one that serves the entire room and other areas of the house. Find the breaker box. If you’re lucky, the circuits are clearly marked as to which breaker protects what area. If not, turn the breakers off one at a time (ignoring any that are rated higher than 20 amps) and check the outlets in the room to see if they’re still hot. When you find one that kills one or more outlets in the room, check outlets in surrounding areas. Make note of which breaker does what, and the amperage rating of each breaker (clearly marked on the breaker itself). NOW you have some knowledge.

Whether the monitors/TVs draw their full wattage or not, they’ll draw steadily when they’re on. For the sake of this discussion, let’s assume 150 watts per, for a total of 450 watts.

An average PC will draw in the neighborhood of 200 watts. This rating assumes a normal assortment of peripherals (HDD, CD drive, floppy, CPU, 256 MB RAM, AGP video, coupla PCI cards…) and a power supply efficiency rating of 75%. It also assumes that every component is running hard simultaneously, an event that is rare to say the least. Though power needs can vary somewhat from PC to PC (Athlons need more juice than equivalent Pentiums, e.g.), I’d say you can figure each PC is drawing an average of 100-150 watts.

Most external peripherals, such as scanners, speakers, printers, etc. draw tiny amounts of current, generally from 5 to 30 watts. One exception to this statement is laser printers, which can draw 200 watts or more when they’re actively printing. (Inkjets use only about 25-30 watts.)

Finally, don’t be fooled by the rating on your speakers! Most computer speakers use a PMPO (Peak Maximum Performance Output) rating, which is meaningless in the real world. RMS (Root Means Squared) is a much more realistic measure of maximum output, which is proportional to power consumption. For example, el cheapo PC speakers typically boast a PMPO rating of at least 160 watts, but actually put out something less than a watt RMS.

Hope this helps, and happy 'puting!

5

One important item left out here!!! The outlets themselves have amperage ratings, and upgrading the circuit to 20 amps would require 20-amp outlets as well as wire of a sufficient guage.

6

The links in a curcuit are the load,the outlet,the wiring, the fuse—or breaker.

Back in the days when we only had the “seat of our pants” as a guide ,the rules were simple.

When you’re all loaded up in your work area,and have been for some time,start feeling things.

Any cord or extension that’s significantly warmer than ambience is borderline trouble.

Feel the cover plate on the outlet to check the temp.

Locate the fuse,or breaker, and check them ,and the wiring to them ,in the same way.

If you have to increase breaker/fuse size------y’d better look into increasing your wire size.

OR-----pull in another circuit and split the loading.

If you do have component overheating -----it might just be sensible to leave the front door open for the firemen who will probably ,eventualy, show up.

Calculations,meters and theories arer good tools-------but your hands and your nose are even better------if it’s warmer that it should be-------or has an strange odor about it----------ACT!

7

Slight hijack.

Yea, I’m well aware audio companies like to quote “RMS Power” in their specs. But this is incorrect, since there’s no such thing as “RMS Power” in the strictest (i.e. engineering) sense…

8

I’m not sure if you have already gone this way, but what about a KVM switch? Allows you to have only one keyboard/video/mouse. And the savings of juice of just one monitor can be of help.

9

In an ideal world, the breaker will let you know when you go over the limit by blowing and protecting your wires. At which point you’d have to think about either shutting down some of your equipment or running a new branch circuit to your computer room. (Not a bad idea anyway.)

We don’t know, however, anything about your existing electrical system. If it’s an older house, the wiring may not be up to snuff. And it’s never a great idea to rely solely on the breaker. So consider running a new branch circuit anyway.

10

I have 3 PCs, 2 printers, 2 - 19" and 1 - 21" CRTs, a scanner, 3 halogen lamps, a router, cable modem switch, 3 large PC speaker systems all with subs, a outboard hard drive, and occasionally a complete 4th system all hooked into one 15 AMP circuit and not even a hint that it’s overtaxed even with all systems on at once.

I have one little 1500 watt cube heater on a completely separate circuit that pops the 15 AMP breaker with regularity.

Go figure. PC’s and peripherals do not normally draw anywhere near their stated max wattage capacity in normal use.

11

Absolutely, positively do NOT do this, unless you first have a qualified electrician look over the wiring and tell you it’s OK. He will probably not tell you it’s OK, though, because if it were, there would have been a 20 amp breaker put in in the first place. In fact, it’s possible that you already have a breaker at higher than the rating of the circuit, so it’s not a bad idea to call that electrician in, anyway.

If a circuit in your house is inadequate for your needs, your only safe options are to decrease the load on that circuit somehow, or to have the circuit re-wired. Doing otherwise is just asking for trouble.

Even if you’re personally willing to live with the risk of burning your house down, you should still keep your house up to code, since it’ll make a big difference in your insurance.

12

Thanks all. I’ll confirm the current wiring, as if the room is split wrt. breaker source, it would be better.
If that’s not the case, I’ll get one of my friends who can handle more than 12V to see about adding a breaker. The current wiring goes through a hanging ceiling, so I should be able to copy what’s there now.

Think I’ll also ask the guy at Revy about any requirements in that area. Thank you! :slight_smile: