I've got to put a number on the sort of load a particular model PC represents in order to determine the appropriate ampacity of the branch circuit feeding them.
How reliable would the reading from a current clamp be in quantifying the load as seen by the breaker feeding the circuit?
I suspect staggering the application of those loads will go a long way towards making the breaker happy (i.e., putting them all on a single outlet strip with a "master power switch" would be asking for trouble?)
Any other advice/suggestions? Looking at the PSU's in the machines is pretty useless as that only determines an upper limit about what they *might* eventually draw over their lifetime (accommodating additional boards, disks, etc.). And, of course, measuring one particular machine isn't going to tell me much about some *other* machine...
Can you attest to their accuracy regardless of load characteristics? And, how their "measurements" relate to the rating on a typical circuit breaker? (i.e., have you just traded one problem for another?)
The devil is in the details. Based on the Kill-O-Watt recommendation below Here's what my 2.8GHz. quad-core intel does.
State Watts Volt-amps off 3 7 booting 120 174 Idle 88 132 Sleep 10 20
A newer system would more likely have a power factor corrected power supply. I don't have anything that new.
That doesn't include either of the two 24" monitors.
93W 95VA each. Or the desk lamp. Or the electric heater I turn on to keep my feet warm when it's cold. Or the laser printer when it fires up.
A current clamp can give you accurate results. Just be sure it's one with a spec and it's measuring what you want to measure. They don't all measure RMS. If you care about peaks, RMS may not be what you want.
I've compared my Kill-O-Watt to my Valhalla 2101 Digital Power analyzer and found them to read almost exactly the same for the power factors encountered here.
How much averaging are we talking about? Averaging over 300 devices is more likely to work than trying to average over 5.
I don't think you have much opportunity to average the peak load that the breaker cares about. There are events that make everything happen at once. 4AM virus updates. 1PM when everybody comes back from lunch. 8AM when they arrive. Power glitch that reboots everything. Heat in the wire won't be as much a problem, but the NEC won't let you put 15amp wire on a 20amp breaker anyway.
Lost work for 15 people due to tripping a breaker is VERY expensive. Your job won't survive many of those.
Back when I was trying to figger out the electrical code, I ran across some anecdotes. Don't trust me, look it up... but as I recall, 3-phase building power is designed around resistive loads. With low power factor loads, like computers, there's a dramatic increase in the current in the neutral. Fires are not uncommon when old buildings get updated with computers. Someone with the actual facts can probably weigh in on this. Looking at the PSU's in
My experience is that most computers needed by an office worker draw about 150 watts or so..just the computer box, not the display and other peripherals. And it's easy to sample what you have now. And there are green forces trying to drive that down. Yes, you can easily build a system that takes 10x that much, but it should be obvious if you have any serious computing going on.
I think your best bet for general advice is a computer vendor. Dell can probably give you case studies covering a wide range of scenarios.
AGAINST WHAT??? Even if I test against a set of 0.1% resistive loads covering a 0-20A range, that doesn't tell me how this correlates to what the *breaker* thinks "trip current" should be!
Circuit breaker is *the* standard. Doesn't matter if your meter claims 19.032A are being drawn and the breaker *shouldn't* have tripped.
I don't have anything that *efficient*! :> (gobs of fans, redundant power supplies, 4-16 spindles, etc.)
I first have to determine what *else* is on the circuit(s) in question. I'll pay them a visit tomorrow and start poking around. Of course, difficult to do when folks are actually *using* the office ("Gee, I wonder what's on
*this* circuit..." "Hey! Who turned off the coffee pot?!")
What I care about is whatever the breaker will ultimately base "its decision"! I don't want to start plugging in PC's to see when/if the breaker trips -- anymore than I'd plug in electric frying pans to determine how many I can support on a given counter circuit! :>
Staggering, not averaging. I.e., don't put all machines on the same "outlet strip". Don't let machines automatically restore power after an outage. Etc.
E.g., when I spin up the drives in my arrays, they don't all try to apply power to the spindle motors simultaneously. You can tolerate a few extra seconds in the power up sequence for the array. And, you can tolerate a few extra seconds in the powering up of multiple computers!
These are used by students. Unlikely that they'll all decide to "drop in" at the same *instant*. Also, I suspect someone else will turn things on in the morning and shut them down at night.
Don't let machines restart unattended.
Not my job :>
But, that doesn't mean I don't want to consider factors that would make them unhappy with the results ("pro bono" doesn't imply "incompetent")
I *believe* they occupy an older, remodeled building. But, I haven't yet looked to see how "current" the wiring is. On my last visit, I had other primary concerns (hadn't even been inside the facility). I will be more observant, tomorrow.
(And, better equipped to get into more pertinent details)
Yes. The problem lies in the fact that people take power for granted. I.e., replacing a 17" monitor with a 21". Adding another printer so B&W output can use cheaper supplies. Tacking on USB accessories. Adding a second disk drive.
When you are dealing with *one* machine, its no big deal (usually). But, adding a second disk to a dozen machines is now an extra amp on the supply...
If you "upgrade" and have a say in that "upgrade"...
Common mains circuit breakers aren't precision devices by any means. There is a time delay built into most mains type breakers. The amount of overload has a significant effect on the length of time required to trip. Just because your breaker has a 20A rating doesn't mean that it will trip immediately if you draw 21A through it. In fact, a small overload could take 1000 seconds or more to trip the breaker. The higher the overload, the faster it will trip. Read the discussion in the paper at
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and check out the graphs relating current to trip delay.
I have a couple of the Kill-O-Watt instruments and have found that they agree pretty closely with my RMS current clamp meter. Besides, they give you instantaneous RMS and V/A readings (with a button press). Pretty good value.
Common sense says that it's not a good idea to load a circuit too closely to the breaker rating. You'll likely be bothered with annoying tripping when devices are turned on in quick succession.
** Providing it is a " true rms " clamp meter AND you open up a three core lead so the clamp goes around only one conductor - it will give you the long term current that operates the thermal part of the breaker.
** The inrush surges produced by most SMPS are a real hazard for breakers, quantifying them and relating that to the published curves is a nonsense idea.
IME, if the RMS current of the total load is close to the breaker's rating, then the inrush surge will trip the same breaker at least occasionally.
The inrush surge even with a given SMPS varies over a wide range, depending on the part of the AC cycle existing at switch on AND the recent history of use.
It also depends on the characteristics of the "overload".
Code requires breakers to be sized at 125% of continuous load. So, a 20A breaker is supposed to see a 16A load.
I don't want to discover that what my current clamp sees/reports is vastly different than what the breaker "thinks" the load is like. E.g., if my readings show 1.2-1.5A and there's nothing else EXPECTED to be on the branch circuit, I can be reasonably confident that all of the loads can be supported (provided they aren't treated as a *single* load switched simultaneously).
Once I've had a closer look at how things are laid out and alternatives that they have for equipment siting (e.g., move the laser printers to another location/circuit), I'll be better able to evaluate what they can and can't do, safely and reliably.
("Well, we have 10 laptops here, why can't we replace them with 10 desktops??" etc.)
I'll see what sort of numbers I get tomorrow. I can always make an educated guess seeing what's inside the box and how well that correlates to "measurements".
Most important thing is to evaluate alternatives so they have
*a* solution -- even if it is not the solution they originally expected.
On a sunny day (Sun, 6 Oct 2013 16:34:27 -0400) it happened "tm" wrote in :
I second that, been using one now for a year. I did use it to see the difference in power consumption of various PC configurations (plus monitor).
I doubt one can do that in a cheaper simpler safer way for the same money, it is a mass product. The total over a year about matched what the big meter says.
Note "rating" on a circuit breaker is NOT one number - most especially not the tag value! Reasonable ratings of circuit breakers are like those of fuses: a set of curves, time VS current.
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