Actual amps vs. what is stated on the power supply

Question

I'm not very knowledgable about EE so I apologize if some of my terminology is off. At work, I need to set-up a bunch of (100+) wireless routers for a test farm and I'm trying to figure out what kind of power strips to use to power them all up.

On the power supplies, I'd say the average amps is between 1 - 1.5. Most consumer-grade power strips for 120v top out at 15 amps. I'm trying to maximize how many routers I can power with each strip. Is there any way to guesstimate the amps vs. the figure that's stated on the power supply?

The routers will most likely have their wireless radio turned off and will be mostly doing general processing and read/write stuff.

Massive thanks!

edit: There are 20A power strips also, but it's still fairly restrictive.

Answer 1

Are you sure your routers take 1.5 amps at 120v? It seems far more likely to me that they use 1.5 amps at 12v or less, which means your AC power strips only have to supply say 0.2amps per router.

Answer 2

How lucky do you feel? What is the cost of being unlucky?

You can play games with likely average power versus maximum specified, but there is a risk in that. The totally safe way is to assume each device will always draw the maximum specified current. Of course they most likely all won't, but the problem is you don't know how far below the maximum they are most of the time and what the worst case really is. You can try to measure to get typical values, but you still don't know from that what the worst case is or when it might occur.

To paraphrase Dirty Harry: Do you feel luck? Well, do you, punk? Go ahead, underrate your power strips. Make my day!

Answer 3

Normally, you would get somebody in to help you with this, because as others have mentioned it's non-trivial. You need to be especially careful with high currents, the same as with high voltages.

As Szymon suggested, perhaps the included PSUs are not the best way to go - they are generally not as efficient as an industrial supply operating at its rated load. Additionally, power strip ratings are generally optimistic, and they are not manufactured with great precision). Do not daisy chain your power strips, ever. Maybe investigate having a custom power system built that will feed your routers?

@Olin, while I would agree that the worst case is the ideal case to use for sizing of system components, in practice determining the worst case is very difficult. Current ratings on components are there to keep the devices (be they power strips or FETs) inside safe thermal limits. Your building's cabling is rated based on the expected heat rise in the conductors for a given load. Almost all building systems are sized based on average load (transients can be ignored because they do not provide a significant heating effect on the cables). Of course, determining average load is the hard part. If it were not this way, you could never cable an office block where people all come in and turn on there computers in the morning...

For the purposes of answering the question, average load could easily be measured using a multimeter on the 12V side and watching the current for a few minutes. (You could use a datalogger if you are so inclined, but it's probably overkill). Then multiply this number (say 1.5A@12V) by the number of devices you wish to power and you'll end up at a magic number of 150A@12V or so. I doubt your routers with the radios off will draw anywhere near that current, btw.

Then you need to check that your building can supply the required current where you need it (say 20A@110V, 10A@230V). You will need a dedicated AC circuit for this project.

Answer 4

Computer power supplies have strong 12V lines and are reasonably cheap. 1kW OCZ Fatal1ty will give you 83A.

Instead of one strong PSU you can use few smaller ones. Cabling will be easier and old PSUs are dirt cheap.

Answer 5

Confirm Power (Volt-Amps/Watts?) used by one router, divide by Input Voltage to get Input amps. Lastly, don't exceed 80% of any OCPD (Over Current Protection Device< fuse or breaker) or the power strip rating.

As far as facility's electrical system, all you have to do is map your outlets/circuits and make sure you are not overloading any single 120V circuit & breaker. I suspect that good grounding, or even isolated grounding, might be a concern. Avoid 'Noise" and Surges.