# [Beowulf] powering up 18 motherboards

Bari Ari bari at onelabs.com
Thu Feb 17 16:36:07 PST 2005

```Alpay Kasal wrote:

> Hello all… I have a question about powering on motherboards simulataneously…
>
>
>
> I have 18 identical mobo’s right now with identical ram, cpu, and hard
> disk. I hooked one up to a kill-a-watt and found that it draws 140-150
> watts when powering on, and stays level at about 90-100 watts
> afterwards. The problem is that I am setting this up at home, where I
> only have 10 amp circuits (and only a couple of them can be freed up).
> Correct me if I am wrong here please…

10 amp circuits? What country is this in? What line voltage and
frequency? What's the wire size and insulator?

> 1 mobo = 100 watts / 115 volts = .87amps each mobo while steady on
>
> 1 mobo = 150 watts / 115 volts = 1.3amps each mobo while turning on
>
>
>
> I won’t include the rest of the math, but needless to say, it’d be a
> pain in arse to turn on the room in piecemeal without tripping a circuit
> breaker. My questions is :
>
>
>
> Will a heavy duty UPS aid in getting me through powering up the room? I
> don’t mind splitting up the 18 machines with 6 outlet surge strips. Any

Circuit breakers are designed and initially calibrated to run with a
continuous load of around 80% of its current rating. Most electrical
codes limit continuos loads on branch circuits to 80% of the current
rating of the conductors and current protection. (This is an over
simplification. The NEC is far more complicated on this.)

Using your rough numbers of 100W/mobo, you're just under 80% with 9
mobo's per 10A/115VAC circuit. The startup current is 17% over the
rating of the circuit breaker. The breakers will hold for a short time
at this load. How long is dependent on the brand and age of the circuit
breaker.

If you stagger the startup of the mobo's on each circuit (let's say to 5
and then a min. later the other 4) it will help to keep the breakers
from tripping.

-Bari

```