How much is too much? - Electrical Power Question

[Jay]

I will be adding a DL145 server with 2 x opti 280s to my home rack and I am now a liitle worried about how much power I can draw from a double socket!

I will have...

Rack - Plug 1 - Total Watts aprox 550

1 x HP ProCurve 1810-24 switch - 30W
1 x DL145 2 x Opti 280 2 x SATA disks - 270W (guess)
1 x QNAP 439 Pro II+ NAS (Atom / 4 x 2TB SATA) - 33W
1 x Cisco 1841 Router - 45W
1 x Draytek 2820 Router - 18W
1 x IP Camera - ?
1 x Cisco WAP4410N Wireles AP - 10W
1 x UPS 700VA -?
1 x Desk Fan- 50W (guess)

PC - Plug 2 - I would say sub 500W

1 x Q6600 @ Stock
1 x 720W Enermax PSU
1 x SSD
1 x ATi 4890
2 x 24" LCDs
1 x UPS (500VA I think)
1 x Creative 7.1 speaker system

I don't actually think both added together would equal 1 fast boil kettle or a clothes dryer but i'm never too sure

[Lucio]

You're usually limited to 30A on any single ring main, and 13A on any one individual plug socket. This translates to ~3Kw per plug socket and 7.2kw across the ring main.

Incidentally plug 2 should be taking at least 500W (since a Volt Amp is just another name for a Watt), and if the battery is rated to output at that, chances are it's drawing at least 625 (if 80% effeciant)

[peterb]

VA is not the same as a Watt (except in DC circuits) - or the load is purely resistive. VA will always be higher than the wattage because the load will usually be reactive. So you should take the worst case and use VA for load calculations. WaTts are real power (which you pay for at the meter) VA are not, but represent real current which has to be allowed for in wiring considerations (because wires have non-reactive resistance and that current causes heating in the wires - and that bit is real energy being consumed)

So in the above calculations, the critical factor is the current on the ringmain/socket with is (usually) 30A/13A which is 7.2KVA or about 5.8KW at 0.8 power factor. The PF is (simplistically) a ratio of the resistive and reactive component of the load.

If you are connecting to a socket on a ring main, you need to take into account other loads on that ring main.

An electric kettle takes between 2.5 and 3 KVA (and as it is resistive that will also be consuming 2.5 to 3 KW).

[Jay]

So at the moment would you say I was safe at that draw?

[peterb]

Yes, probably (without knowing what else is on the ring main)

You could get one of the plug in wattmeters (I linked to one in another thread) that will give you both power and VA.

[Jay]

I did a test today on plug 1 minus the fan and its only 300VA. I did replace the two UPS with one 1350VA device at the current load it will give 20 mins with the DL145 on and over 50 mins with it off.

[BEANFro Elite]

Ring-main circuits are usually 32A, a single length of 2.5mm cable can carry just over 20A, but of course being a ring circuit the current carrying capacity is effectively doubled allowing of course variables such as cable length, atmospheric temperatures etc and is typically able to carry about 40A or there abouts.

Ring circuits can therefore cope better with having clusters of load at certain points without compromising the rest of the circuit.

So in other words, your power requirements are more than catered for.