I'm preparing to build the computer of my dreams. Unfortunately, it's going to top out at 1350 Watts when running at peak performance. I live in an 80 year old bungalow and the power situation is less than optimal. Our circuit is rated at 12 AMPs. As I'm still renting, I'm not sure I want to invest in upgrading the circuit (or at least I'm weighing the tradeoffs). Are there any other options (i.e. battery storage, UPS, solar) that might be possible.

All the best/

  • 2
    Where in the world are you? 12A is not a circuit size typically used in the US (15A is the minimum for general purpose receptacle circuits). OTOH, if you're in Europe, you have 240V power, and 12Ax240V=2880W, more than enough for your computer.
    – Nate S.
    Dec 3 '19 at 17:48
  • How are you getting to 1350 watts? Even a top of the line system should be half that, and monitors aren't that bad either. Dec 3 '19 at 17:50
  • 4
    @user3757614, I'm guessing with multiple-GPU compute capability, i.e. for bitcoin mining or heavy simulations -- those are pretty power hungry. Either that, or OP bought a 1350W power supply, and assumes the system will use all of that capability.
    – Nate S.
    Dec 3 '19 at 17:54
  • Also, how much power does your computer actually need, and how efficient is your current power supply? You might be able to replace that power supply with one that is less powerful and/or more efficient, and then you could use your current circuit.
    – Nate S.
    Dec 3 '19 at 20:47

TLDR: Pay your landlord to add a 20A dedicated circuit for this PC.

Fuggedaboutit. 1350W is just too much.

Remember, you're talking 1350 watts at the low voltage DC side - you have conversion losses. You must provide what the AC-side requires. That will be clearly stated in the specs, or placarded on the unit. Here's a Rosewill Photon 1350.

enter image description here

Let's be gracious and assume this mark is saying 15 through 8 amps, not 15.8 amps. That is the actual power it will be drawing. 15 amps = 1800 watts, which reflects the internal inefficiency of the supply, and that's about right. This is a continuous load, so you must provision 125% of that power, so that's 18.75 amps. This will take the entire output of a 20A circuit.

You don't have one of those.

If your "12 amp" figure is based on a 15A circuit you are applying the same 125% derating to, then good on you for working it backward like that. Regardless, a 15A circuit cannot power this. Running a circuit at 100% load continuously will result in breaker trips, which results in death. Permanent death if you're playing Diablo hardcore.

Beware old service panels

If your 80 year old building has a Federal Pacific, Zinsco or Challenger panel, then maxing out a circuit is madness. Those breakers are not reliable. This could result in fire, and perma-death of you.

A Challenger panel can have its breakers changed out to Eaton BR/C, (C for Challenger), and it'll be fine. Zinsco and FPE are beyond recovery, as not only are the Connecticut Electric breakers worse than the originals, but the buses themselves are defective.

Landlord permission + licensed electrician is mandatory

You cannot alter the wiring in your rental unit. I live in one and the most I've ever done is change a cover plate. All work requires landlord approval because of a legal concept called "mine / yours". And all work in a rental unit must be done by a licensed electrician, to disallow low-quality work by cheap slumlords or sloppy tenants. So if you want to upgrade the electrical service to the unit, you'll need to have a sit-down with the landlord and pay a 3-digit sum for the electrical work. Or move.

Another snag is that older buildings are often laughably underpowered. I have seen units with 30A/120V for the entire unit. You will not be able to add a 20A circuit to such a unit.

Further, if the landlord pays for your electricity, expect a rent increase that may not be fair, because the landlord does not want you bitcoin-mining.

Don't forget air conditioning

Putting 1800 watts into a room will make a lot of heat. 5300 BTU/hr to be precise. You will need an equivalent amount of A/C just to break even. That will need its own circuit, since sharing the circuit with this PC is right out.

  • 5
    Actually, it isn't 15 through 8 amps or 15.8 amps. It is 15 Amps at 115V, 8 Amps at 240V. Which is actually messed up rounding - I'll bet someone said "15 @ 115, so 7.5 @ 230" --> "Can't show a fraction, so round up to play it safe" --> "8 @ 230" --> "Oops, it isn't 230, it's 240" --> "8 @ 240". Whereas if they talked to the engineers, they likely would have said either "It isn't really 115 any more, it is 120" and/or "Actually, @ 240 it is a little more efficient and 115 * 15 = 1,725 / 240 = 7 and yeah, 7 is good enough". But in the end, it really doesn't matter - 15A = 20A circuit. Dec 3 '19 at 20:21
  • 1
    Is it in the code that a computer's power supply nameplate rating must be treated as a continuous load of that value? Because that's not really how they work in reality -- the supply will draw only as much as the computer needs at that particular point in time, up to a maximum value of the nameplate rating, but often 90+% of the time it's drawing dramatically lower. I'm pretty sure that power supply is intended to be run on a standard 15A circuit.
    – Nate S.
    Dec 3 '19 at 20:21
  • 2
    The IEC C14 inlet is only rated for 15A anyway -- power supplies that need more than 15A use an IEC C21 inlet instead.
    – Nate S.
    Dec 3 '19 at 20:23
  • 2
    The nameplate doesn't say 'continuous' though -- I'm not sure the 125% derating is needed, since it does not truly draw it's nameplate current continuously. For most such computers, if you averaged their power draw out over 24hrs, it would be more like 600W or less.
    – Nate S.
    Dec 3 '19 at 21:00
  • 2
    @NateS-ReinstateMonica Nameplates don't decide whether a load is continuous. Usage does, unless Code overrides. What you say of "most" computers is true, but Code doesn't have anything waiving "all" computers. This one's for gaming, and 3+ hour gaming sessions are common. So it would come down to developing a use-case, supported by measurements. I'd put a Kill-a-Watt on it and max the system out, and see what happens. Dec 3 '19 at 21:19

Best suggestion I can think of is to do what you don't want to do which is upgrade the circuit. I realize you specifically say that you don't want to do this but if you're living in the US you will be too close to popping the breaker to call it safe. Where I work we always leave a decent margin of safety to keep from overloading the circuits. If you still don't want to upgrade the circuit I would suggest a UPS just in case you do blow the breaker. However, if you're running this in not the US where you have a 220v 12A you shouldn't need to worry about any of this because you'll be well below the draw maximum of the circuit.

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