I'm trying to figure out the maximum wattage my breaker panel can supply. If I look at the breaker panel I see 2 large breakers, one with 40A and one with 50A (plus the test button, etc). I'm going to assume the 50A is the main circuit breaker (the kanji reads chief utility breaker I think). Not sure what the 40A is for.

It's also rated for 100/200V apparently, so by my calculations that would be 5000W maximum capability. So would running one high-end gaming PC, a microwave, aircon and a hair-dryer trip it?

For reference, I'm in east Japan which runs 100V at 50Hz.

I've taken photo's of the breaker panel from left to right, 3 photos in total for clarity of reading the kanji and figures.




On the right picture, (1) - (4) are for sockets. (Centre, South, North and ???) (7) - (11) are for Aircon Units and (12) is for the interphone system. (5) I'm not sure of, and (6) is for an Electrical Oven Range... but this place has a gas range.

  • 1
    Not familiar with the normal setup there. Picture might improve the question, though local knowledge would be the best input. IF the power delivery is similar to the US model (but no real reason to think it is) you might have 200V at 50 A split into two legs of 100V, so you'd have 10Kw available.
    – Ecnerwal
    Jul 28, 2015 at 14:23
  • I'll get a picture of the breaker up tomorrow. The kanji would be confusing, but perhaps the numbers may help indeed.
    – Elarys
    Jul 28, 2015 at 14:43
  • Try to get a good photo of the kanji, that may also help (if it's labeled, must be for a reason).
    Jul 28, 2015 at 17:32

2 Answers 2


OK, I am not 100% percent sure of this, but what I understand from the labeling on one hand fits in with what I know of electricity on the other. So, let's say 98% sure.

40A : this is the circuit breaker itself, standard three-phase AC.

50A : this looks to be a residual current breaker, rated 30 mA.


  • If something in your house has a short circuit between live and neutral and starts sparking, the 40A breaker should trip protecting you from starting a fire. 40A is the maximum draw you have.
  • If, on the other hand, you have a short circuit to earth -such as live current getting in contact with an appliance's external surface- then the residual current breaker cuts in.

The residual current is there mostly to protect the user from becoming the shortest path to earth, or in other words getting electrocuted. This is why it needs to be at a very low differential rating of 30 mA. It also makes sense that it has a higher current rating (50A) than the main breaker, since we want to make sure the main breaker trips before the residual current breaker gets damaged.

To complete the individual circuit identification:

(4) is for the washing machine

(5) is for the fridge

The other circuits are as the OP indicates.

To answer to original question: 40A x 110V gives you 4400 VA. Since this seems to be a 3-phase + 1 neutral setup, you will in reality have three times the current, 40A on each phase. So, in total 13200 VA.

This is not quite the same in Watts, but with a hypothetic cos(phi) value of 0.95 you should be good for some 12000+ W. You should be OK with the devices you want to connect, as long as the circuits are well balanced (equal draw on each phase). I would suppose this is so -the installation looks really professional quality- and try it out before requesting changes, since adding more power will mean changing the main breaker and the residual current breaker (as well as paying a larger monthly charge).

Hope this helps.

  • You mentioned that it was three phase, so you need to take that into account for the final power calculation. It also depends on if it's delta or wye connected... For delta, I figure 7 kW, and for wye, 12 kW, assuming 3 phase 100 V, with 40 A line currents.
    – Pigrew
    Jul 30, 2015 at 18:38
  • @Pigrew I took a second look at the labels on the 40A breaker, then a third even longer look. It sure seems like they are using a Y connection, with 3 phases + 1 neutral - at least, this is how I interpret "単3中性" on the residual breaker. Have corrected my power estimate, which is now more agreeable with the amount of A/C lines that have been installed.
    Jul 30, 2015 at 20:52
  • Sorry for the super late reply. This has been an incredibly useful post and answer, my difficult remaining will be to identify the phases. But this has been a great starting point.
    – Elarys
    Nov 9, 2015 at 0:04

Ok, so this is completely rough(and most likely extreme) numbering, but might help...

If you don't already know this site (or a similar one), you can probably get more accurate numbers from it yourself: http://extreme.outervision.com/PSUEngine

It's a wattage-per-part calculator. Not the most up-to-date, but close enough to get the range roughly right.

Some quick clicking(not particularly paying attention to it being possible to fit this in a case or not):

  • dual 5960x
  • quad TitanX
  • quad flash SSD (assuming raid 0)
  • dual 10k sata HDD (again, raid 0)
  • watercooling
  • the usual assortment of extra cards/drives/fans/lighting/...

Gives me roughly 1700-1800W. Let's say 2000W just for the sake of argument.

Realistically, your machine is probably much less than that, assuming relatively new parts(600-800W power supply?), but that should give roughly the worst-case scenario.

I'm also going to assume for the sake of realism that the computer and the microwave aren't running constantly, and probably rarely run at the same time at all.

Even if that is too much for the circuit, you're going to find out as soon as you turn it on and it tries to pull max load. Lucky you, you have breakers... Plug the computer into a different circuit(preferably lighter-use), flip the breaker back, try again.

[Edit: Just realized it's also not a sane build money-wise, but hey :) ]

  • Useful information for sizing a computer's power supply, but I'm not sure that you addressed the actual question that was asked.
    – mjohns
    Jul 31, 2015 at 12:57
  • Not directly, but with @Alan's post, might give an idea... I'm guessing power requirements are probably on a sticker/etc on the other appliances in Japan, same as in Canada, so the only unknown would have been the computer.
    – s1ns3nt
    Jul 31, 2015 at 14:34
  • Actually just a quick note on the realism thing, with a wife and a baby around; I'm likely to have even more devices running. Calculating the power draw for the PC is interesting. I had planned to have 2 Dual-Xeon Workstations running, with a NAS Server with around 20-30 SSD's in addition to the 2 gaming rigs. Seperate rooms (different phases) may be best for this it seems.
    – Elarys
    Nov 9, 2015 at 0:08

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