This question takes place in Japan.

In Japan, domestic electrical service is metered, but there is also a monthly fee that increases with larger service capacity. Units are wired for (typically) up to a 60 A service, but the installed breaker is limited to the rate plan the customer pays for. A 30 A service is less expensive than a 40 A service, and upgrading to a higher rate plan is done with a simple breaker swap which can be done at most once per year.

Rate plans start at 10 A and move in 10 A increments up to a typical maximum of 60 A. Power is single phase, 100 V only, so 10 A = 1000 W, etc.


How can I calculate what service size I need? I would like to select the smallest amperage plan necessary.

My home has a 30 A main fuse installed. The maximum sub-fuse is 40 A. So upgrading to 40 A seems logical. But actually I'd like to calculate the maximum fuse needed if I'd run (all/multiple) of devices. Some like the air-conditioner have a current written on their body. Other appliances just have maximum power and voltage.

Would I be correct in doing the following?

  1. calculate maximum current with voltage and power of each device
  2. summing up the current of all of the devices (i.e. 10 A + 20 A + 7 A + 5 A)
  3. finding thus the maximum current of the main fuse
  • 1
    30A main? I've never seen a service that small. Could you provide some context, or maybe a picture? Is it possible you have a split-bus six main disconnect panel, and the fuse marked 30 is feeding a bank of 15 and 20 amp fuses? Dec 11, 2019 at 19:55
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    @ThreePhaseEel No, it's not like NEC220 at all. OP's question will have a different answer depending on who is living in the apartment unit. NEC220 doesn't care if you plan to make toast and dry your hair at the same time, for example. In Japan, however, it can save you a lot of money if you don't need to do both of those things at the same time. This isn't about service ratings or protecting conductors or meeting any sort of electrical code - it's strictly a budgeting question about what a particular person's anticipated peak loak is going to be.
    – J...
    Dec 12, 2019 at 12:53
  • 1
    @Harper-ReinstateMonica This isn't a code question at all. It's strictly about utilization, not about infrastructure.
    – J...
    Dec 12, 2019 at 13:37
  • 1
    Related : Japan Times - How to conserve energy
    – J...
    Dec 12, 2019 at 14:05
  • 1
    @J... The last paragraph of that article includes the text "Tepco says you can only change your breaker once a year since that is the minimum period for a billing contract," so I guess the try-it-and-see method won't work with Tepco. Dec 12, 2019 at 14:15

6 Answers 6


To answer the actual question here :

The maximum sub-fuse is 40A. So upgrading to 40A seems logical.

No, the 40A device you're looking at is probably this one :

enter image description here

This is a GFI/RCD trip device and its main purpose is to provide ground fault protection for the dwelling. It does have a 40A rating, but this does not impact your user load calculations in any way. What you need to consider is only the applicances you will be using, and the schedule that you will be using them on.

As an aside, the service breaker here is the green one on the left - this is the breaker TEPCO will change if you upgrade your service. I suspect if you ask for a service upgrade to something greater than 40A they will also replace the ground fault unit to match the service rating.

Would I be correct in doing the following?

calculate maximum amperage with voltage and wattage of each device
summing up the amperage of all of the devices (ie. 10A + 20A + 7A + 5A)
finding thus the maximum amperage of the main fuse

You can do this if you anticipate that you will be using all of your electrical devices at the same time. If, however, you are trying to make a better guess of your peak load, you should consider all your appliances and figure which combinations of them you will likely be using at the same time, and then select the highest load use case you expect you will encounter.

For example, if you never plan to use the microwave while drying your hair, you don't need to consider the load of both of those appliances at once. If you're really trying to save on a rate plan, you can also then develop a daily routine that deliberately places certain activities at different times to reduce your peak electrical usage. If you're fine turning off the A/C, for example, while you use other high-load devices for a short activity, this can really cut down on the service size (and cost) for your electricity in Japan.

This kind of mindful load balancing is very much encouraged in Japan, and the steep rate pricing for larger service is very much designed to promote this kind of peak-shaving behaviour. It reduces the peak grid load if everyone is mindful to not dry their clothes and make rice at the same time, so this kind of lifestyle hacking is very much the norm in Japan, especially since the setsuden push following the loss of Fukushima.

There is a good article at Japan Times that discusses these types of strategies.

Service Types

Also, keep in mind that there are two types of common electrical service in Japan. The panelboard in the image above (which we now know looks "exactly" like yours) is showing a two-pole service feed, or what is called in Japan a "single three" (単3) service. "Three" here refers to the number of wires - two hot and one neutral. A 単3 service at 30A provides two hot legs at 100V, so 6kW of total power, or effectively 60A of 100V power. This type of service also allows for 200V loads for larger appliances, A/C, etc (note the 200V breaker top-right above, for example). 200V loads draw from both legs, so they must be budgeted separately.

The other common type of service is a "single two" (単2) service, or single pole, two wire service. This type of service provides only one hot leg and 100V power only. A 30A 単2 service can only provide 3kW, not 6kW like the 30A 単3 service, so when calculating how large your service amperage needs to be you need to be sure which type of service you have. Harper's excellent answer below provides more detail on how to calculate your loads for both situations.

  • 1
    The Japan Times article gave very helpful background on this question. Dec 12, 2019 at 16:23
  • 2
    @Roijan actually I found it to be practically "made of wrong". It essentially advocates for intentionally undersizing ones service, for the express purpose of causing nuisance trips, making people conserve. It's true, Japan is very different... Dec 12, 2019 at 20:02
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    @Harper-ReinstateMonica It's just a question of economics - electricity is very expensive in Japan and infrastructure work can be very expensive in Japan, at least compared to the cost of conductors. It makes sense to provision enough ampacity to future-proof the infrastructure and then use the breakers to actually protect the grid from wild peaks rather than the conductors.
    – J...
    Dec 12, 2019 at 20:47
  • 1
    @Harper-ReinstateMonica Yes, that seems accurate. I find it interesting that in the US, breakers are used to protect one's home, but in Japan it seems they use them to protect the grid as a whole. Dec 12, 2019 at 21:05
  • @RoijansaysreinstateMonica The breakers do have that function also, to be fair, and the branch breakers are all there to protect those conductors. It's only the service breaker that is a load limiter first and conductor protection second.
    – J...
    Dec 12, 2019 at 21:09

The main fuse in your electrical box is put there to protect the wiring that brings the power feed into the panel. The size of the wire conductors determine its safe current carrying capacity so it does not overheat and catch fire. For this reason you cannot just decide to change the size of this fuse without taking the feed wire size into account.

If those feed wires are 10 awg then the 30A fuse is the proper size to protect those wires.

So as you can see, if you need more than 30A total simultaneous current draw from your panel upping the fuse size is not the first thing to consider. Evaluate the feeder wire sizes and where those wires come from. That wire may require replacement to achieve a safe feed over 30A.

  • I updated the question. It seems it is no problem to update it.
    – steros
    Dec 11, 2019 at 22:43
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    @steros - Never assume. Never apparently. Make 100% sure before going to be changing fuse sizes.
    – Michael Karas
    Dec 11, 2019 at 23:53
  • 1
    I assure you the Japanese do that ^^
    – steros
    Dec 11, 2019 at 23:53
  • 1
    @steros: I assume what Michael Karas is trying to point out to you is that 1) it's really hard to actually be sure without having an electrician thoroughly inspect all your wiring, and 2) a priori it seems unlikely that your home would've been fitted with wiring capable of safely handling 40A but only a 30A fuse, since wiring is expensive but fuses are cheap. Although I guess an underspecced fuse might make sense if your local power company based its pricing (partly) on the size of the main fuse. Dec 12, 2019 at 1:05
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    @IlmariKaronen The first rule about Japan is to recognize that everything you think you know doesn't apply there in most cases for just about everything. It is common in Japan for units to be fitted with service conductors large enough to support the highest rate plans offered by the power company. The breaker is then installed not to protect the wires, but simply to load-limit the customer to the service level they are paying for. It's not what you're used to at all. A bigger breaker comes with a higher monthly fee.
    – J...
    Dec 12, 2019 at 13:45

The crux of your question is "How do I calculate my highest possible amp usage?"

Everything else about your post is irrelevant because you will have a certified electrician doing the work.

If you only know the wattage of the product then divide it by the volts and you get amperage. So a 600 watt item such as a microwave running on 100 volts is 6 amps.

600 watts / 100 volts = 6 amps

Small electronics such as cell phone chargers or wireless routers will usually have a special adapter with numbers on it so read those numbers and do the math.

If you have a traditional incandescent 60 watt light bulb then that is running .6 amps.

A 60 watt equivalent LED light bulb is actually using just 9 watts so that's .09 amps.

Do note that items with a motor such as a vacuum cleaner or A/C unit will have a "starting amp" rating which could be almost double its running amperage so if you're running an electric kettle and the A/C then hold off on vacuuming until the kettle is done or turn of the A/C unit.

Some devices might not list any information such as a window or box fan but these typically run on about 50-100 watts so just divide by the voltage and remember that low, medium, and high will have different power draws.

If you're really curious about exact figures and do not want to mess with calculating volts, watts, and amps then buy a P3 Kill-A-Watt meter (or equivalent) and test all of your devices.

enter image description here


  • OP has updated that he is in Japan. So it's 100 v. not 120v Dec 12, 2019 at 14:31
  • @ChrisCudmore Thanks, updated. The voltage wasn't mentioned in the question :-/
    – MonkeyZeus
    Dec 12, 2019 at 14:42
  • Thank you for the answer. Unfortunately I have to accept J's answer in terms of fairness as he actually answered first in the comments already and just turned it into a real answer. I hope for your understanding.
    – steros
    Dec 12, 2019 at 21:44
  • @steros There is no fairness except the answer that is most useful to you - please do accept whichever answer you find most helpful. It doesn't need to be mine!
    – J...
    Dec 12, 2019 at 22:13
  • @steros You should accept whichever answer best helped you to solve your issue :-)
    – MonkeyZeus
    Dec 13, 2019 at 13:30

The first task!!!

The first task of breakers and fuses is to protect wires and downline equipment.

In order to enlarge a fuse/breaker, you must inspect all the wiring and pieces of equipment to ensure it is listed/rated for that use. For instance, take an American panel with a 100A breaker feeding a subpanel whose buses are rated for 125A, fed by wire sized 1/0 AWG. You want a 125A breaker. That checks out, because the wire and subpanel can handle it, as can the wire. However you can't do a 150A breaker because the subpanel can't handle it.

Sizing the main when minimum sizing is valuable

I understand in Japan, you are flat-rate-per-month charged based on the size of your main fuse/breaker. You have incentive to specify the smallest possible. (honestly, anyone sizing an off-grid solar system has the same dilemma.)

To start with, you need to ascertain whether your service is 1-pole (100V) or 2-pole (200V).

In the case of 1-pole service, it is straightforward; you simply do the below calculations overall. However for 2-pole service, it is more complicated. You must ascertain which pole a circuit is on, and do the below calculations independently, for each pole. 200V loads count against both poles at once.

So, per pole:

You need to ascertain which circuits are on that pole (rather easy on a 1-pole service), then determine which appliances are on that circuit. Then, you list all the appliances on the circuit, and compute the practical ampacity of each of them.

Now, here's the tricky part: Group all the circuits into which pole they are in. Now, factor out appliances you don't expect to use *simultaneously** except, (the tricky part) do this across the entire pole, not per-circuit. Say you have a case where circuit 3 has a 13A washing machine. You never use it at the same time as a 10A bathroom heat-lamp on circuit 5. So you cross off the smaller load, and consider only the 13A load - even though they're not on the same circuit, they are on the same pole.

You obtain amps either from the appliance's nameplate amps, or from its "watts" or "VA" figure divided by the voltage - typically your voltage is 100V. However, sometimes, appliance ratings don't reflect actual draw - take a desktop PC. It might have an 850 watt power supply with a nameplate of 12 amps. However, in actual usage, it might only draw 3 amps. You can measure this with a device such as a "Kill-a-Watt", shown in another answer.

I actually recommend going to the extent of labeling circuits, so you can mark which breaker/outlet a load is on. Say, So for instance, since poles are so important here, circuits on pole 1 might get called Thor Hulk Ironman Hawkeye Cap etc. And pole 2 gets called Ariel Belle Cinderella Jasmine Mulan etc. Then turn each circuit off and go see which outlets went dead, and label all those "Thor" etc...

Balancing a 2-pole service

Now, you may discover that your pole loading is lopsided -- you may find your maximum simultaneous usage on pole 1 is 45 amps, and on pole 2 only 13 amps. Or you may discover the washing machine and bathroom heat lamp are on opposite poles, so you are unable to dismiss the heat lamp. It would improve balance if you could move things from one pole to another.

In that case, you (or your contractor) can do just that. However, you can only do this on a per-circuit basis, not a per-appliance basis. Hence, caring about circuit numbers/names.

  • Sorry we already have that answer and it is already irrelevant. I will not check anything, the company will do that. I have the problem to tell them which level of fuse/breaker I want.
    – steros
    Dec 12, 2019 at 6:00
  • I've seen something similar to what @J... describes in France. I believe there's some kind of dial behind the tamper proof door that EDF can set to a higher value if you choose to upgrade your contract (and pay more). If I remember correctly there is a separate panel with another main breaker and individual circuit breakers that you or your electrician would size to suit your wiring.
    – Carl
    Dec 12, 2019 at 17:54
  • @J... I picked that up from Google research about Japanese panelboards. One link for instance said if you had 100V service you probably have 40-70A, and if you have 200V service probably 30A. Also, your pictured panel looks 2-pole, the main and RCD being awfully large for 1-pole... Dec 12, 2019 at 20:04
  • @Harper-ReinstateMonica The pictured panel is, in fact, a single phase 3-wire (単3 on the breaker), but the 2-wire breakers (単2) are pretty much the same size. Fair enough, though - we don't know what's in OP's box, so two pole is a possibility.
    – J...
    Dec 12, 2019 at 20:39
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    @steros I have rewritten this answer for you. Dec 12, 2019 at 20:42

I'm not a professional technician (just a hobbyist), however, two things to take into account:

  • When switching on a motor, it temporarily uses (much) more power (current spike I think it's called). So you have to take this into account. See remarks below for more information:
    • isherwood: most breakers and some fuses allow momentary over-current situations to prevent nuisance trips
    • NoSparksPlease: time delay fuse or standard (inverse time) circuit breaker will hold 200% for 2 minutes
  • However, if you would be able to calculate or measure all current spikes of all devices, than you probably overshoot your goal for calculating the fuse.
  • Also, I think it's not just summing up all currents, but also the safety regulations in your country that decides a maximum power, and the electrical installation and wiring inside your house able to handle that current.
  • @NoSparksPlease That seems not so safe to me; unless of course it trips immediately when crossing that 200% (which probably a short circuit will do). thanks .. I didn't know that; I added your comment (with your credits) into the answer. Dec 11, 2019 at 17:20
  • For fuses page 2 , second paragraph cooperindustries.com/content/dam/public/bussmann/Electrical/… Dec 11, 2019 at 17:29
  • 2
    @MichelKeijzers It's not a case of 200% = 2 mins, 201% = 2 milliseconds. Instead the breaker breaks faster the more over the rated limit the actual current is. Note that breakers are mostly about preventing fire, and a few 10's of amps probably isn't going to set fire to anything that quickly. Dec 11, 2019 at 17:38
  • 1
    Sorry, but this doesn't seem to answer the question. The load side of the fuse isn't relevant, because that's not what the fuse is protecting. OP needs to decide what amperage they want to support, update the relevant conductors, and then almost by happenstance are they now permitted to touch that fuse.
    – GManNickG
    Dec 11, 2019 at 20:20
  • My point about the "time delay" and the link to Bus Fuses was to point out accommodation for the spikes are built in to the fuses. Dec 11, 2019 at 21:45

Calculating the maximum load beforehand will be difficult, because it depends a lot on your usage patterns. As noted in other answers, you can reduce the maximum load by avoiding using several high-power appliances at once.

In my opinion, the most practical approach is to start with the lowest-rate plan. Then see how often you trip the main breaker. If that doesn't happen, the plan is sufficient. If it only happens rarely, you may just consider avoiding those appliance combinations.

If the breaker trips so often as to be a nuisance, you'll need a higher rate plan. Then e.g. if you switched on your washing machine and it caused the breaker to trip, and you would have wanted to also switch on your clothes dryer and start vacuuming, you can add up the load of those appliances and sum it to the current breaker rating. That should give a fairly good estimate for the rate you need, without needing to separately calculate the load of each individual small-load appliance.

  • TEPCO only allows a breaker change once a year. If OP gets it wrong they're stuck with a small service for a full year before they can upgrade again.
    – J...
    Dec 13, 2019 at 13:20

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