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I've got some very high-amperage devices (Bitcoin miners. ugh.) coming into my apartment; and although I'm going to pains to make sure I don't over-load any single room's breaker on the wall sockets … I'm wondering what the maximum amperage on the entire apartment (or, I suppose, the entire breaker panel?) is.

The wall-socket breakers for each of the three adjacent rooms I'm planning on loading (bathroom, spare bedroom, and living-room) are each 15A. Can I safely load each of those up to nearly 15A (so, a total load of 45A)? The breaker box has an inscription of “125A MLO”; what does that mean? Does that encode the maximum load of the breaker-box, or am I way off-base?

(Additionally: should I look for a particular brand of extension-cord, to ensure it can carry a full 14A over 𝑛 feet? I don't want to cheap out, if it could set my apartment on fire. :P)

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  • You're planning on putting high-current electrics in the bathroom? – pjc50 Mar 19 '15 at 15:44
  • Questions about house wiring are usually a better fit on DIY.SE (home improvement stack). – Nick Alexeev Mar 19 '15 at 16:33
  • Are you sure all the rooms are on separate circuits? Do you plan on not plugging anything else in on each circuit? How old is the building (wiring)? – Tester101 Mar 19 '15 at 17:19
  • Your box may be rated for 125A, but it should have a main breaker in it that is probably rated for less than that. Incoming circuits are usually multiples of 100 these days (at least from the outside). If your building is really old, or if it has many individual units sharing a lot of common utility loads (such as shared laundry), it could be as low as 60 in an efficiency. Also, extension cords are a bad idea for fixed loads. If you need separate circuits to power devices to connect them to one computer, do the safe thing and spend $50 on Raspberry Pis to talk to each device. – nstenz Mar 19 '15 at 18:10
  • @pjc50 well, a high-amperage extension cord from the socket in the wall the bathroom shares with the spare bedroom, where I'll be putting 'em. That's the plan, at least. ('specially because the bathroom's fed 20A, instead of 15A.) – ELLIOTTCABLE Mar 20 '15 at 0:54
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I don't know about your local code requirements, but in almost all countries at least the building has some breaker to connect to the power grid.

For example for my house this means \$3 \cdot 63A \cdot 230V = 43.47kW \$. Check it for your house. That is a whole lot of power that could easily set lots of things on fire.

However there are some things to note that do not make all this available for me:

  • There are other persons/electric devices in my house
  • the power company allows at most 40kW to be drawn continuously (due to limitations of the installed measuring equipment)
  • Drawing so much power will lead to a voltage drop that depends on many factors.
  • Most breakers do not like to be run on their exact amps value and trip after a while (sometimes hours). Use some safety margin.

Depending on the wiring you use (inside the walls and as short, non chained extension chords), they will introduce some more or less noticeable voltage drop, further decreasing the usable power for your devices.

For safety reasons you should check (or if you do not have the tools and knowledge to do so, let it be done by someone else) all equipment involved. There could also be other limiting factors involed, e.g. an RCD/GFCI.

125MLO indeed means that the panel is rated for max 125A.

For extra safety install multiple kill switch methods and a smoke alarm...

  • You beat me to it! And I'd also like to mention that effectively paralleled breakers/fuses/etc usually don't work very well. One takes more than its share of current, trips, and dumps its load onto the remaining ones which causes them to go also. Not to mention that adjacent rooms might be on different phases and so they can't be paralleled anyway. If you have a bunch of independent loads, then that's not a problem. And yes, residential breakers tend to trip at about 70% continuous, while allowing for some inrush. – AaronD Mar 19 '15 at 15:43
  • @AaronD: true that, though I was thinking of the OP not essentially paralleling them but attaching one PSU that draws what it can from one circuit and from that run N bitcoin mining computers. I doubt he has a multiple kW PSU for these things... – PlasmaHH Mar 19 '15 at 15:58
  • @AaronD That 70% figure is useful! And these loads are all separate; hell no I'm not trying to wire multiple circuits from my house in parallel. O_O – ELLIOTTCABLE Mar 20 '15 at 0:57
  • @PlasmaHH 3x EVGA 1600W PSUs, one on each wall-circuit. – ELLIOTTCABLE Mar 20 '15 at 0:58
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Continuous loads - 80%

Your question is all about "continuous loads". That is any load which is likely to be run continuously - including heaters, air conditioners, lighting, and some things you might use serially, like dryers. And of course, Bitcoin miners!

You can only load a circuit to 80% for continuous loads. **

That means on a 15A circuit, you can only load that to 12A (1440 watts) of continuous load, counting all loads on the circuit. I would buy a $20 power monitor (e.g. Kill-a-Watt) and see if that power supply really does draw 1600W; it may draw less.

Whole house (well, 2 half-houses) matters too

Your entire apartment has a 125A breaker, but that's two legs of 120V. Given the 80% rule, each leg can load to 100A only, so 12,000W per leg, or 24,000W total if you balanced it perfectly. That's for all continuous loads in your house, including all the other stuff. That leaves you no margin for non-continuous loads.

This is tougher to monitor, and requires an understanding of which circuits are on which legs. Do not carelessly put 150A of load on one leg, and 50A on the other. Once you learn how your type of panel is laid out, it's fairly obvious which breaker is on which leg. You can also buy a whole-house power monitor, but that requires installation into the panel, as it permanently installs clamp ammeters around your main power feeds.

Talk to your power company!

Ask them about rates which fit your usage. One asker had a totally crazy power plan, where he paid for his highest monthly spike ($6.50/kw), but power was nearly free (0.9 cents per KWH, not a typo). Usually that's a sucker bet, but it's a perfect plan for you. Even if you did nothing to mitigate peaks and ate the extra spike charge, that still totals out to 2 cents/kwh effective. If you do have some power management to back off bitcoin load when other load is peaking, you could tuck it all under the normal peaks and pay the penny rate. Crazy.

So talk to your power company and see what plans they have.

Lastly, if the landlord is paying for this power, clear it with him, or expect big consequences.

** The gory details: the circuit's capacity must be not less than the noncontinuous load plus 125 percent of the continuous load. NEC 210.19A1, 215.2a1, 215.3.

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Since your location is in the USA, you are subject to the NEC, and the NEC limits continuous loads to 80% of the circuit capacity (actually expressed as the load must be multiplied by 125% , but it comes to the same thing.) So each 15 amp circuit can support no more than 12 amps of CONTINUOUS load (which is defined as any load lasting over 3 hours, IIRC.)

Since you are also indicated to be in Mrs. O'Leary's Cow Town, you may have special restrictions on extension cords that the rest of the country does not. I know IBM shipped shorter power cords to Chicago than anywhere else back in the 1980s...

You'll also need to allow for air conditioning load if you don't want the place to be a sauna.

Ugh, indeed - why ARE you doing this?

  • The "80% rule" doesn't matter since he's not designing the system, he's just plugging things in. That's why circuit breakers are so important, they protect you from unplanned overloads. – Tester101 Mar 19 '15 at 17:28
  • Also note that the NEC does not limit continuous loads. They require you to size a circuit 125% of the continuous loads, so that the circuit can safely support the load. – Tester101 Mar 19 '15 at 17:44
  • I think I know the 'WHY'. Current market price of a bitcoin is roughly $265 USD. Crack a couple of those and you can pay for the equipment, crack a couple more and WHAM - Profit. – BrownRedHawk Mar 19 '15 at 18:05
  • And pay for electricity, listen to and live with noisy hot machines, and wildly varying price - the real money, as usual, is in selling equipment to folks who think they will make money at it, few of whom do. Given a concern expressed about "not setting the apartment on fire" and the reason for the 80%/125% rule, the wiser course is to follow it. – Ecnerwal Mar 19 '15 at 18:38
  • @Ecnerwal I'll be turning a $US profit against electricity costs (albeit a small one, and that'll disappear over the hellish summer months O_O. In fact, I'm probably going to shut them down whenever the efficiency-costs dip into the red, possibly automatically. Another project!); but I'm actually unlikely to even flip the mined coins for $US. I ‘paid’ BTC for these (at cents on the invested-dollar from, like, 2010.) It's pure fun for me at this point. =) – ELLIOTTCABLE Mar 20 '15 at 1:02

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