# How many kilowatts (or amps) can I safely draw from the AC grid? [closed]

I rent a house in the SF Bay Area. The house is not exactly new, but it's in good condition; hopefully the AC installation is the same. Most AC outlets are without grounding; but fortunately, there are two outlets in the backyard that do have ground connectors.

I am thinking to build a kiln to fuse and slump glass. The problem is, these things draw a lot of power. I was told I may need 8 kW, perhaps more. Well, that's 72 A on a 110 V grid.

Is it safe to draw that much power from a regular outlet? If not, what other options do I have? Keep in mind, I rent this house, so I can't do any hacks.

The heating elements will probably be controlled via some nice hefty triacs (with an Arduino behind them, or something), which will open earlier or later during each AC cycle depending on how hot I want to make the kiln. Probably doesn't matter, but anyway, it's not a purely resistive load.

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## closed as off topic by Kellenjb, Leon Heller, Kortuk♦Nov 14 '11 at 6:23

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Are there kilns that run on propane tanks or something like that instead? –  endolith Nov 11 '11 at 12:47
No, it's not an electronics design question. But it is an electrical engineering question. –  Jason S Nov 11 '11 at 16:51
Yes this question is not a good fit here, it belongs at diy.stackexchange.com - see electronics.stackexchange.com/faq#questions and diy.stackexchange.com/faq#questions –  BlueRaja - Danny Pflughoeft Nov 11 '11 at 16:52
@endolith: temperature-controlled kilns are much simpler w/ electric power. (a friend & coworker uses one, has done wood-fired work) –  Jason S Nov 11 '11 at 16:52
@Florin: You could probably reduce your power needs by spending additional \$ on insulating the kiln (get some good refractory brick) -- at the cost of it being somewhat slower to heat up. –  Jason S Nov 11 '11 at 16:54

As others have said, 72A can't be drawn from a regular outlet and will need special wiring. Legally, this wiring will need to be done by a licensed electrician, and of course will require the landlord's approval.

As to whether the grid feed can support that, that is something you should be able to ask the electric company. My house is in Massachusetts, was built in 1985, and has 200A service. That's fairly typical of new construction, but less is common in older houses. Again, this should be a easy answer for the electric company.

As for the Adrduino controller, that's a bad idea. Since you obviously aren't expert at either electric power handling or microcontrollers (that's not a bad thing, just a fact), this is not something for you to hack around at. This is in no way a beginner project, since it's handling high power levels and lethal voltages (you will certainly be using 220V for this not 110V).

If you do somehow end up trying to do the temperature contol and power handling yourself, I would definitely not use phase angle control on a cycle by cycle bases as you mentioned. First, this will cause nasty harmonics on the power line current, present a bad power factor, and maybe get you into trouble when neighbors report excessive radio interferance. Second, you don't need to control the heater power anywhere near that fast anyway. The first time constant of the heater system is probably measured in a few minutes at least. The control system can easily decide to switch the heater on or off for multiple seconds at a time.

Plain old relays are appropriate here because the currents are substantial and you don't need to switch often. As I said above, every 10, 20, or 30 seconds is probably good enough. Unfortunately relays aren't synchronized to the power line zero crossings. A possible solution is to use zero crossing solid state relays for the actual on/off switching, with a mechanical relay taking over shortly after the solid state switch goes on up to shortly before it turns off. The reason for not using the solid state switches during the steady state on phase is that they drop some voltage. At your current levels, that will result in significant heat, which will be a lot of trouble to deal with. By having the solid state switches only dropping this voltage for a cycle or two at each transition means you can operate them at their peak current rating and not worry about cooling them that much. This also reduces the stress on the relays since they won't be switching high voltages. The relay output is in parallel with the solid state switch output, and the relay is only switched when the solid stat switch is on. This will greatly reduce wear on the contacts and thereby increase relay life.

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The mechanical relays in parallel with the SSRs is a good idea. –  Oli Glaser Nov 11 '11 at 15:32
So, let me TLDR a bit: [This is likely too much power for my current situation; I need to ask the provider. Won't work anyway with a regular AC socket, it requires special wiring. Sample the temperature and make decision once every few (dozen of) seconds, instead of cycle-by-cycle; maybe even include some hysteresis, but still trigger the SSS at zero-crossing, because that's nice. Use relays in parallel with solid state.] - Excellent advice, thanks. I'll do a small-scale test first, at a power level that my AC can handle, probably using a cheap electrical heater. –  Florin Andrei Nov 11 '11 at 18:51
@Florin: Keep in mind that a small scale system will probably also have a shorter primary time constant from heater input to temperature change. For example, finding that 2 second switching works in the small system might still mean 20 second switching is fine for the full size system. In each case, it's a good idea to measure the step response from cold to stabalized warm at fixed heater power to get some idea of the system response time. –  Olin Lathrop Nov 11 '11 at 19:27
I checked the panel, there's 200A service to it, but it's split 100/100 with the neighbor - each of us has a 100A breaker. i.imgur.com/TTCpm.jpg –  Florin Andrei Nov 11 '11 at 21:28

Here in Belgium a house has a main automatic switch (before the fuse cabinet) approved by the electricity utility. The switch's rating is the maximum you can draw for the whole house. For my apartment that's 40A (at 230V).
I guess in California things will be more or less the same. I wouldn't worry about the wiring outside the house. It's the wiring inside the house which will be the limiting factor. You can't use the standard wiring, that's not up to the high currents you need. Rewiring (even if your landlord agrees) will be cumbersome, since the tubes for the wires may be too thin for the thicker wires, let alone that you can pull them through. My guess is that you may need at least 6mm$^2$ wires, I once installed a 3$\times$19A heater which required already 4mm$^2$.

I mention the 3$\times$19A because 3-phase is the way to go for this kind of power, really. If the utility supplies it, and the landlord allows for the required changes you get 3$\times$208V instead of 1$\times$120V, which makes a world of difference: the 72A will be reduced to 13A per phase.

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I agree 3 phase would help if it is available, but likely it would be very expensive to provide if it's not already entering the building. If this is a stand alone house, it would not have three phase unless it was built with some very unusual circumstances in mind. I want to point out however that the OP is really dealing with 220V, not 110V. Houses here in the US are normally fed from a 220V center tapped transformer secondary. One side to center is used for most circuits, but accross the ends for high power stuff like a water heater, dryer, or electric range. –  Olin Lathrop Nov 11 '11 at 15:59

This is a project for a licensed electrician. There are many things to consider that only a professional would know. Most apartment feeds are sized according to their present needs such as electric stove, air conditioning, electric hot water, heat and square footage of living area. What you want to add (8KW) is huge, and needs special consideration. You did not tell us the size of your service panel, or what electrical demands are already in place, so one can only make assumptions.

The power lines from the utility are sized for the main panel breaker. If you exceed that rating, the utility must change those wires. You can not always just add an additional 40A 2 pole breaker just because there is room in the panel for it, a load study of your apartment must be done. If you had the power to add such a circuit, it would mean a dedicated 2 pole (230V) circuit with appropriate heavy wiring to a single dedicated outlet (or maybe hard wired). The NEC has strict rules (and local codes as well) about such installations.

You may want to look into safe alternatives such as, a generator - 15KW should do it, or if there are any gas kilns available.

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+1 for the generator. It's a better investment since it can be useful for emergencies as well, and you can take it with you if you move. –  Jeanne Pindar Nov 11 '11 at 16:21

There is absolutely no prospect of drawing 72A from a "regular outlet".

As long as the feed to the house is adequate - and that level of power is within the range that MAY be available in many US residences, but not all, if would be possible to have a purpose provided circuit and outlet made available.

The closest that you will find available in a "normal" house is the feed to the range (electric stove), but even this is very unlikely to provide the power level required.

An internet search for USA range fusing provisions suggests 40A, 50A, and 60A in use in various installations. 50A seems common. A look at the fuses or circuit breakers on your house's distribution board will show you what the Range circuit is currently "fused" to provide. The actual circuit MAY be rated at slightly above the fuse rating, but probably not.

The relevant regulatory standard is often NFPA70 - this is non binding and optionally adopted in the US and may be adopted in revised form in a given area, BUT it's a good guide.

Here's a quick comment on US range wiring. Newer ranges may be 40A and older ones are apparently more likely to be 50A. They say

• 50 Amp Electric Range: Old range ovens require a 50 amp breaker because of the massive load that the stoves use for baking. Six AWG size copper or aluminum wiring is acceptable for this size circuit. If aluminum wiring is used, it has to be installed into a CU/AL breaker. This means that the breaker accepts copper or aluminum wiring. The outlet that the stove is plugged into also has to accept aluminum wiring if it is used.

Assume you have a 50A circuit. At 110 VAC that will provide 110 x 50A = 5.5 kW.

It's likely that if people are achieving adequate results with 8 kW then you can get the same result with 5.5 kW plus additional insulation and more care.

Kilns may be wired to allow the use of 2 or 3 phases. Elements are liable to be connected Phase-Neutral and not Phase-Phase. So technically it may ell be possible to supply a kiln made fro use with 2 or 3 phases from multiple circuits or outlets all supplied from the same phase. It is reasonably likely that such an arrangement would not meet regulatory requirements.

Kiln elements are liable to appear mainly resistive to 50 or 60 Hz mains.

You mention power control by switching earlier and later in a cycle - with kilns you will get acceptable results by switching whole cycles with on/off controls. This allows zero crossing TRIAC control ICs and much simplified control electronics.

Very basic zero crossing / random phase tutorial here.

Nice Fairchild zero crossing TRIAC controller application note AN3004. More detail than you'll want at present but a worthwhile reference document.

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+1 for switching whole cycles. At that power, switching in the middle of the cycle will generate a massive amount of noise, AFAIK there also some regulations regarding that issue by electricity suppliers if you draw that amount of power. –  0x6d64 Nov 11 '11 at 12:09
@Russell: High power appliances, like electric ranges, here in the US would run at 220V, not 110V. The feed for a single house is a 220V center tapped secondary. Normal circuits run between one end or other and the center tap, which makes them 110V. High power circuits run accross the whole secondary, which makes them 220V. –  Olin Lathrop Nov 11 '11 at 17:59
@OlinLathrop - Thanks. So is a 50A circuit actually a 220VAC x 50A = 11 kW cct? In which case he still can't use "regular" outlet but can use the range circuit? –  Russell McMahon Nov 11 '11 at 20:18
@Russell: Yes, a 50A circuit is as you descibed if it is specifically stated to be 220V. The assumption here is that it's a 110V circuit unless you say differently. Such a 220V circuit would have a ganged breaker, which is two breakers mechanically tied together, one on each hot leg. In this case, both legs are hot, each at 110V from ground. That all said, I've never seen a outlet, even a 220V one, for that kind of current. I expect that needs a very special outlet or more likely must be a fixed installation that is directly wired. –  Olin Lathrop Nov 11 '11 at 20:53

The average 110 or 120 volt socket is only good for 15A, and sometimes you see 20A or even 30A sockets at this voltage. These have different orientations of the prongs to prevent insertion of a 20 or 30 amp load into a 15A socket.

It must be a code thing, because while it's technically possible, you just never see 120V sockets for more than 30A. At least, I haven't seen any. The usual solution when that much power is needed is to go to higher voltages. You may have to get your power from a 240V socket, in which case you could use a transformer to make 120 at higher current from it, or, if you're building the circuit, you could design it to work at higher voltage.

Another thought is, 8kW is a lot of power even at 240V. If you have a 30A/240V outlet, that only gets you to 7.2kW; beyond that it would be impossible to say where you might find that much power in your particular house. Your distribution panel will be the final word on whether you can get that much power - while 200A service is not uncommon in newer houses, there was a time when 100A service was the norm, and even older houses might even have 60A panels or less. If you look at your main panel, the rating on the main breaker (or main fuses, if it's that old) will tell you what the whole-house limit is.

Lastly, since you say you rent the place, you might actually consider having an electrician look at your setup. That way, if the landlord raises an eyebrow you can say you've exercised due diligence.

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No, you can't draw that much current through a regular outlet. Typical outlets in Europe supply 10-16 amps (at 230 volts) and the limit is because of how contacts are manufactured and how thick wires are, so the same limitation will apply regardess of mains voltage.

If you need to draw 72 amps through a single outlet that should be a special high-current outlet rated for that much current. It will be a beefy unit with huge terminals and it will require a beefy connector. It will have to be connected with a beefy wire (something like 10mm2 - exact cross-section should be local codes compliant) all the way from the power meter box (joints are allowed, but they also have to be capable of passing that much current).

Also note that there may be a restriction on how much a single house/apartment can draw - for example in the apartment building I live that's 50 amps (at 220 volts) per apartment and the limit varies greatly depending on exact location and state of the grid at that location.

You'd better consult the professionals before you considering the following option but I guess in your case the best bet is to run a separate cable from the power meter box to the appliance and connect it through a properly rated circuit breaker. When you leave house you can remove all that stuff. One important thing is that you have to attach all cables so that they don't lie on the floor. Leaving cables lying on the floor is very dangerous - they are prone to damage and any damage to a live cable can trigger a fire. Please note that this will be considered "temporary wiring" in many jurisdictions and having that in a house could impose severe consequences - for example, if the house burns down and that's attributed to having temporary wiring the insurance coverage might be refused. Again, you'd better consult the professionals before you consider this option.

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Re: I rent a house in the SF Bay Area. ... I want to draw 72 amps at 110 v.

Re: draw from the grounded outlets in the backyard? Absolutely not. They will be wired with either 14ga (15 amps) or 12ga (20 amps).

Re: Capabilities of the AC service to the house? Easy to determine: check the ampere rating of the master breaker on your main distribution panel. It will most likely be between 75 and 200 amps. That number is the maximum current supported to the house from the electric company.

Any higher loads will require new "service" from the electric company to the property.

The electric company doesn't charge much for the new service since they want you to spend more money on electricity. But you will need an electrician to upgrade the house's panel for the new service.

Re: connecting the kiln to the electric supply -- normally it would be hardwired (no plug/socket). An electrician would do the job. Large current plugs and sockets are available, and I used to use them to plug television remote trucks into the power supplied by arenas, etc. But in your installation there would be no benefit. Hardwired would be the way to go. Of course, the wire would need to be rated. You'd need at least gauge 2. See table

Re: I am thinking to build a kiln...uses 72A Bad idea! Whoa! That's the sort of thing you buy, not make. If you want to try to make it, do it on your own industrial property, and not in a rented house in the SF Bay area. You'd be breaking your lease and fire safety laws by trying it. And it would be a great way to start a fire and/or electrocute yourself and perhaps others. So please don't do it. Powerful electric kilns are bought not made.

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