How to get more than rated current from 240v home GPO sockets? - Page 1
Forum section: electronics / general

author: ZeroCubic
author_rank: Contributor
timestamp: September 08, 2021, 01:55:47 am
content: Hi guys, first time posting so apologies if this has been discussed elsewhere or if I should be posting in a different topic area.About me: I live in Australia. I work in robotics as a contractor design engineer.The Brief: I am working on a project which requires casting metal parts from 3D printed molds, and sintering 3D printed metal powder objects. For this I need an electric muffle furnace which goes to 1700c and can hold a hydrogen atmosphere (luckily alibaba has them at a good price). This will be installed at an office which has only 240v 10A sockets.A furnace with the specs I need requires 4kW of power continuously. Clearly a 10A socket at 240v can't do this. and even a 15A socket (which I dont have anyway) can't either (3600W).So is there any practical way of getting 4kW of power (for up to 48hours, for some refractory metals, so generator is out) without having 3 phase installed?My ideas so far are:Idea 1: Couple 2 or 3 powerpoints together (each on separate circuits) into a single 20A single phase? I don't have much experience with AC power circuits but my intuition is telling me if I simply wire them together directly they would get out of phase and do something unpleasant and not work. Is there a specialised circuit I don't know about which is designed for this sort of thing?Idea 2: take a feed directly from the distribution box, use high amperage cable, and install a 20A fuse. I imagine as long as the distribution box can handle 30A total (the furnace + other devices running concurrently) I dont see why this wont work. Also I dont know how to tell what the maximum current my distribution frame can handle; it isn't written anywhere obvious.If anyone familiar can tell me why this wont work and if there is another way that will work, I would be very gratefulfor the curious, this is the kind of furnace I'm talking about:https://www.alibaba.com/product-detail/1700C-laboratory-chamber-H2-hydrogen-CH4_62361982245.html?

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2021, 02:40:14 am
content: What is the capacity of the circuits you have available? I have only very limited knowledge of Australian electrical codes but electrons behave the same anywhere. Forget about the sockets you have, none of those are adequate for such a large load. If the circuit is wired with sufficiently heavy cable then you can either install a socket of sufficiently high amperage rating or you can hardwire the furnace. A better option if you are able to do so is install a dedicated circuit with the breaker and wire sized to handle the load and run it directly to the location where the furnace is. Here code would require a dedicated circuit for a large appliance like that, I don't know what your local codes say but one of our Australian members can probably provide more info.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 08, 2021, 02:42:11 am
content: You could use a transformer on each circuit to turn the 240V into 120V at double the current, then connect the two in series. Transformers that big are going to be heavy and expensive.Quote from: ZeroCubic on September 08, 2021, 01:55:47 amThat's the right way to do it, read the label inside the access door to find out the rating, then check the size of the main breaker. In addition to the new breaker and wiring, you'll also need a disconnect for servicing the equipment, that can be a properly rated plug and socket or a disconnect box.

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2021, 02:54:57 am
content: Using transformers is a kluge at best and probably not legal, even if you connect lower voltage secondaries in series there is the possibility of backfeeding a circuit that has been shut off at the breaker/fuse, this is not going to be safe and probably is not legal.No matter what you do you are going to be limited by the maximum capacity of the panel since all of the circuits are in there. I would be very surprised if the capacity is not well over 30A. What is it fed from? Surely there must be a main breaker somewhere, if not in the panel itself then upstream. What size wire is feeding it?Check the local codes in regards to the need for a disconnect at the equipment, I'm fairly confident it will be permitted and it is probably a good idea, but it may or may not be required.

author: David_AVD
author_rank: Super Contributor
timestamp: September 08, 2021, 03:21:01 am
content: A typical power circuit in a vaguely modern office should be rated at 20A. That's 4800W @ 240V.Depending on what else is on that circuit, you may be able to have a 20A outlet fitted.The better solution is to run a new dedicated 20A circuit just for the purpose.

author: tggzzz
author_rank: Super Contributor
timestamp: September 08, 2021, 07:28:20 am
content: Quote from: ZeroCubic on September 08, 2021, 01:55:47 amAs a contract design engineer, you should be aware that the traditional technique is to replace the fuses with nails. As a professional design engineer, you will also specify installation of a good fire suppression system, and ensure that the client's insurance covers them for damage when the circuits have been modified so they don't meet the local codes. Good luck with that.For the avoidance of doubt, your questions indicate that you shouldnotbe attempting this work: get a qualified professional electrician to do it.

author: ZeroCubic
author_rank: Contributor
timestamp: September 08, 2021, 11:20:11 am
content: Thanks to everyone that replied. I mostly deal with small DC things so I just wanted a sanity check from my more enlightened friends on the eevblog. I ran the problem by a sparky friend and he agrees with what you guys have said, to just run a new circuit. So I’ll be getting him to wire it up so I don’t fry myself. Turns out the distribution board is good up to 100A so plenty of head room.As for fire suppression, insurance etc we have that covered already.

author: f4eru
author_rank: Super Contributor
timestamp: September 08, 2021, 12:04:17 pm
content: In many countries, you have to update one of your sockets (and wiring behind it) to one with more capacity.An industrial P17 (32A blue) may do the trick.Consult a proper electrician on how to do that, respect the local code.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 08, 2021, 03:04:20 pm
content: Quote from: f4eru on September 08, 2021, 12:04:17 pmNote the wire needs to handle the 32A also.Hire an electrician.Be careful with the hydrogen.andy

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2021, 08:21:59 pm
content: Quote from: ahbushnell on September 08, 2021, 03:04:20 pmThe wire needs to be able to handle whatever the fuse or breaker protecting it is rated for. I don't know about Australia but in the USA there is nothing the NEC prohibiting installing say a 40 or 50A receptacle on a 30A circuit, provided the breaker protecting the circuit is correctly sized for the wire. Likewise code allows something like 12 or 13 15A duplex receptacles on a single 20A circuit, again because the wire is protected by the breaker if the total capacity of the circuit is exceeded.Sounds like he is already consulting with an electrician.

author: EPAIII
author_rank: Super Contributor
timestamp: September 09, 2021, 05:21:29 am
content: The very fact that you are asking this question says that you need professional help with this. Questions like this have not only electric engineering aspects, but also those of safety and even legal things such as your PERSONAL liability if others are hurt or there is property damage or loss. Insurance companies are well known for finding reasons not to pay and home-brew wiring by a person who does not have the proper qualifications is one that has been used a lot. I really hate to see anyone who feels unsure enough about electrical wiring to ask such questions make the installation themselves after reading one or several internet replies.Please do get an electrician to install the proper wiring and outlet for your oven. It is THE safe way to proceed. And it also should provide you with a lot of protection against those pesky other aspects.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 09, 2021, 05:32:48 am
content: The only safe option is to install a socket with enough capacaty for the load.Your load will require just short of 17A and to get a bit of overhead I would recommend going for a 25A socket.If you do any creative stuff like paralelling outlets and get a fire you can just forget to get anything of your insurance.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 09, 2021, 10:39:56 am
content: Quote from: geggi1 on September 09, 2021, 05:32:48 amIt's more then the socket. The wire in the wall also.

author: dmills
author_rank: Super Contributor
timestamp: September 09, 2021, 11:05:21 am
content: The scariest thing about this whole project is the combination of Hydrogen Atmosphere Muffle Furnace and "from Alibaba"!Be careful with that stuff it is a small molecule and making connections for H2 gas tight is notoriously tough, at 1700c hydrogen embrittlement is going to be very much a thing!

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2021, 08:51:52 pm
content: Hydrogen is also extremely light so if there is any ventilation in the room it isn't going to collect, and I'd assume it's a small quantity in the first place. That's all beside the point though, he came here asking for electrical advice and he got the information he needed. I'm not going to concern myself with whether or not he knows how to safely work with hydrogen.

author: jonpaul
author_rank: Super Contributor
timestamp: September 10, 2021, 06:19:45 am
content: OP:Like others, i strongly Suggest to read and learn about electrical safety, and hazards of hydrogen.Then find a LISCENCED electrician to run NEW WIRING and NEW RATED sockets to supply power.Notice that your insurance WILL NOT cover unlicensed and dangerous electrical wiring or experiments.YOU will be liable for any destruction of property or injury or death.GOOD LUCK,Very Sincerely Yours,Jon PAULMSEE

author: ZeroCubic
author_rank: Contributor
timestamp: September 10, 2021, 08:28:03 am
content: thanks again for the extended replies.I can appreciate the skepticism of my abilities, when I ask questions that might sound stupid. Nonetheless when it's something potentially dangerous and I'm not certain about it, I try to err on the side of caution and ask the questions even if they sound stupid and simple.Like I said earlier, I don't tend to work with mains AC. Also, as I mentioned, I'm getting an Electrician friend who deals with this stuff for a living to do it, so you guys don't need to worry about safety.

author: tggzzz
author_rank: Super Contributor
timestamp: September 10, 2021, 09:31:32 am
content: Quote from: ZeroCubic on September 10, 2021, 08:28:03 amBoth are good attitudes to have!Trying to answer the right simple question can be very enlightening

author: james_s
author_rank: Super Contributor
timestamp: September 10, 2021, 07:02:32 pm
content: Quote from: jonpaul on September 10, 2021, 06:19:45 amThat depends on the local laws. Here in the US for example it is perfectly legal for somebody like myself who is not a licensed electrician to do wiring. It has to be inspected and signed off just like if it is done by a licensed electrician but as long as a person knows what they're doing this is not a problem. Electrical wiring is not particularly difficult.

author: tggzzz
author_rank: Super Contributor
timestamp: September 10, 2021, 07:59:13 pm
content: Quote from: james_s on September 10, 2021, 07:02:32 pmMore or less the same here. A key question is whether a professional would be prepared to accept liability for putting their stamp on the work of a "random stranger".

author: james_s
author_rank: Super Contributor
timestamp: September 10, 2021, 10:35:17 pm
content: Quote from: tggzzz on September 10, 2021, 07:59:13 pmDo they have that choice? Here the work either meets code or it doesn't. The inspector can't just fail it because they don't like the person that did the work, they have to call out what's wrong with it. There are lots of nitpicky things they can call out but if you dot your i's and cross your t's it isn't too hard to get it passed. A large part of it is just attention to detail, use the right materials and do neat, tidy work and it will be safe, reliable and legal.

author: tggzzz
author_rank: Super Contributor
timestamp: September 10, 2021, 11:26:59 pm
content: Quote from: james_s on September 10, 2021, 10:35:17 pmThere is a big difference between saying something has a fault and saying that something doesn't have a fault. The former is easy; the latter very problematic. That's often summed up by the concept that it is more or less impossible to prove a negative.

author: themadhippy
author_rank: Super Contributor
timestamp: September 11, 2021, 12:40:42 am
content: QuoteThe cartel dons who run the the uk scam all tell there members not to sign off third party work,you can register the work with the council for a fee,,but they often wont sign off the work unless you obtain a test certificate from,you've guessed it,an approved cartel member.

author: james_s
author_rank: Super Contributor
timestamp: September 11, 2021, 02:47:38 am
content: Well if they make it too difficult to get a well done job through an inspection all that happens is people stop pulling permits and stop getting work inspected and then nobody catches the stuff that isn't done properly either. A certain percentage of people either can't afford or refuse to hire a pro and are going to do it themselves. I would prefer they have the information, materials and other resources available to do the job right. Personally I get a great enough satisfaction out of doing the work myself that it is extremely rare that I would ever hire a tradesman. Not to mention some of the absolutely horrid work I've cleaned up that was done by "professional" tradesmen.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 11, 2021, 07:34:23 am
content: Quote from: ahbushnell on September 09, 2021, 10:39:56 amYes that is implied if the cable sizes is the same as her in Norway you will have to install a 4mm2 or 6mm2 depending on the cable lenght.

author: tggzzz
author_rank: Super Contributor
timestamp: September 11, 2021, 07:51:16 am
content: Quote from: james_s on September 11, 2021, 02:47:38 amYup, that's a classic dilemma.Unfortunately the Dunning Kruger effect is real and has to be addressed, however imperfectly.

author: Brumby
author_rank: Unknown
timestamp: September 17, 2021, 12:00:00 am
content: Quote from: james_s on September 10, 2021, 07:02:32 pmIn NSW at least, local laws prohibitanyonewho is not a licenced electrician from doing any work onfixedelectrical wiring - which covers everything inside the walls of buildings and other permanently wired fixtures such as split system air conditioning. Equipment that plugs into a wall socket has no such restriction.Quote from: ZeroCubic on September 10, 2021, 08:28:03 amYou asked. That is testament to your abilities to think.QuoteWe see inquiries that have grave safety implications and offering advice through a global forum has its challenges - both in differences of regulations and the ever problematic process of accurately communicating the (potentially) not-so-straightforward details of a subject to someone without the feedback an apprentice would get with the mentor at their side. All this to someone of unknown skill.QuoteThat is the correct direction to take.