Current limiting mains-based heater element - Page 1
Forum section: electronics / beginners

author: jeremy
author_rank: Super Contributor
timestamp: October 08, 2014, 04:31:43 pm
content: Hi all,So I've been looking at how one would connect a commercial crepe iron from France to an Australian power point. The major problem is that the device is rated at 3kW, where Australian powerpoints are only supposed to have 2.4kW of load. Give than I'm pretty sure it is just a dumb heating element inside with a rheostat, what do you think the best way is of limiting the current to the device is?

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2014, 04:46:30 pm
content: Use a bigger plug, rated for industrial power connections would be about the best. At 3kW you are probably going to cook a standard socket outlet in any country, they are not really rated to actually carry the full rated current for long periods. You would need a dedicated socket outlet with a proper cable to supply power and appropriate breakers, as this will be left running for long periods, and a standard outlet will fry in under an hour.If you want to limit power best is to replace the heater element with a lower power version. However you will then run into it not being able to cook the crepes properly.

author: jeremy
author_rank: Super Contributor
timestamp: October 08, 2014, 05:58:41 pm
content: It is interesting that you say that, because it comes factory fitted with a standard EU plug. Are they rated for higher currents than the rest of the world?

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: October 08, 2014, 06:08:44 pm
content: Quote from: jeremy on October 08, 2014, 05:58:41 pmI'm not sure about the rest of the world, but over here (in the Netherlands) we usually have 16A circuit-breakers at 230VAC.That makes 3680W. Every wall socket has to be able to deliver that amount of power without the risk of fire.Obviously this requires 2.5mm2wiring.

author: Monkeh
author_rank: Super Contributor
timestamp: October 08, 2014, 06:26:41 pm
content: Quote from: sunnyhighway on October 08, 2014, 06:08:44 pmObviously, despite 1.5mm² being adequate, subject to this calculation concept.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 08, 2014, 06:51:00 pm
content: Quote from: sunnyhighway on October 08, 2014, 06:08:44 pm#10 (2.6sqmm) gauge wiring for less than 15A? The NEC rating for #14 (1.6sqmm) is 20A.

author: Monkeh
author_rank: Super Contributor
timestamp: October 08, 2014, 06:54:14 pm
content: Quote from: DanielS on October 08, 2014, 06:51:00 pm14AWG is 2mm². 10AWG is over 5mm².

author: bktemp
author_rank: Super Contributor
timestamp: October 08, 2014, 07:28:47 pm
content: In Germany 1.5mm² is used for 16A 230V. I think 16A circuit breakers are quite common in central europe.http://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#CEE_7.2F7_plugIf you do not have a 13A power source you could use a 24V 250VA transformer and connect the secondary in series to the 240V to reduce the voltage and therefore the power consumption to about 2.5kW.

author: jeremy
author_rank: Super Contributor
timestamp: October 08, 2014, 07:59:56 pm
content: We use 16/20A breakers in Australia too I think, but it is the socket itself that can't take the high current.

author: David_AVD
author_rank: Super Contributor
timestamp: October 08, 2014, 09:27:47 pm
content: Converting it to a 15A plug and wall outlet should be fine. Any vaguely modern house power circuit will be rated for either 16A or 20A and have 2.5 sq mm wiring.That said, you should be able to use an appropriated rated TRIAC based dimmer to reduce the current consumption.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 09, 2014, 04:34:46 am
content: Quote from: Monkeh on October 08, 2014, 06:54:14 pmOops, I was looking at diameters

author: SeanB
author_rank: Super Contributor
timestamp: October 09, 2014, 04:51:31 am
content: Appliance cord at 1.5mm can be rated for 16A, because it is able to be cooled by being in free air. In the wall 2.5mm is needed because of the limited cooling available in either a conduit or a direct buried situation. As well in a wall situation you can have a 20A load split over 2 or more socket outlets.Seems Aus sockets are rated for less than the cabling can handle, so you probably will have to replace the outlet for this appliance with a dedicated socket specifically for it. Not going to suggest socket types, but look at the range available locally from eg Crabtree in Aus, or look at the 16A industrial socket outlets and plugs.

author: IanB
author_rank: Super Contributor
timestamp: October 09, 2014, 05:07:58 am
content: Seems odd that the socket would be so limited. Australian plugs look rather similar to US plugs, and here in the US the standard plugs and sockets can handle either 15 A or 20 A depending on the type.Also I have seen a UK market 240 V / 3000 W electric kettle fitted with a 1.0 mm2flex and it doesn't get the slightest bit warm in operation.So if the plug doesn't get hot and if the fuse doesn't blow, I wouldn't be that concerned about operating a 3 kW device where you are.

author: David_AVD
author_rank: Super Contributor
timestamp: October 09, 2014, 05:12:19 am
content: The active and neutral pins on the Australian 10A plug are the same as the 15A version. Only the earth pin is different.

author: ludzinc
author_rank: Unknown
timestamp: October 09, 2014, 05:37:40 am
content: Cabling is rated to higher current than the socket, so you can run more than the one appliance on the run at a time.You can have a dual socket in the kitchen, for your toaster and kettle, as a typical example. Both 2400W, 20A in the cabling.

author: richard.cs
author_rank: Super Contributor
timestamp: October 09, 2014, 08:15:01 am
content: A note on cable sizings:The required cable size is governed by two concerns, the allowable voltage drop to the load (which depends on cable size, load current and length, and weakly on cable operating temperature), and the maximum alowable cable temperature, usually limited by the softening point of the insulation used. In this discussion people are really talking about cable temperature and for fixed wiring the loads are assumed to be continuous allowing equilibrium temperature to be reached.In the UK a 1.5 mm2twin and earth cable is considered capable of 16 amps in free air but only 8 amps if inside an insulated wall. In both cases the maximum condictor temperature is 70C but the ability of the cable to loose heat is different. We would use 2.5 mm2for 16 amp circuits here too. :-)

author: Monkeh
author_rank: Super Contributor
timestamp: October 09, 2014, 02:01:53 pm
content: Quote from: richard.cs on October 09, 2014, 08:15:01 amTosh. 20A in free air, 16A under <100mm thermal insulation if touching a surface, 10A if totally surrounded by insulation, unless those numbers have changed, which I think unlikely as they're already fairly conservative.QuoteOnly if you're buried in insulation. You could use 1mm² in open air.

author: richard.cs
author_rank: Super Contributor
timestamp: October 09, 2014, 04:59:00 pm
content: That'll teach me to trust some random website rather than getting out the BRB (which was at home) :-P . 1mm would be pretty close to the line and I wouldn't personally do it: 17A on a tray, 15A clipped direct, 11A in an insulated wall. For 1.5mm the numbers are 22, 19.5, 14 so I'd give you that one though I would argue that common practice is to use 2.5mm even though it's not required. Note that 16A on a 1.5mm cable would drop nearly 0.5 Volts per metre.

author: Monkeh
author_rank: Super Contributor
timestamp: October 09, 2014, 05:40:48 pm
content: Quote from: richard.cs on October 09, 2014, 04:59:00 pmFrom which table are you getting those numbers?All of this, of course, is a bit loose without decent discussion of cable type, installation method, bunching/conduit fill factor.. My point was that throwing numbers out without assessment and calculation is daft.

author: richard.cs
author_rank: Super Contributor
timestamp: October 09, 2014, 06:14:39 pm
content: Quote from: Monkeh on October 09, 2014, 05:40:48 pm4D2A,"Multicore 70C thermoplastic (PVC) insulated and thermosetting insulated cables, non-armoured (copper conductors)", from the columns for "1 two-core cable (with or without protective conductor) single-phase a.c. or d.c.). Methods 11, 1, and 4.I wasn't really trying to get into a detailed (and very UK-centric) discussion of cable types and ratings but it does seem to have drifted that way. BS 1761 gives a whole load of pre-calculated ratings with a generous safety factor but nothing stops anyone from doing the thermal calculations themselves if the situation justifies it. You can then get an answer for all kinds of odd questions such as this real situation I hit recently: "A three core 10 mm2PVC-insulated SWA cable is direct-buried. two cores are paralleled for neutral and the third used for live. What is it's current rating?". There definitely isn't a table for that one.Coming back to the OP's question it's a 25% overload and would likely be fine for tens of minutes at least. I personally would try it and see if anything (other than the iron) gets warm. Given that the iron almost certainly regulates temperature, probably by cycling on/off around some setpoint, it's likely that once warmed up its average load is under a kilowatt and it would just be fine. Failing that a 16A circuit with whatever cables/connector the relevant Australian standards require for such things.

author: Monkeh
author_rank: Super Contributor
timestamp: October 09, 2014, 06:20:39 pm
content: Quote from: richard.cs on October 09, 2014, 06:14:39 pmAh, okay.. 4D5 for twin and CPC. Slightly different ratings. Is that a BGB rather than a BRB? Methods are different.

author: richard.cs
author_rank: Super Contributor
timestamp: October 09, 2014, 06:51:11 pm
content: Quote from: Monkeh on October 09, 2014, 06:20:39 pmTechnically BBB as the copy to-hand is 16th with amendments 1 and 2 (so 2004) but I didn't think there have been any changes to those numbers.

author: Monkeh
author_rank: Super Contributor
timestamp: October 09, 2014, 07:07:51 pm
content: Quote from: richard.cs on October 09, 2014, 06:51:11 pmAh, okay. The figures are right, but the methods appear to have changed designations.. BYB next year apparently.Oh, hey, I think that was the topic a few miles back.