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The appliance is a quartz oven, 1000W @ 250V. I am trying to obtain a max transient figure for the current (no oscilloscope on my bench). Opened the oven and checked the writings on various parts - they are all around 15A @ 250V.One of them is weird; its less than perfectly legible engraved writing has two entries for 250V, one reads 10A and the other 16A. Not sure what to make of that...

Another thing is its power cord is a 0.75 mm^2 multicore cable. Looking over https://www.engineeringtoolbox.com/wire-gauges-d_419.html it looks as if this cord won't be taking anything above 9.5A (more likely half of that). Is this figure what I should be working with ?

P.S. I need to put my controller PCB in between the mains and this oven. I am building a variation of http://andybrown.me.uk/2015/07/12/awreflow2/

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  • \$\begingroup\$ Elementary math: 1000W @ 250V is just about 4A, and the oven should be a purely resistive load. \$\endgroup\$ – Turbo J May 27 '19 at 12:54
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    \$\begingroup\$ It is a resistive load, but it may have a surge transient current like an incandescent bulb until it heats up. For the purposes of cord size, don't worry about this transient, it is too short. \$\endgroup\$ – Mattman944 May 27 '19 at 13:05
  • \$\begingroup\$ @TurboJ taht may not be so according to shift-controls.com/blogs/technical/power-controller-selection \$\endgroup\$ – kellogs May 27 '19 at 13:14
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For most purposes, the steady-state current, 1000 W / 250 V = 4 A, is all you need to be concerned about. If you can't see the transient with an analog ammeter, you probably don't need to worry about it. Even if you can see it, a transient that is on the order of 150% for a couple of seconds is probably not of much concern. The transient magnitude and duration should be no problem for the power cord, branch circuit wiring or branch circuit-breaker. The internal part markings indicate standard parts that have ratings that exceed the requirement. Many components are made only in a few, fairly widely spaced, current ratings.

For estimating purposes, look up the temperature coefficient of resistance for tungsten and see how much the resistance changes between room temperature and incandescence.

It appears that a driver for an incandescent lamp needs to handle 10 times rated current for 3 to 5 ms. PDF Link A controller may need a current limiting scheme.

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  • \$\begingroup\$ Okay, maybe my quartz elements area heated by... not tungsten ? Seems it isn't something else - tansun.com/gb_en/technical/how-quartz-heating-works.html. I don't know, but, Is it possible for the ehating element not to be tungsten ? \$\endgroup\$ – kellogs May 27 '19 at 15:12
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    \$\begingroup\$ It is certainly possible that the heating element is something other than tungsten. I saw something that seemed to indicate that tungsten is commonly used and that it has a higher increase in resistance with temperature than other heating elements. You could check the cold resistance with an ohmmeter or a low DC voltage an a DC ammeter. \$\endgroup\$ – Charles Cowie May 27 '19 at 16:34

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