I am not an engineer nor an electrician but I just installed an Atmor 5.5kw water heater with a 12mm wire and a 30amp breaker at 220 voltage.

I install it separately with plastic molding outside my house from my bathroom at the attic because there is no outlet for the water heater down to my main panel.

This is a single phase only but I observe that the wire is getting warm whenever I turn on the heater.

Is this nothing to worry? Can the wire tolerate the resistance of my water heater up to it's highest point?

  • 12mm wire? That's either 0000 or 6 AWG depending on which size the 12 mm refers to. Both of those are huge for residential wiring.
    – cde
    Apr 11 '16 at 0:04
  • 2
    I'm voting to close this question as off-topic because this belongs on DIY, the Home Improvement stack.
    – cde
    Apr 11 '16 at 0:05
  • Please edit in which country you are in, and confirm the wire size you used.
    – cde
    Apr 11 '16 at 0:06
  • This should be migrated to DIY Home Improvement, but I assume that your '12mm' refers to the outside dimension of the multiconductor cable - we (or they) are going to have to know the diameter of the individual copper conductors (sans insulation) and all the markings on the wires to have any hope of figuring this out. Apr 11 '16 at 0:41
  • Philippines. The hint was the mishmash of terminology, but the dead giveaway is that Atmor 5.5kw heaters are marketed (in English) mainly there. Philippine wire is AWG. '12' wire that gets a little too hot on 25A, well, 12AWG makes perfect sense. Apr 11 '16 at 2:54

I am guessing you are in the Philippines, in a postwar development with Euro-style 220V/neutral power.

12 square mm is not a common Euro wire size. Philippines use American Wire Gauge so "12mm" is actually 12 AWG. This is too small for your job. You need 10 AWG.

The early electrifications were done to the American standard of 120/240VAC split phase. In newer areas, they use the more efficient 220VAC single phase, still 60Hz, which allows sourcing appliances from nearby neighbors. However AWG remained the standard for wire size. In America, NEC allows 20 amps on 12 AWG, and 30 amps on 10 AWG.


A 30 amp circuit would require #10 AWG wire or 4 mm diameter wire. #10 is 13.6 mm2 area wire #12 AWG is listed as 8.5 mm2. Make sure you have at least #10 AWG wire.

I don't know what you mean by 12mm wire but if you can hang on to the wire with a bare hand it is not getting that hot. Wire normally heats up while in use and a water heater puts a fair amount of current through the wire. Consequently, it will get warm but not overly so.

Like I said, if you can hang on to it, then its heat is not out of the ordinary. If you can't keep your hand on it then shut it off until you have larger wire.

Good luck!


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