I bought a new in-wall timer rated 20 Amps. and it says '1350 w Tungsten', so I'm wondering if there are watts which are measured different then other watts? In my calculation it should be 20 x 120 = 2400.

  • What is the make and model of the device you're referring to? – Tester101 May 5 '16 at 17:20
  • Intermatic timer iw700k – aofkj May 6 '16 at 3:13

Yes, apparently "tungsten-heater" load type is measured differently than a ideal/purely resistive load type!

Check this for example - http://www.nkkswitches.com/pdf/electricalratings.pdf - you will see chart of the current v time "inrush" curve for tungsten filament electrical light bulbs - while cold, their resistance is very low - so they spike when switched on (not coincidently that's the time when these burn out - during power-on).

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And thus differences in ratings, for example this smart switch allows only 1/2 tungsten load than resistive:

• Electrical rating: 15A, 120Vac, 60Hz
• Resistive: 1800W
• Tungsten: 960W
• Electrical Ballast: 500VA
• Type 1.B Action

Different devices have different characteristics when the timer breaks the circuit.

They usually rate the contacts of the timer by resistive (tungsten) or inductive (motor) style loads.

Even though the contacts are rated for 20 amps a resistive load may be more likely to pit the contacts so they derate the timer for a resistive load.

Good luck!

EDIT - The particular timer you have is only rated for a 1 HP motor which translated to 746 watts and 6.2 amps of inductive load. 1350 watts of Tungsten resistive load which translates to 11.25 amps of standard lighting. Even though the specs say it has 20 amp contacts it is not rated to switch a 20 amp load. Apparently over-promising and under-delivering. 😞

  • Thanks @ArchonOSX So I understand that the load on it cant exceed 1350 w, correct? – aofkj May 5 '16 at 16:47
  • @BenWelborn Hmm not quite. Tungsten is the element used for standard lighting filaments so it is a resistive load. So, the 1350 would be for resistive lighting. The timer is also rated for 1 HP or 746 watts of inductive load so you were right on with the motor comment. It is possible to switch fluorescent or LED lighting at a higher rating but I certainly wouldn't try to push the total of 20 amps. I clarified somewhat in my edit. – ArchonOSX May 6 '16 at 9:52
  • Woops, I got that mixed up alright! Pardon the deletion of my previous comment- I don't want to leave misinformation out there. – Ben Welborn May 6 '16 at 12:14
  • Resistive load is usually the easiest load to switch. Highly inductive or capacitive load (like fluorescents and LEDs) is usually far more damaging to the contacts. – SomeoneSomewhereSupportsMonica Apr 29 '18 at 2:47
  • Tungsten/incandescent lighting has a huge inrush current on make. Whereas a motor/inductive load has a huge inductive kick voltage on break. The latter is harder to deal with for a relay, hence the more conservative inductive rating. – Harper - Reinstate Monica Jul 30 '20 at 4:55

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