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I just installed a Reliance furnace transfer switch model TF151W to my main circuit panel. I tried to test it by running an extension cord from another outlet to the transfer switch. I put the P3 International P4460 Kill A Watt EZ Electricity Usage Monitor in the line to test the watt consumption of my hot air furnace.

After switching from Line to Generator and cranking up the heat, I noted the wattage was only 3.7 watts? Is this because the common neutral is not switched? If I was to connect the Honda EU2000I 2000 Watt inverter generator would I have any problems?

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    What model is your furnace? – ThreePhaseEel Nov 8 '16 at 1:54
  • Low capacity furnace; no problem. Blowers, for such, are typically rated at 1/3 or 1/2 HP (Max SS draw about 400 watts). Larger furnaces may trip the circuit when they try to start the blower, due to the surge current. – Brock Adams Nov 8 '16 at 2:06
  • It is a Tech-4 True Blue 90 MOD USA 054NH3R, But with a 1/3 hp blower I would think it would draw 400 or a bit more in watts? Why would the meter read only 3.7 watts? – Andrew Fischer Nov 8 '16 at 2:20
  • @AndrewFischer -- is the blower kicking in? – ThreePhaseEel Nov 8 '16 at 2:23
  • yes it is working normally – Andrew Fischer Nov 8 '16 at 2:24
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Well, from the tests you ran (see comments above), your setup is correct.

The Kill A Watt EZ should show a reasonable reading, BUT it appears that that kind of device requires balanced current.
So, because the furnace neutral current is not flowing through the Kill A Watt, it does not compute a valid power based on the current and voltage it does see. (Note that the meter is seeing a "ground fault" from its perspective.)

EG:
          Meter limitations

This is an apparent limitation of this model of meter; your setup is correct.


To determine the furnace's power draw, if you don't trust the label plate :), and, since you've access to the wiring at the furnace, measure the current and voltage there with standard AC meters.

Multiply the voltage and current to get KVA. The KVA will be slightly more than the actual power used (thanks to power factor), but your generator needs to be able to supply about twice that KVA anyway (allows for motor start).

Since your furnace has a 1/3 HP blower (max 250 watts, steady state), your Honda 2000 Watt generator should be just fine to power your furnace.

  • Thank you, You have been very helpful! I was thinking the neutral loop and the neutral straight to the circuit panel would cause a problem with the meter. I am going to pursue this further with the Kill-A-Watt manufacturer. – Andrew Fischer Nov 13 '16 at 2:37
  • You're welcome. If you hear anything good from the KAW people, you can post it in a comment or self-answer here. – Brock Adams Nov 13 '16 at 2:43
  • I would say it's the meter working as intended. I'd also say move the Kill-a-watt so it's on the switched side, or reroute the neutral so it goes through the Kill-a-Watt. That shouldn't be THAT hard; neutrals are required to travel with their companion hots, so that the magnetic fields cancel each other out. Otherwise you can get eddy current heating in places you do not want that. – Harper - Reinstate Monica Nov 24 '16 at 1:28
  • @Harper, this is no requirement to switch the neutral here, nor any logical or practical reason to. This scenario occurs only during the kind of momentary testing that the OP did, whereby testing the switch from another AC socket instead of a generator. Using an actual generator, the neutral is paired with the hot. In the test scenario, there's a benign ground loop. Other than this kind of meter, there is no drawback and the configuration is extremely temporary. – Brock Adams Nov 24 '16 at 4:29
  • You wouldn't switch the neutral. I said to make sure the neutral also loops through the Kill-a-Watt, so that it works properly. This could happen on either side of the switch. Easily done, lop a 3-prong extension cord in half and now you have cheater cords. That's how I test hardwired things with a KaW. – Harper - Reinstate Monica Nov 24 '16 at 5:30

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