Must furnace be hardwired?

[monipit]

I have a Coleman generator (5000 W) which I use extension cords with to power a sump pump, refrigerator and some lights during power outages. I would like to use the generator to run my forced air furnace, but of course the furnace is hard wired to my breaker box.
Is it possible to wire a 12 gauge pigtail (cord) at the furnace turnoff switch (located on the side of the furnace), connected with wire nuts, and have the romex (which is 12 gauge) from the breaker box end in a receptacle box where the pigtail could stay plugged when things are normal?
Then when the power goes out the male end of the furnace gets plugged into the extension cord of the generator. There is no interaction with the grid, but is it legal? I've heard that hardwiring isn't done everywhere, and pigtails are used. Also, the furnace is one of those high performance ones, will the generator hurt the control board?
Any suggestions are appreciated.

[HayZee518]

furnace and generator

take a look at the following diagram. it uses two three way switches. the odd colored screw(s) connect to your furnace. on the first switch one brass connects to your house supply, the other one to your generator. on the second switch your house neutral connects on the same side as the first switch and the second brass goes to the generator neutral. mount both switches on a double toggle plate 4 inch square 1/2 inch raised. nipple a four inch square by 2 inch deep junction box to your existing single pole switch box. use a strain relief fitting on your generator cord and feed this through a bottom knockout on the new box.

[monipit]

Thanks for the reply and the diagram HayZee. I am in western New York and I wonder if it is within code to use a plug, instead of being hardwired. It can't hurt a lineman this way, it doesn't have a way to involve the grid. The only thing I worry about is damaging the furnace control board with the "dirty" electricity of the generator compared to the power company's. Any ideas?

[HayZee518]

I used to live in Buffalo [Western NY] I know each town/city jurisdiction uses its own inspectors i.e. Lackawanna, Kenmore, Batavia etc. but they all follow the NEC. I don't see why the furnace can't be wired to a pigtail with a plug. It IS a utilization device as defined in the code. Normally it isn't cord n plug connected but IMO I can't see why not! If your feeder is #12 the use #12/3 SO or SJO cord.

[BBurger]

3-way switches are not allowed to be used for switching a neutral conductor, period. And neutrals are not allowed to be switched at all unless the hot is simultaneously switched. See NEC 404.2

[paulhiggs]

The only thing I worry about is damaging the furnace control board with the "dirty" electricity of the generator compared to the power company's. Any ideas?

Could you please elaborate on what you mean by "dirty electricity". I'm thinking of configuring my finished basement including furnace and small (15amp) air conditioning unit to take generator power when there is an outage (staying cool in summer and warm in winter). The comment about electricity created by a generator worries me, is there anything that can be done to make it better? Are some generators better than others?

[paul52446m]

Could you please elaborate on what you mean by "dirty electricity". I'm thinking of configuring my finished basement including furnace and small (15amp) air conditioning unit to take generator power when there is an outage (staying cool in summer and warm in winter). The comment about electricity created by a generator worries me, is there anything that can be done to make it better? Are some generators better than others?

There is nothing wrong with running a furnace off a generator as long as you keep the wires right. If you cross the hot and ground it will not prove flame and keep going. Also if the furnace is not grounded it will not work.
The furnace is like a lot of electric units, it will run ok up to 10% over or under its rated power. So make sure you check your gen. to make sure it is putting out right. Paul

[HayZee518]

"dirty electricity" refers to the output of a generator. Line voltage off the grid is regulated insofar as frequency and voltage. A portable generator can provide up to 135% more or less than line voltage and its frequency depends on the generator's rotation speed. Motors of a furnace utilize 60 cycle or hertz for frequency as well as control boards [microprocessors] A portable generator's "frame" is never grounded, even though the instructions say to ground the equipment through a ground rod. Fixed generator systems are grounded.

[paul52446m]

"dirty electricity" refers to the output of a generator. Line voltage off the grid is regulated insofar as frequency and voltage. A portable generator can provide up to 135% more or less than line voltage and its frequency depends on the generator's rotation speed. Motors of a furnace utilize 60 cycle or hertz for frequency as well as control boards [microprocessors] A portable generator's "frame" is never grounded, even though the instructions say to ground the equipment through a ground rod. Fixed generator systems are grounded.

I keep my 5000 watt gen so with nothing plug in the RPM are set to put out 120
volts and if the governor is working it will keep the voltage right as i plug more
items into it as long as i don't go beyond the rating of the generator out put.
A lot of guys don't keep check on the outputs. I do because i don't want to
burn up tools. Later Paul

[HayZee518]

do you monitor frequency too? it is a well known fact that as you add loads, a generator will tend to slow down, so you increase speed to compensate for the load. keep adding loads and the generator reacts. add too much load and the generator will overload, reducing its output. no matter how much speed you add it won't overcome the loads. eventually the windings can't take the increase so they heat up and you endup losing the winding and the gen burns up.

[paulhiggs]

The generator has a 4 wire (10ga each) connection to the box on the house then into the sub-panel with one of those being a ground wire that is then connected through the grounding of the house.
Does "floating ground" mean that its not connected inside the generator?
The transfer switch only switches the hot wires, the neutral and ground from the generator go to the busses in the sub-panel.

[HayZee518]

the way the generator is wound determines the neutral. home generators use two windings, each for 120 volts, 180 degrees out of phase with each other so you can run 240 volt equipment. the end of each winding is connected to the other and one lead is brought out which is your neutral. from either leg to the neutral you will read 120 volts. some diagrams show the end of a winding terminating at its circuit neutral - in reality they are connected together. floating ground means the neutral point is NOT connected to the machine equipment ground.