electrical / generator questions

cinobyteJanuary 5, 2010

I have a 5500 watt(6500 surge) genset im a little confused on how it handles loads. the generator has a 120V/20A duplex and a 4 prong twist lock and a breaker for each. the breaker specs are 125/250V/20A.

say I want to use all 5500 watts. is it best to split the load 2500w to the duplex/20 amp breaker and 2500 to the 4 prong/20 amp breaker?

I plan on back feeding in to the house and I am trying to use the genset as efficiently as I can. any and all info is greatly appreciated.

side note: I know how every one feels about back feeding with out a transfer panel but i plan to only back feed my basment. My basement currently has its own service panel which is NOT connected to my main panel. during emergency power outages we move to the basment where we have a wood stove but need electricity.

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stinkytiger(z5 - z6)


Backfeeding is bad so do not do it. Enough said ...

As to the power bit the exact specifications will depend on just what generator set you have.

However I will make some general comments about portable gen sets. What usually confuses people is "phase". Huh? What is that? Phase is the difference in timing of two signals. Imagine two waves. Now also imagine they are perfectly aligned. The two signals are said to be in-phase. Now imagine one of the waves shifted along such that when one wave is at its peak, the other is at its trough. This is when they are out-of-phase.

Phase is usually measured in degress. In-phase = 0, out-of phase = 180.

OK enough science, how does this affect a generator. Your generator probablly has two phases. On the four prong plug, the connections are most likely Ground, Neutral, Hot 1 and Hot 2. Hot 1 and Hot 2 are 180 out of phase with each other. The voltage between Hot1 and Neutral is 120V, and the voltage between Hot 2 and Neutral is 120v. The voltage between Hot1 and Hot2 is 240v. 120V + 120V = 240V because they are 180 out of phase.

To get the most power out of the genset, you want to split the load between the two phases. About 2500w per phase using the 4 prong plug. You do not want to use the other sockets as well on your genset because they are essentially just connected to the four prong wires internally anyway.

Some important points. If you back feed using a suicide cord (naughty, do not do it, not recomended), you can only back feed one of your phases. Hence only use 1/2 power of your genset. Internally in your house you have some circuits on one phase, and some on the other phase. If you short the phases together by mistake, by trying to back feed both phases from your gen set very bad things happen.

If you load only one phase on your genset, that is bad for your genset. It is unbalanced. It is like having a flywheel with an uneven weight distribution. Your genset will run rough.

I therefore strongly recomend that you get yourself hooked up properly with a transfer switch. This enables both phases to be used and safely as well. Also a correctly hooked up transfer switch will enable the Bonding and Grounding of the genset vis-a-vis your house to also be done properly, also enhancing safety.

Warmest regards, Mike.

    Bookmark   January 5, 2010 at 9:43AM
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Mike knows his stuff but unfortunately, he did not understand what you wrote. If your basement has its own panel that is totally isolated from the grid system, then just run a 4-wire extension cord from your genset to that panel. I would use 10/4 cabtire Type SJO for this purpose.

Put the green wire to the panel ground, the white to the panel neutral and the black and red to the main breaker in that panel if it has one. Make sure the bonding screw has been removed because there is no reason to tie the neutral and ground together.

If you are using wood to heat, then what are your loads other than lighting? If you have a basement refrigerator, that would likely be the heaviest draw. Are you transferring other loads over to this emergency system you wired up? Important things would be a well pump, sump pump, septic system discharge pump and basement freezer to protect stored food. Entertainment draws very little these days. Motors will be your biggest concern.

If you don't have all those motors, then I don't see you using more than 20 amps unless I'm missing something. You should do a survey of your power requirements. Look at each branch leaving the panel to see what the wattage or amperage is. Try to balance out the load between the two phases as Mike suggested.

    Bookmark   January 5, 2010 at 10:12AM
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