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# Understanding 240 Volt Circuits

#1
01-10-08, 02:15 PM
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Join Date: Jan 2008
Location: South Central Florida
Posts: 8
Understanding 240 Volt Circuits

Four years ago I bought an old lake home in central Florida. In mapping the circuit pannel, I discoverd that I have a (4-wire) circuit which goes out to a boat house at the edge of the water (~350' from house) I assume that it is a 240v circuit since it has 4 wires; however, there is nothing out there that uses 240v. There is a 120v outlet and a 120v boat-lift motor. Why did they run 240v out there? Another 240v circuit which goes to my well pump has only 3 wires and runs a 240v irrigation-well pump and 240v timer. Somehow they are picking off 120v from the well pump. How is this possible? Another (apparent 240v circuit) has on it only an outside lamp post and outside recepticle - appears to be 240v potential because of the circuit breaker design. Please, explain to me 240v circuits.

#2
01-10-08, 04:28 PM
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Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
Originally Posted by lnlillard
I have a (4-wire) circuit which goes out to a boat house at the edge of the water (~350' from house) I assume that it is a 240v circuit since it has 4 wires; however, there is nothing out there that uses 240v. There is a 120v outlet and a 120v boat-lift motor.
This is actually called a multi-wire circuit; there are hot, hot, neutral, ground. It provides (2) 120V circuits which share the neutral and ground wires and can be more efficient over long distances which is presumably why they chose that design.

Another 240v circuit which goes to my well pump has only 3 wires and runs a 240v irrigation-well pump and 240v timer. Somehow they are picking off 120v from the well pump. How is this possible?
This is a "pure 240V" circuit; the three wires are hot, hot, ground. There is no legal or safe way to get 120V from this circuit unless you also have a subpanel breaker box at the well pumphouse.

Another (apparent 240v circuit) has on it only an outside lamp post and outside recepticle - appears to be 240v potential because of the circuit breaker design. Please, explain to me 240v circuits.
You are probably looking at a tandem (dual/slim/mini) breaker which looks like a 240V breaker, but is actually two 120V breakers in a single package. It's also possible that this is another multiwire circuit.

#3
01-10-08, 07:51 PM
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Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
It takes two hot wires to make 240 volts. Straight 240 volt circuits have two hot wires and one ground.

Combination 120/240 volt circuits need two hot wires and one neutral. They will have those wires plus a ground. (Some old installations, such as for electric dryers and electric ranges may only have three wires and are grandfathered.)

Circuits to a sub panel in an outbuilding may be either three wire or four wire. A ground is only required sometimes.

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