question on 240 wiring

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Old 06-10-02, 06:13 PM
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question on 240 wiring

How come some 240 take four wires like the range and dryer and some take three wires like the water heater. How does one know if it takes a fourth wire. On the three wire do they just drop the netrual wire or is it the ground.
 
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Old 06-10-02, 07:08 PM
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Some appliances (like a water heater) require only 240V, so the neutral is not required. The heating element is simply connected from one hot to the other. Other appliances (like a range) have some 240V (heating elements, again connected across the two hots) and some 120V (light, clock, timer, connected from one of the hots to neutral) requirements and therefore need the neutral.

In older installations it was common to use the equipment ground as the neutral for things like ranges and ovens (the current drawn by 120V sections of these appliances is pretty minimal), but this is in violation of current code for new installations.
 
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Old 06-10-02, 07:17 PM
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Any appliance that uses 240 volts, but uses no 120 volts, only requires three wires: hot, hot, and ground. A electric hot water heater is one such appliance, as are many air conditioners.

Any appliance that uses some power at 240 volts, and some power at 120 volts, requires four wires: hot, hot, neutral, and ground. Most electric clothes dryers and electric ranges are such appliances.

Just to confuse you further, until about six years ago, code allowed electric ranges and electric clothes dryers (just these two appliances) to be installed on 3-wire service (with a number of restrictions). Furthermore, installing new ranges and clothes dryers is still allowed on 3-wire connections, if the connections are preexisting. New installations in new homes and remodels, must provide all four wires for these appliances.

To fully understand all of the above, you need to know how 240 volts and 120 volts are derived from your panel. If you need more help with this, post back and we can try to explain further.

Note that in European systems without any 120-volt power, 240 volts is always supplied on only three conductors.
 
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