[Amps] what is a 4 wire 240VAC service?

[Its from KE4VYN]

To understand how a 240/220 volt household circuit works you should first know a little bit about how a regular 120/110 volt circuit works. If you are at all familiar with residential electrical wiring then you probably already know that in most cases appliances, and fixtures connect to three wires: 


1) A black wire which is often known as the "hot" wire, which carries the current in to the fixture. 
2) A white wire called the neutral which completes the electrical circuit. 
3) A bare copper wire called the ground, the sole function of which is to enhance user safety. 

 

When the circuit is in use current is energized through the fixture by way of the "hot" wire and then to ground by way of the neutral, and unless something goes wrong the bare ground wire doesn't do anything except to remain ever vigilant in case of a problem. 

 

Now for 240/220 volt house current; Appliances that use straight 240 current also have three wires: 


1) A black wire which is often known as the "hot" wire, which carries the current in to the fixture. 
2) Another "hot" wire which may be blue, red or white (if it is white the code actually requires it to painted or otherwise marked one of the other colors, but often it is not) which also carries current in to the fixture. 
3) A bare copper wire called the ground, the sole function of which is to enhance user safety. 

 

That's it, no neutral. Now, if you are paying attention, then you are probably wondering "If there isn't a neutral wire then how is the circuit completed?" The answer is that when one hot wire is negative, then the other is positive, so the two hot wires complete the circuit together because they are "out of phase". This is why 240 volt circuits connect to double pole breakers that are essentially two single pole breakers tied together. In the main panel, every other breaker is out of phase with the adjoining breakers. So, in essence 240 volt wiring is powered by 2 - 120 volt hot wires that are 180 degrees out of phase. 

 

I previously mentioned "straight" 240 volt appliances, but there is another class of 240 volt equipment; some appliances (such as clothes dryers and ranges) use 240 volt current to power their main function (drying clothes or cooking food) but use 120 volt current to power accessories such as the clock on your stove or the light inside the oven, or the digital readout on your dryer controls. That is why some 240 volt circuits have four wires: 


1) A black wire which is often known as the "hot" wire, which carries the current in to the fixture. 
2) Another "hot" wire which is red, which also carries current in to the fixture. 
3) A white wire called the neutral which completes the electrical circuit for the 120 volt accessories only. 
4) A bare copper wire called the ground, the sole function of which is to enhance user safety.

 

At one time, the code allowed for one insulated wire to function as both ground and neutral in 120/240 volt combo circuits, but now all such circuits must use the 4 wire scheme. This is why your new dryer (or electric range) might have 4 prongs on its plug and your old dryer receptacle only has 3 holes. 

In which case article 250.140 of the 2005 N.E.C. (National Electric Code) allows for the "pigtail" (the cord and plug assembly) to be changed to match the old 3 wire receptacle as long as certain conditions are met. The National Electric Code allows that, but your local code might not, so check first.



73,

Lee

KE4VYN