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Joined: Nov 2004
Posts: 138
How come some 240V single phase loads don't require a neutral; just 2 hot legs and a ground?

Joined: Dec 2001
Posts: 444
Load is Balanced. No neutral req.

Joined: Nov 2004
Posts: 138
So in a 240 V oven the load isn't balanced?

Joined: Dec 2003
Posts: 524
... Clydesdale,it's not that the load is unbalanced in an electric oven,..but that the timer,clock and light requires 120 volts,..ergo the much needed neutral.. [Linked Image] [Linked Image]

.."if it ain't fixed,don't break a Licensed Electrician"
Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Clyde, let me try to give a VERY simple explanation.

You have two "hot" wires. They are "hot" with AC. Measure either to ground, and you have 120 volts; together, and you have 240.

This is because the 120 originates at opposite ends of the same transformer. Since they come from the same transformer, they are exactly opposite in timing of the "60 hz."

In a manner of speaking, the first wire is at +120 at the exact same instant as the other is at -120. So they can be considered to "cancel each other out;" were there a neutral wire, it would have no current flowing on it.
Another way to look at it is that one wire is "hot" while the other is "off." In this model, you imagine each wire using the other as a neutral, in turn.

Joined: Jul 2004
Posts: 9,791
Likes: 14
I still wonder why the appliance folks didn't just use 240v bulbs, motors and timers in dryers and ranges.
They sell millions every year so there is plenty of volume. These parts are just used in the specific appliance anyway.
There may have been a big cutover but that is nothing compared to the change in a quarter billion dryer and range receptacles and a mixed cord standard.

Greg Fretwell
Joined: Feb 2005
Posts: 219
Let me see if I understand this theory correctly. Since I don't have an oven I'll just hold the 2 hot legs together to simulate the element, Timmy are you ready?.........Will they still "cancel each other out" or will see a really huge flash, oh well here it goes.

Ah Oh, I think we need a new Timmy!

Joined: Nov 2004
Posts: 138
ok. so, the clock/timer in the 240V appliance is only using one hot leg(120V), therefore my resoning tells me that there is unbalanced current.

Joined: Feb 2005
Posts: 693
Correct - the current is unbalanced by an amp or two, which is why this type of appliance (dryer, range, etc.) now requires a neutral.

A dryer, for example, uses 240 only for the heating element, which is why they function fine on 208v with slightly less heating power.

Larry Fine
Fine Electric Co.
Joined: Aug 2005
Posts: 49
Like said they balance the load between eachother, and there will always be an amp or two differance, just the way the phase works.

Then you get into 3-phase wich has an even less amount of time between pulses. In 240 you only have 2 pulses per cycle, at 3phase you get 3 pulses, so less amount of time that a device's voltage is lowering.
Wich is why 3-phase is more efficient.

Now all new range/dryers require a nuetral, but still not on air handlers, A/C's, water heaters etc...

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