240v question.

[Mark42]

The control lights and clock on my electic stove stopped working. So I pull the stove out, unplug it and open the back expecting to find a blown fuse or burned connection, etc. But everything looked fine. So I plug it back in and using my electic meter, start tracing the power. Funny thing it showed 240 volts across the two hot lines, but showed zero volts from each hot to ground. I was confused. So I unplugged the stove, and tested the outlet. Same results 240v across the hots, and zero from each hot to ground. A quick check in the circuit box showed that each side of the double breaker had 120v when tested to ground and 240 across them. I checked the ground side of the circuit and it was not connected to the ground bar in the circuit pannel. It had come loose and was just hanging in the air. So I turned off the mains and re-connected the ground and everything worked fine. <br /><br />I understand why the 120v parts of the stove (clock, timer and lights) did not work. Because there was not a complete circuit with the ground disconected. But why did the 240V heating elements still get hot without a ground? Is it because alternating voltage is cycling back and forth between the 120 feeds so a ground is not needed? I just want to know how the heating elements worked when the 120v parts didn't.<br /><br />Mark.

 

[rwise]

Re: 240v question.

the heating elements would be on the 220/240 volt and not need the ground except for safety, one end of the element to one line of 120 the other end to the second line of 120.

 

[Paul Moir]

Re: 240v question.

I'm not really up on my New Jersey wiring codes, but by 'Ground' do you mean the white neutral wire? As opposed to the bare Ground conductor? I hope so!<br /><br />The elements and controls don't use the neutral wire for anything. Current flows out one hot wire (say black) and into the other hot wire (red). 1/120th of a second later, it flows the other way. As a matter of fact, when wiring 240v devices that have no need for 120v - like electric baseboard heaters and electric water heaters - often a wire is used that has no neutral conductor at all.<br /><br />Hope that explains it. I would like to find some graphs of single phase power and two-phase to help explain it, but I can't seem to find anything anywhere...<br /><br />EDIT: Post collision; rwise puts it simply.

 

[Mark42]

Re: 240v question.

Thanks guys!<br /><br />The 240 cable used for the stove has a black and red and bare ground. No white neutral. The house is all copper wire except for this one 240 circuit is aluminum. I checked and they breakers are coded for aluminum wire on that circuit. The house was built in 1992, so I'm sure the wiring met code at the time.<br /><br />So if I understand this right, the two 120 hots have to come from the separate 120 volt leads coming into the box. IE two 120's from the same "leg" won't work.

 

[chuckz]

Re: 240v question.

House power is single phase power. It comes from the secondary of a center tapped transformer. From each side of the transformer's secondary winding to the center tap is 120VAC. Across the secondary winding is 240VAC.

 

[Paul Moir]

Re: 240v question.

Yes, you're correct. To simplify it a bit, lets forget about AC and pretend it's DC at the panel. There's three lines entering it from the pole out on the street. One hot line is at +120 volts (say, black), the other (red) is at -120 volts, and the neutral line is at 0 volts. Inside the panel the neutral and ground wires are connected together, partly to ensure that the neutral is at the local 0 volts.<br /><br />So, between the black line and the white neutral you have 120v. Between the red line and the white is 120v, which is to say -120v. And between the black and red wires is 240v (120 - -120 = 240). Since the ground is at the same potential as neutral, the voltage between the black line and ground is also 120v. Between two wires coming from the same leg is 0v, since 120 - 120 = 0.<br /><br />Actually, that's pretty close to what's going on here. It's just that polarities (+ and -) are switching 60 times a second. <br /><br />It seems our codes are quite a bit different from yours in New Jersey. Up here we're very limited in the amount of current we're allowed to normally send down the ground wire. So appliances like stoves which have 120v components must have a completely seperate neutral wire - normally the same size as the supply wires (#8-3). Also, aluminium wire is now really rare and, to the best of my knowlege, no longer sold.

 

[Mark42]

Re: 240v question.

Paul,<br /><br />I think the electrical contractor used the aluminum wire to save money. Copper in 8 guage is expensive and there is about 50 feet of it between the circuit panel and the stove. If I had known when they were wiring the house, I would have insisted on all copper.<br /><br />My understanding is that aluminum is a better conductor than copper, but because of the oxidation and reaction problems with other metals it fell from favor in household wiring.

 

[Paul Moir]

Re: 240v question.

Copper is actually a better conductor, but just (11 ohms per circular mil vs. 17). And the places I've seen aluminum wire used here they went up to the next size on the heavier conductors - which would make it a better conductor overall. Like you said, the only real problem is the connections.<br /><br />So you think #8 2 wire (red&black) copper is expensive? We've got #8 3 wire running stoves in just about every house up here.

 

[sangerwaker]

Re: 240v question.

Copper wire is the highest I have ever seen it right now. I'm sure Paul can attest to that.

 

[Scaaty]

Re: 240v question.

Just an aside, but after reading all these posts, I still see why I got a D- in Electric shop, and a A+ in Auto shop! Anyone need to let the smoke out those wires, call me!

 

[crab bait]

Re: 240v question.

1992 was about the time the code changed from 2 hot wires an a ground ( 3 prong plug/an or outlet )<br /><br /> to 2 hot wires an neutral/white an a ground ( 4 prong plug/an or outlet)<br /><br />alum wire is still permitted over 10 gauge wire.. an NOT to serve mulitple devices.. <br /><br />anything below 8 gauge must be copper..

 

[Mark42]

Re: 240v question.

My wife wants to replace the GE stove with a nice glass cook top and separate oven. That means some changes to the cabinetry. I told her to wait until after the basement is finished this spring, and when the counter tops need replacing we will modify/extend a section of cabinets to take a glass drop in cook top and wall oven. <br /><br />So last week she burned the Formica top with something that was burning in the oven. I feel this is a highly suspicious event in light of the aforementioned* kitchen upgrade conditions.<br /><br />Anyway, when I do make the change to the stove, would it be worth replacing the aluminum with copper or just leave well enough alone.<br /><br />Mark<br /><br /><br />* I do believe that is the first time I have ever used that word.

 

[crab bait]

Re: 240v question.

whatever.. to go to copper,, can drop a size.. <br /><br /># 6 alum .. # 8 copper<br /># 8 alum... # 10 copper..

 

[Mark42]

Re: 240v question.

Thanks again to everyone who gave input here. I learn something from this website almost every day.<br /><br />I was telling a coworker about the stove problem. He said I didn't know how lucky I was to be able to trace the problem myself. He had a similar problem with his dryer a few years back. First he called an appliance repair man and paid him $75 for a house call just to be told that the dryer is OK and he needs an electrician to check out the house wiring. So an electrician comes in and replaces a faulty circuit breaker and charges him $180. Total cost $255 paid out (duly and rightly charged by the service techs) because he had no knowledge of home repairs.<br /><br />I know that electricity is dangerous and requires proper precaution. Still, people who have the talent and knowledge to make household repairs are saving themselves big $$$. <br /><br />With this stove problem, I never once stopped to think that I was saving myself money, I was just annoyed that the guy who installed the circuit never tightened down the ground properly, and how much time it took away from my evening to fix it. My coworker pointed out I should be grateful for my God given abilities, not angry over someone else’s shoddy work.

 

[Indymike]

Re: 240v question.

Change to copper wire when you do your remodel. Aluminum loves to heat and creep. That ground connection may very well have been tight at one time. <br /><br />Do the current connections at the breaker have an anti oxidizing compound on them...they should...it is a dark gray viscous compound kind of like grease. Aluminum terminations need to be periodically retorqued.<br /><br />(EDIT) And change the range receptacle and cord to four wire. <br /><br />Mike

 

[crab bait]

Re: 240v question.

your exactly right INDY..