Burnt connection on a dedicated 240v line

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Old 02-10-15, 01:14 PM
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Burnt connection on a dedicated 240v line

Hello, I'm hoping someone can lead be in the right directly. I'm trying to find the source of a burnt connection.

Here's some background info...

I had a 240v~30 amp dedicated circuit installed by an electrical for a spa unit in my basement . A few weeks later, we needed to move the unit to another area so I ran a new wire from the old outlet to a new one. I used the old outlet as a junction box and attached the wires in the junction box using wire nuts.

Ohho moment...

Once I was done, I plugged in the unit and turned it on. The unit started up fine. However a few seconds in, I started to noticed a burning smell. I turned the unit off and was able to tracked down the source of the smell. One of the connections (red wires) in the junction box were damaged. The wire nut was slightly melted and the wires were a little black.

At first I though that the damage was due to a loose connection, maybe I didn't secure the wires tight enough. However I got some readings that I did not suspect (#4 & #5) and now think maybe the problem may be coming from the unit itself.

Any comments on the readings would be appreciated. Also, below is a diagram of the circuit.


1- If I take a reading from the wires @ the junction box going to the beaker I get 242v, as expected

2- If I take a reading from the wires @ the outlet I get 242v (unit not plugged in), as expected

3-With the damaged wires (red wires) disconnected, if I take a reading at the red wire going to the outlet to ground I get 0v, as expected


4- If I plug the unit into the outlet (but not turned on) and then check the the red wire going to to the outlet (the red wires are NOT connected) to ground i read 120v. (I expect to read 0v)

5- Also, while the unit is still plugged in (but not turned on), if I check the voltage between the two red wires in the junction box ( NOT connected together) I read 242v







Attachment 46300
 
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Old 02-10-15, 01:22 PM
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What size wire did you run for the new leg?
 
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Old 02-10-15, 02:03 PM
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I used a 10/2 wire, same as the one installed by the electrician
 
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Old 02-10-15, 03:11 PM
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Red to ground should be ~120 volts. Black to ground should be ~120 volts. Did you use "red Romex" with just black, red, bare wires? Was the original cable "red Romex"?
 
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Old 02-10-15, 03:37 PM
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Once you plugged the spa in it became a junction (a bridge) between the red and black.
So you're measurement of 240vac thru the spa is quite normal.
 
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Old 02-10-15, 06:57 PM
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The original wire was Romex, I used BX cable for the extension. The lead wires are insulated and the ground is bare.

PJMax: So you saying, even though the red wires @ the junction box are disconnected. If I plug the unit in and take a reading on the red wire that's going to the outlet side (again not connected to the other red wire going to the breaker) that I should be get a reading of 120v to ground.
 
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Old 02-10-15, 07:03 PM
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The black wires are connected and the unit is plugged in. That means you'll measure 120vac on the red wire coming back from the spa because the spa is creating a bridge from the black wire to the red.

Now... if you measure between the two red wires you'll measure the 120vac coming thru the spa and the other 120vac leg coming from the panel and you'll see 240vac.
 
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Old 02-10-15, 07:51 PM
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Thanks for clearing that up.

Should I contact the manufacturer of the spa to see if the unit should be creating a bridge. Or is it most likely that the wires were damaged because they weren't connected properly.
 
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Old 02-10-15, 08:06 PM
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You may just not have had a good connection under the wirenut. You may be better off to twist the wires first and then cap them.

If you created a short in the spa..... the breaker would have tripped.
 
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Old 02-11-15, 05:21 AM
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The problem was definitely a poor connection at the wirenut that got burned.

Were any of the wires "old"?

I may be paranoid but when I wire things up, if the wire ends are not shiny I will gently scrape or sand the ends to get rid of oxidation, that can introduce resistance in the connection that in turn will lead to overheating.

Over a period of decades, the surface of the wire conductor can become tarnished under the insulation also.
 
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Old 02-11-15, 09:45 AM
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All the wires are new. I'm gonna attempt to reconnect it tonight.

I'll sand off the black residue on the damage wires and twist them together before adding the wire nut.

How do I know when to stop twisting the wire nut? I think I've read somewhere that the wires on the outside of the wire nut should twist around each other 2 times. Does that sound right??
 
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Old 02-11-15, 07:34 PM
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I tightened up all the connections and all is working well. Thanks for the help!
 
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Old 02-11-15, 08:50 PM
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Good news.

When installing wire nuts...... the insulation part of the wire does not generally get twisted.... just the copper wires.
 
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