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Old 12-13-2010, 02:35 PM   #1
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240v conduit with different gauge wires...


Hello all

I have a seemingly simple question, but the more I think the harder it becomes for me to answer it.

I am re-using a conduit at the front of my house that was installed underground (apparently) to feed an electric barbecue on my patio. What ever it was is long gone.

It is live, and the end was a receptacle box with the receptacle missing and 4 wires sticking out of it. I found 240v on 2 #10 wires and 110 between the two #10 black wires and a #12 white wire. I also found a #12 green wire that also provided 110 ground to the two 240v wires...

The limit of the safety was two wire caps twisted over the 2 black wire ends....

I tested the circuit breakers in my panel and found a dual 40 amp breaker that shut the power off.

I need to power my new AC system that runs right past the installation location, so I dug up the conduit, cut it open and pulled the wires out of the end.. and rerouted the conduit to a 2 wire fuseable AC disconnect box... that is when I decided I needed more help.

1. I need 20 amp - 240 volt for my mitsubishi heat pump (no electric heating) unit. That is from the manufacturer specs.

2. I need a GFCI 20 amp circuit to meet code... OR.. I may be able to use the existing outdoor plug on my patio to meet that requirement. I don't know if it is GFCI protected, but it has been there previously and it is within 25 feet.

3. The return (white) wire is obviously smaller than the two black wires. I have been trying to decide the best way to wire up a new, dedicated GFCI right at the condensor, and I would "like" to provide one more circuit (120v) for future use for a fountain motor, not to mention outdoor lights, christmas tree lights, power for an outdoor radio etc....

I was thinking this "may" be code acceptable.. but I understand it may be a big no-no also. Please don't get angry with me, I am trying to understand the original circuit and why it was installed with different gauge wires, and if I can take advantage of that fact. That is why I need your help.

If I put the two black wires to the condenser, and do not use the wite common wire, I have 240 volt 40 amps. I only need 20 amps, so I am thinking to run the two black wires to 2 - 20 amp fuses, and then the output of the fuses / disconnect switch (and ground of course) goes to the condensor.

If I then tap one black wire (before the fuse), and the common 12 gauge wire, I can pull 20 amps of 120 from it. I suspect strongly that this will require sub panel and a master breaker for the sub panel for what I am thinking.. am I right? or can I simply wire it inside the disconnect box?

Also.. could I (don't get mad.. this is why I am asking). run a third 20 amp circuit from the same common white wire and the other black wire? The 40 amp circuit breaker in the main panel will keep either black wire from going over 40 amps, but... I don't understand the reasoning of 240 vs 120 and the 2 seperate phases involved. IE, here is my real question....

For my question, forget about the 240 volt part for a moment and think only of the two black wires, and the one white wire. I know this is common in electrical panels, but if 1 phase runs on 1 wire, and a second phase runs on the second black wire and both phases return back to ground through a single white wire.. is the white wire truely carrying 40 amps? Or is a portion of the circuit (the other black wire) carrying some of the return current and that allows the white wire to only carry 20 amps? This is my question. Also, I don't understand why the previous electrician pulled a smaller white wire (and ground) and 2 larger black wires. I am not even sure if I have a 2 phase 240 volt or 2 phases of a 3 phase 480 volt system in my house. I get a bit confuses about phases.

I guess what I really want to know is the simplest, way I can get 1 - 240v circuit and 1 or 2 (preferably) 120v circuit from this particular selection of wires. I think I will have to have a sub panel and feed the disconnect panel and a GFCI box from it... but I am not sure if there is a safe and code approved way to do this....

I can't pull more wires (they fit like a glove inside the 1/2" conduit now...., and it isn't worth it to dig up the conduit and make it bigger

I did some electrical contractor work 30 years ago, but nothing major like this since then.

If you have any questions, please contact me for more information. I am not trying to get anyone killed includeing myself, just want to find out the true facts so I can re-work this wiring system with the least amount of work and cost....

Thanks for reading this "too" long posting for my question. Hope some of you decide to help me to understand this (and make is safe).

Biospot

PS, I forgot, the main water copper water pipe that feeds the house is available 15 feet away. I could run a ground wire to it with minimal work.....

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Old 12-13-2010, 06:04 PM   #2
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240v conduit with different gauge wires...


Lots of reading, but basicly, you can not do what yoou are asking.
If the black wires are #10, then the 40 amp breaker must be replaced with 30 amp breaker.
The #12 white wire must be replaced with a # 10 wire.

I would install a small 4 circuit panel at the end of the conduit.
Feed the AC with a 20 amp 2 pole breaker, and then add another 20 amp single pole breaker to hit the required receptacle, and use this feed for the christmas lights.

The water main has nothing to do with a subpanel.

The smaller ground wire is ok.

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Old 12-13-2010, 07:24 PM   #3
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240v conduit with different gauge wires...


Quote:
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....The smaller ground wire is ok.
Not so. On such a setup, one would be required to use a #10 EGC as well.

See table 250.122 in the NEC

UNLESS the job is in Canada, then all bets are off.
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Old 12-13-2010, 11:29 PM   #4
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240v conduit with different gauge wires...


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Originally Posted by kbsparky View Post
Not so. On such a setup, one would be required to use a #10 EGC as well.

See table 250.122 in the NEC

UNLESS the job is in Canada, then all bets are off.
You are right.
Thanks for catching that.
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Old 12-14-2010, 04:09 AM   #5
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240v conduit with different gauge wires...


Quote:
Originally Posted by jbfan View Post
Lots of reading, but basicly, you can not do what yoou are asking.
If the black wires are #10, then the 40 amp breaker must be replaced with 30 amp breaker.
The #12 white wire must be replaced with a # 10 wire.


I would install a small 4 circuit panel at the end of the conduit.
Feed the AC with a 20 amp 2 pole breaker, and then add another 20 amp single pole breaker to hit the required receptacle, and use this feed for the christmas lights.

The water main has nothing to do with a subpanel.

The smaller ground wire is ok.
----------------------------------------------------------------------
thanks jbfan. I appreciate the help you give to us!

OK I have done a bunch of reading and I see lots of things to investigate, but as a certain long term answer... can I do this?

1. replace the 2- 40 amp circuit breakers with 2 - 20 amp breakers then use the 12 gauge ground wire that is existing? I can simply disconnect the neutral wire at both ends, or possibly re-color it with tape from white to green. From what I see in the install guide, I don't need a neutral, just 220-240 AC and a ground..

2. replace the existing 20 amp outdoor receptacle with a GFCI receptacle to meet the code requirements of a GFCI within 25 feet of the AC.

I will give up the small circuit panel, though that would be nicest. I really can't pull a new cable except in extraordinary conditions......

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Old 12-14-2010, 07:01 PM   #6
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240v conduit with different gauge wires...


Anyone?
Can anyone explain to me about NEC 220-22? Can this apply in my case or not? I have not found any clear (simple) explanation for me to understand.

I have single phase 240, and the majority of the current will be used by my 240v heat pump. No connection to neutral will be used to the AC unit at all. It is designed for Japan and 240v power
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Old 12-14-2010, 07:36 PM   #7
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240v conduit with different gauge wires...


Change to a 20 amp breaker, ditch the neutral and you should be fine on the 240V.
Make sure that you use a 2 pole breaker.

Depending on when the house was built, the outside receptacle should be gfci protected now.
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Old 12-14-2010, 08:07 PM   #8
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240v conduit with different gauge wires...


Thank you very much. I feel much releived on this...




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