double wiring an overloaded circuit

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  #1  
Old 12-05-05, 07:36 PM
robprime
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double wiring an overloaded circuit

Is it ok to add a second, parallel, wire to a 240 VAC, 60 hz home power supply line to handle increased current demand?

The second wire must also be about 10 ft. longer than the original one, due to not being able to access the area the original is run through. This won't cause any waveform overlap, phasing problems, will it?

I remember reading somewhere that double wiring a circuit like that is ok but both wires must be the same length. However 10 ft. (about 10 ns) waveform overlap at such a low frequency would seem to be negligible.

Am I overlooking something or can I run my wire?

thanks,
Rob
 
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  #2  
Old 12-05-05, 07:45 PM
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You are remembering some of the rules for paralleling conductors. The big part is that the conductors must be at least 1/0 before this is allowed. Your circuit will not be anywhere close to this size.

You will need to replace the current wire with the correctly sized wires.
 
  #3  
Old 12-05-05, 07:49 PM
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I'm not sure what exactly you're doing, but it's almost certainly not allowed. Can you provide any more details? The length of the wire is immaterial, but the size might be. And where exactly is this wire running to and from?
 
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Old 12-05-05, 08:12 PM
robprime
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Thanks for the answer.

No, the wires are #6-3 w/gnd, about 40 ft. in length. 32 on the original. Expensive to replace with what? A single #2? I need about 75 amps.

Out of curiosity, is the waveform overlap thing the problem? I can see how a significant percentage of overlap, say 1% or even 1 tenth % could cause problems by broadening the duty portions of the combined waveform, maybe causing some voltage adding or subtracting problems. But 10 ns in a 16.67 ms waveform is like less than 1 thousandth of a percent.

I'm not arguing, just asking. Are there some other considerations that I'm overlooking?

Rob
 
  #5  
Old 12-05-05, 08:21 PM
robprime
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Just caught John Nelsons reply.

This is to power some additional tools in an already loaded 30 amp garage service.
 
  #6  
Old 12-05-05, 08:23 PM
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This has nothing to do with wire length, nor with waveform overlap. It has to do with one of the wires overheating and starting a fire if the other wire should fail for any reason.

I'd still be interested in knowing exactly where this wire would run from and to.
 
  #7  
Old 12-05-05, 08:37 PM
robprime
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You are right, John. I was overlooking that. Thank you for pointing that out.

What got me going on the overlap thing was that I recall the post/article that I had read insisting that the wires be 'exactly' the same length and I didn't understand why.

Equal length obviously has no bearing on one wire failing and the other overheating.

I guess safety dictates replacement, even if I must tear up some walls and stuff.

Thanks guys,

Rob
 
  #8  
Old 12-06-05, 04:27 AM
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Why not just abandon the first wire and add the new wire (large enough for the new demand) and run it the slightly longer path?
 
  #9  
Old 12-06-05, 06:59 AM
robprime
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This garage has 2 #6 wires running to it. One is to the 30 amp subpanel (the 32 ft. one) and the other was a dedicated line for a clothes dryer (which has been moved inside the house to the utility room).These wires enter the garage wall at two different points, about 8 ft apart.

These wires run from the main panel in the utility room through bottom story ceiling/ 2nd story floor joists accross the house to the garage. Removing and replacing them would entail quite a bit of wall and ceiling trashing.

My thought was to make a splice inside the old dryer outlet box and jumper over to the subpanel, thus adding up to the approx 8-12 ft length differential.
Disconnectng them and bypassing with a new heavier wire would also mean the same thing. With no attic to run it through, it's that or running it through the living area exposed (not happening), as a wire that size is not one you tuck behind a baseboard.

Rob
 
  #10  
Old 12-06-05, 07:30 AM
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This is how I construe the description of the problem---- a load ( type unknown) is supplied by an "individual" Branch-Circuit with conductors with an ampacity of X amps. For some un-known reason, the load has increased?, and the ampacity of the Branch-Circuit conductors is inadequate. An example would be replacing a 3HP motor with a 5HP motor, or replacing a 4KW heating element with a 8KW element.

Perhaps a more definite description of what has caused the load to increase would assist in a possible solution.
 
  #11  
Old 12-06-05, 08:11 AM
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As I understand your posts so far, you have a #6/3 with ground NM-B cable from your main panel to a garage subpanel which is breakered at 30A.

If this is the case, there is a good chance you can just replace the garage subpanel with a 60A (or greater) subpanel and increase the subpanel feeder breaker in the main panel to a 60A.

Are you absolutely sure you have #6 wire feeding the subpanel? Cloths dryers are usually wired with #10 wire which is rated for 30A, and I see you have compared the two in an earlier post. Also, you must verify if you have copper or aluminum wiring. Aluminum wire has lower ratings than copper.
 
  #12  
Old 12-06-05, 08:16 AM
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You may have another possibility with the abandoned dryer circuit if and only if the circuit has 4 wires.

If the abandoned circuit has 4 wires, you can replace the breaker with a 20A double-pole and create a multi-wire circuit for receptacles in the garage. This is essentially two 20A circuits that share a common neutral and ground.

To see if this is feasible, tell us exactly what you have in terms of wires at the old dryer receptacle. We need to know colors, gauges, and material. Do the same thing for the subpanel feeder as I described in my previous post.
 
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