Wiring Questions & dispell myths

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  #1  
Old 09-07-05, 07:21 PM
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Wiring Questions & dispell myths

Several issues / questions. Background - I am finishing my basement, new house is only 2 years old & I have pulled a building permit on it. I am now in the planning stage of my wiring runs. This will be my second complete basement finish & I also completly rewired an old Knob &Tube house

My father-in-law and I had a discussion last weekend about my wiring of the basement. We disagreed with the basic way to run the wires. He is a good DIY type builder & has a good general background, but I thought his suggestions were serious overkill & would make little or no difference. Don't get me wrong, he & I have worked together on alot of projects and get along well.

First he suggested for my general lighting & outlet circuits that I run 10/2 to somewhere in the middle of each circuit, then branch off it w/ 12/2 to each outlet or switch. If I did that, the 10/2 section would be around 20' & no more than 40'. Father-in-law said I would have less voltage drop. I agreed it would theoretically be less, but not enough to make any difference. I also did not want to mix wire gauges. I was planning on using 14/2 on these general circuits He was especially adament about the Sump Pump circuit & my shop circuit. Shop circuit begins right next to the panel. Both these circuits I was planning on using 12/2.

Regarding the Sump Pump circuit, currently it is on a 15A breaker w/ 14/2 wire with nothing else on that circuit. Pump label says 115V, 50/60 Hz, 9A. It used to be on the same circuit as our Home Office, but messed up the computers & we had the electrician come back & rewire. However the lights in the house still dim briefly / slightly when it comes on. That run is 50 -60' long. Thought I would rewire w/ 12G and place on a 20A breaker. I would also place on that circuit 2 -3 little used lights & a couple little used outlets. Will the 12G help with the light dimming issue?

Same brief light dimming problem with the AC when it turns on. We have two seperate A/C units, each on a 30A double breaker w/what appears to be 10G wire. Father-in-Law said taking the wire to 8G or even 6G sould help here too. Again I said probably not since the wire run is maybe 8' from panel to A/C unit. I am willing to do it if it will noticably help, but 8'?

Your comments & experience are greatly appreciated. I look forward to your responses. Sorry for the long post but I wanted to give a complete picture.
Mike
 
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  #2  
Old 09-07-05, 07:45 PM
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The idea of using 10ga from the panel to the circuit, followed by lighter gage wire for the immediate vicinity of the circuit is a fine idea _in theory_. The only way to figure out if it is worth the extra cost, extra effort, and required increase in size of things like junction boxes etc. is to do a voltage drop calculation. I rather doubt that it is worth it for runs in the basement right near the panel.

The distance that you can run 12ga wire, carrying a full 20A, with a 3% voltage drop on a 120V circuit is about 55 feet. 3% VD is a pretty severe design criteria, and 20A is far more these circuits will carry the vast majority of the time.

As far as the lights dimming when various motor loads start, increasing the size of the wire to them won't particularly help. You will reduce the voltage drop to the load, which means that the load will start faster...but during this slightly shorter starting period, the load will draw slightly more current, making the dimming deeper.

If you want to eliminate this dimming, you could use a 'soft start' hardware (possibly not available for these loads, but available for _some_ air conditioning system) which decreases the current required for starting the motor and increases the time it takes to start the motor. The other possibility is to get the power company to increase the size of the service wires to your house, possibly increasing the size of the supply conductors. Needless to say, pricey.

If you do go with 10ga wire, remember that larger wires require more room in electrical boxes, meaning larger boxes, larger wire-nuts, etc. 10ga NM-B cable is so stiff to work with that you might prefer to run THHN wires in conduit; you can get 10ga stranded THHN, which is quite easy to work with.

-Jon
 
  #3  
Old 09-08-05, 04:42 AM
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I don;t think using 10 gage wire will help you at that distance. You probably won't notice any difference.

I would not use 15 amp circuits at all. Just buy 12 gage wire and do everything with that, on 20 amp circuits.

Put the sump pump on it's own circuit. Don't ask for trouble. You probably can;t do much about the dimming, except to live with it.
 
  #4  
Old 09-08-05, 06:41 AM
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I also would recommend all 12-gauge wire. It's also simpler because you only have to stock one size for your whole project, and you can buy it in larger rolls.

Reminds me of the time I built a step-ladder out of all 2x4s. It was the sturdiest ladder you'll ever see, but so heavy as to be impractical to carry around and actually use. The use of 10-gauge wire is similar--overkill. But your FIL sounds like my kind of guy. Better to overengineer than underengineer.
 
  #5  
Old 09-08-05, 06:59 AM
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I partially disagree with the statement "Better to over-engineer than under-engineer." and believe that a caution is in order.

Clearly, the ladder built of 2x4s is going to be sturdy and heavy. So why not go even further, and make an even 'stronger' ladder out of 4x4s! But then you have to think about the nails that you use to hold the thing together. If you don't increase the size of your nails, then you could end up with a situation where the thickness of the side rails eats up most of the nail length, and the nails don't 'bite' into the rungs. If when you 'over-engineer' the thickness of the wood, you do not correspondingly increase the strength and size of most _everything_ else associated with the wood, you might actually end up with a _weaker_ ladder.

The same caution applies to 'oversizing' electrical components. Sure a 10ga wire will have less voltage drop than a 12ga wire. But that wire needs to be terminated somewhere. If your receptacles cannot directly accept 10ga wire, then you will need an additional splice; going from the 10ga down to a 12ga pigtail. This additional splice could eat up any voltage drop savings that you get in a short run! Using 10ga wire probably means larger junction boxes, different receptacles, and much more difficulty folding the wires into the boxes.

I am not saying that over-engineering is bad. Just saying that if you ever choose to over-engineer a component of a system, that you consider the entire system into which the component fits.

An additional caution is that sometimes armatures decide that they know better than code, and want to over-engineer in ways that are not supported by code. When ever someone suggests over-engineering something, make sure that the result is actually code compliant; sometimes it isn't!

-Jon
 
  #6  
Old 09-08-05, 08:58 AM
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Excellent points, Jon. In my experience, overengineering occurs most often in the absence of information. You sit there thinking to yourself, "I wonder if a 2x6 will be strong enough there?" After pondering this for a while and realizing that you don't have enough information to answer your own question, you put in a 2x8 or 2x10 "to be safe."

There's an old expression that says that an engineer is someone who can do with a dollar what any fool could do with two.
 
  #7  
Old 09-08-05, 09:05 AM
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Originally Posted by John Nelson
There's an old expression that says that an engineer is someone who can do with a dollar what any fool could do with two.
I think this is a compliment. Thank you.
 

Last edited by racraft; 09-08-05 at 12:18 PM.
  #8  
Old 09-08-05, 11:50 AM
Sparky375
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One other prob you will have is that 10ga wire can be fused at 30 amps and 12ga at 20amps so at some time some one could change out the breaker on this ckt and over fuse the 12ga on down the line which in turn is a code violation
 
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