Efficiency comparison

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Dave6

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Deployed...FL when I'm not
How cost effective is it to put residential wiring underground (slab) versus through the attic? Underground will leave the wire cooler which will decrease the resistance which = less $$$. Understood that the material and labor costs will be a factor, but not a concern right now. I?m just looking for a direct comparison?the other numbers I can figure out much easier.

With the same mindset?is going from a 12 to a 10 wire cost effective? I?m on Chap 9, table 9 but am not sure how to exactly use this info.

These questions come from the last code class I took where the instructor touched on the code only being the minimum and not efficient (90.1 B). Any help here would be greatly appreciated.
 
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ty

Senior Member
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3rd Rock from the Sun
I don't think it would be cost effective at all.
Since you say 'slab' then your wiring method would change from nm to uf minimally.
Also, your labor cost would increase.

Going from #12 to #10 would also not be cost effective.
The difference in wire cost alone would be too high to see any monetary gain from any effects of heat loss.
The boxes would have to be bigger, spicing wold be harder, and in general the labor cost would be more to install.

This is, of course, discussing effects of general branch circuits.
There might be some benefit to oversizing the wire on your a/c, but that would be a stretch.
 
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Dave6

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Better question; how many kWh would be saved annually between these two ctks? Ctk A, is #10 in the slab with an ambient temp of 70. Ctk B, is #12 through the attic with an ambient (avg) temp of 120. Both 100' long, everything else the same. This will give me a cost comparison which is what I'm looking for.

I would also like to learn how to do this.
 
J

jghrist

Senior Member
It isn't normal procedure to compare losses in residential construction to determine conductor size. One of the reasons for limiting voltage drop in circuits is to have an efficient circuit. More voltage drop means more losses.

If you want to do it, then calculate the conductor resistance of the two alternatives, R1 and R2, accounting for length differences and temperature difference. Peak loss difference is I??R1 - I??R2 where I is the peak current. Energy loss difference is more complicated because it depends on how the current varies with time. Calculate the losses in kW for each hour with the currents in each hour for a year, then add up the results to get kWh energy losses for the year.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Electrical Contractor
Dave, in order for a larger wire to save money, you'd have to be doing something different without it. For example, if you need a 100w bulb because a 60w is too small due to voltage drop, then it would help. If you'd use the same bulb either way, then it won't.

On the other hand, an electric space heater would have to run longer to meet a certain heat demand, so, unless the heat lost to voltage drop contributed to heating the space, minimizing voltage drop could possibly pay for itself with lower running costs over time.

As for under-slab vs above-floor wiring, I don't think a single penny would be saved by under-slab wiring. Running directly through the studs would result in even less wire being used, and a 10-degree difference in wire temperature isn't going to make any difference.

Added: You'd lose less energy to voltage drop by making straight runs wherever possble, rather than using the earth to cool the wire. Use efficient wiring methods and use the right sized wire for the job. Don't waste money on useless gestures.
 
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Jraef

Jraef

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Despite the side issue of large or smaller wire, the specific answer to your question needs the "temperature coefficient of resistance" (tempco) of the copper wire, meaning how much the the resistance changes as the temperature increases. The answer is 0.00393 ohms per ohm per degree C.

So for example, 12ga copper wire has a resistance of 0.00521 ohms per meter at 20C (68F). So applying the tempco to it at a 30C difference in temperature (50C = 122F), you have 0.00521 x 0.00393 x 30 = .000621 ohms per meter increase, so + 0.00521 = .00583 ohms per meter at 50C.

So at 100ft (30 meters), the difference in resistance is .1749 ohms in the attic vs .1563 ohms in the slab. From that you can do a simple Ohm's Wheel calcs on voltage drop from your 10A load**. E = IxR, so 10A x .1749 = 1.749V drop in the attic, vs 1.563V drop in the slab, making a difference of .186V.

Now calculate the power consumed at those two values; assuming 120V and a resistive load for simplicity, running 10 hours per day, at a rate of $0.10/kWh. **

Slab = 120V x 10A = 1.2kW
Attic = 120V - .186 = 118.437V, x 10A = 1.184kW
Difference = 16W
Value = .016kW x 10 hours = .16kWh per day, x .10 / kWH = $0.016 per day in savings,
x 365 days per year = $5.84 per year.
Now amortize that out against the difference in cost to see the payback.


**We also have to ASSUME the load will actually consume 10A either way, as opposed to just consuming less and doing less work because of the slight increase in voltage drop, i.e. a dimmer bulb or a motor with slightly less torque, which is the reality here.
 
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Dave6

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Deployed...FL when I'm not
All points taken. Thank you. When I have time, I will crunch some numbers as well.

I am now kicking myself for not asking more questions to the instructor of the last code class I took. I also shot an e-mail off to copper.org for a better explanation.

I will be retiring from the Air Force relatively soon, and if we do build I would like it to be as effecient as pratical. I'll also be moving back up north...
 
Jraef

Jraef

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Also, pursuant to their website suggestion of using #10 wire instead of #12 on a 20A lighting circuit.

Good luck pushing #10 into a back-stabbed switch or outlet. Are screw terminals even listed to accept #10? I'll have to look but I don't think so, I seem to recall they say #12 max. So that would mean adding pig tails to every connection. Fun...
 
infinity

infinity

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New Jersey
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Jraef said:
Also, pursuant to their website suggestion of using #10 wire instead of #12 on a 20A lighting circuit.

Good luck pushing #10 into a back-stabbed switch or outlet. Are screw terminals even listed to accept #10? I'll have to look but I don't think so, I seem to recall they say #12 max. So that would mean adding pig tails to every connection. Fun...
Click to expand...


Back-stab holes only accept #14 conductors. Binding screw terminals accept up to #10 conductors.
 
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Dave6

Member
Location
Deployed...FL when I'm not
I was about to just let all this go...then found this; http://www.mikeholt.com/mojonewsarchive/EC-HTML/HTML/Voltage_Drop_Calculations~20030326.htm

Here is the line that perked my interest; "System efficiency. If a circuit supports much of a load, a larger conductor will pay for itself many times over in energy savings alone." Now in a residential setting most of your loads are minimal by comparison...but running my HVAC, water heater, dryer etc. with a larger wire be worth it?

Jraef I follow your math...I think anyway. Thank you for the time and effort.

My house in FL has all Romex, all run through the attic...where I am quite sure it gets close to 140 if not above in the summer. Now if I am reading the code correctly (334.8) NM cannot be used if you exceed that temp. But I don't really want to open that can of worms.
 
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broadgage

Senior Member
Location
London, England
Upsizing wires beyond code requirements will result in less voltage drop and therefore either a saving in money, or more effective operation of the connected load.
In most cases however the saving is minute and will take decades to cover the cost of the thicker wire, that is becuase most residential branch circuits are either lightly loaded, or heavily loaded for relatively few hours a year.

If a circuit is expected to be heavily loaded for thousands of hours a year then the position is different, and upsizing may well be worth while.
For example, consider a 120 volt 20amp circuit this is routinely used for an A/C unit drawing 15 amps, for 4,000 hours a year.
At 3% voltage drop that is a loss of 54 watts, or 216 KWH a year.
The cost of the wasted power might amount as much as $50, which would pay for the larger cable in a few years.

That however is an extreme case.
 
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