Proper Wire Size

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awin65

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Need help....I have been tasked in running lighting in a class 1 div 1 location. I have 120V branch circuits at 900watts/140 feet and 2 @ 1500 watts/240 feet. I used the standard voltage drop calculation to determine proper wire sizes for each branch and came up with #6 awg based on 3% drop max per branch(5% max total feed and branch). This seems to be to large???? Other long distance lighting runs on site are using #12. What am I missing??
 
1500 watts @ 240 feet will be a #6 and 900 watts at 140 feet is #10.

You may want to just add a sub panel at the far end for the 2 - 1500 watt circuits.

Also check 250.122(B) you will need an EGC of #6 and #10 in the first scenario.
 
awin65 said:
Need help....I have been tasked in running lighting in a class 1 div 1 location. I have 120V branch circuits at 900watts/140 feet and 2 @ 1500 watts/240 feet. I used the standard voltage drop calculation to determine proper wire sizes for each branch and came up with #6 awg based on 3% drop max per branch(5% max total feed and branch). This seems to be to large???? Other long distance lighting runs on site are using #12. What am I missing??
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900 W @ 120 V is 7.5 amps. Using that number in the voltage drop equation gets you #10 AWG wire with 2.17% drop. No problem.

1500 W @ 240 V is 6.25 amps. Using that number in the voltage drop equation gets you #8 AWG wire with 1.95% drop. No problem.

I'd say what you're missing is the correct voltage drop formula. :D
 
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drbond24 said:
900 W @ 120 V is 7.5 amps. Using that number in the voltage drop equation gets you #10 AWG wire with 2.17% drop. No problem.

1500 W @ 240 V is 6.25 amps. Using that number in the voltage drop equation gets you #12 AWG wire with 1.44% drop. No problem.

I'd say what you're missing is the correct voltage drop formula. :D
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Or you are reading the op wrong-- 240 feet not volts. :D
 
Show your calcs. I did them and your close on the long runs but over.

Welcome to the Forum.

Also, why did you mention a classified location?
 
Dennis Alwon said:
Well good morning to you. :D
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Why thank you. I'm not exactly bright eyed and bushy tailed, but I'll get by.

I can work like this, right?

asleep-at-desk.jpg
 
A couple of things that you may have missed:

1) Voltage drop is a recommendation, not a requirement. The real requirement is that your loads tolerate whatever voltage you supply. One caution is that loads may appear to tolerate excessive voltage drop because they function, but on closer examination they are functioning with unacceptable performance. A case in point would be lighting, which might function but be too dim.

2) Many voltage drop problems are not caused by _excessive_ drop, but by _changing_ drop. An example in a residence would be lights on a receptacle circuit. The lights function just fine, and loads function just fine when plugged in...but the lights flicker when loads are plugged in. This is probably not a problem for the circuits that you described.

3) Voltage drop is caused by resistance, and wire resistance changes with temperature. Many people calculate voltage drop at the 75C resistance of the conductors, because that is what is tabulated at the back of the NEC. But the real voltage drop may be 20-30% lower than that prediction (eg a _drop_ of 4% rather than a calculated 5%).

4) If you can use multi-wire branch circuits then you can effectively reduce voltage drop on your shared neutral, as long as you have a number of circuits going in the same direction. This has to be balanced against issues such as breaker ties. You might consider splitting some of your longer circuits into MWBCs; for example your 1500W at 240V could be split into 3 500W circuits, all run together sharing a neutral. Instead of running 2 #6 conductors and getting a 2.5% drop, you would run 4 #12 conductors and get a 2% drop. For reference, a #12 has only 1/4 the copper of a #6, so by doubling the number of wires and using a MWBC you are using only half the copper. Of course the breaker costs more, but terminating will be easier.

-Jon
 
Thank you for all the help

Thank you for all the help

Part of my question is what exception allows them to run #12 for 250 feet and not have to compensate for the voltage dro
 
awin65 said:
Part of my question is what exception allows them to run #12 for 250 feet and not have to compensate for the voltage dro
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This may help. The FPN is not a code requirement.

210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.
FPN No. 1: See 310.15 for ampacity ratings of conductors.
FPN No. 2: See Part II of Article 430 for minimum rating of motor branch-circuit conductors.
FPN No. 3: See 310.10 for temperature limitation of conductors.
FPN No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See FPN No. 2 of 215.2(A)(3) for voltage drop on feeder conductors.
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