Derating for fill and VD

[Lucidrez]

I have a small load on the end of a very long run. (4 amp / 600') Wire is already oversized 3x to deal with the distance. This run also has 4 current carrying conductors in the same conduit for the first 300' Boss is telling me that the 80% needs to be taken off before the voltage drop calculation, which might cause the wire to be upsized again. (significantly more expensive) I have always assumed that upsizing for Voltage drop "covered" you on the fill, as if it was a non-coincidental thing. Is there anywhere in the code that spells out this order of operation?

 

[infinity]

If you've upsized the conductors by 3X already 4 CCC's won't have any affect on your final conductor ampacity. You can find the minimum conductor size with the 4 CCC's and then see if that size is at least as large as your conductors after the VD calculation.

 

[winnie]

Derating for number of conductors and adjusting for ambient temperature 'stack', and you have to do both to determine the code required minimum wire size.

Voltage drop is a separate calculation that also gives you a minimum wire size.

You pick the larger of the two results.

You don't need to 'stack' one on the other.

Jon

 

[charlie b]

I agree with Jon. The NEC has very few instances that have voltage drop "requirements." This isn't one of them. You didn't mention a concern with ambient temperature, so I infer that that does not come into play here. I would pick a wire size for 4 CCCs first. If that gives you an acceptable VD (a separate calculation, as Jon mentioned), then you are done. Do keep in mind that whatever wire size you pick, the EGC will likely have to be the same size.

 

[Lucidrez]

Thanks all that is what I thought, wanted to make sure I hadn't been doing this wrong my whole life. Now he wants me to add 20% to the load I am using to calculate VD. I want to say " Thats not how the Force WORKS!" but having a bit of buffer won't' hurt anything. and AT is not a factor.

 

[kwired]

I have a small load on the end of a very long run. (4 amp / 600') Wire is already oversized 3x to deal with the distance. This run also has 4 current carrying conductors in the same conduit for the first 300' Boss is telling me that the 80% needs to be taken off before the voltage drop calculation, which might cause the wire to be upsized again. (significantly more expensive) I have always assumed that upsizing for Voltage drop "covered" you on the fill, as if it was a non-coincidental thing. Is there anywhere in the code that spells out this order of operation?

minimum ampacity is what comes first. You said you have a 4 amp load, if it is continuous load you need minimum ampacity of 125% so 4 x 1.25 = 5. You need no less than a 5 amp conductor at the termination temp rating before you factor in adjustments or voltage drop.

Then comes adjustments for ambient temp and/or number of conductors in raceway. But those are almost entirely separate from minimum I mentioned above because often they can use 90 degree conductor insulation rating where the above minimum is usually going to be based on 60 or 75 degree terminations. You need to pick whichever calculation result yields the larger conductor.

Voltage drop kind of is the last order of operation in sizing this. There is circumstances (usually short circuit lengths) that if you only considered voltage drop you may come up with smaller conductor than what is allowed otherwise. Upsizing because of adjustments sometimes takes care of voltage drop, sometimes you still need even larger conductor to get whatever VD result you are after.

 

[infinity]

Now he wants me to add 20% to the load I am using to calculate VD. I want to say " Thats not how the Force WORKS!" but having a bit of buffer won't' hurt anything. and AT is not a factor.

Why would he want you to add 20% (not that it matters anyway)? The code required minimum conductor size for a 4 amp load even after derating is #14 AWG. You cannot use anything smaller for a branch circuit so any conductor size #14 or larger is code compliant.

 

[Lucidrez]

Yes, even a 5 amp load would be well under, 25% of the rating for #12 wire (minimum size for commercial and the 20A breaker and way under factoring 80% for the fill) but because of the length, after calculating VD I need a #6 .. at which point any extra calculation for table 310.15(B)(3)(a) seems silly

 

[ggunn]

Yes, even a 5 amp load would be well under, 25% of the rating for #12 wire (minimum size for commercial and the 20A breaker and way under factoring 80% for the fill) but because of the length, after calculating VD I need a #6 .. at which point any extra calculation for table 310.15(B)(3)(a) seems silly

Well, yes. If Vd calculations yield a larger wire size than do the ampacity calculations (which you do first), then you are done. There is no need to derate them again. You will need to upsize the EGC by the same ratio as you did the CCCs for Vd, of course.

 

[wwhitney]

after calculating VD I need a #6

Southwire's voltage drop calculator says 4A at 0.9 Power Factor, steel conduit (worse case than non-metallic or cable or direct bury), would give you a VD of 3.497V on #8 AWG copper. Which is under 3% for a 120V load. Do you really need less voltage drop than that?

Cheers, Wayne

 

[kwired]

Southwire's voltage drop calculator says 4A at 0.9 Power Factor, steel conduit (worse case than non-metallic or cable or direct bury), would give you a VD of 3.497V on #8 AWG copper. Which is under 3% for a 120V load. Do you really need less voltage drop than that?

Cheers, Wayne

Agreed. The load in this case might very well see voltage drop caused by other loads along with source impedance anyway and it won't matter how much you increase the conductor size of that branch circuit.