is it ok to gradually downsize home run conductors on a branch circuit of 325ft

Status
Not open for further replies.
I am pulling circuits in excess of 325 feet in length and I am concerned about voltage drop, because the length of the run. Is it OK for example to pull a number 4 awg conductor for the 1st 150 ft of the circuit then downsize to a number 6 or 8 for the rest of the run until I get really close to the outlets then obviously drop down to 12awg wire. Or would this not be good enough to negate any possible voltage drop. It is a 120 volt circuit and everything at the end of the circuits is 20amps or less. Thanks
 

iwire

Moderator
Staff member
Location
Massachusetts
It would be allowed but I would not bother changing sizes till I got to the first of the outlets, then I would adjust my wire size.

Keep in mind that 250.122 requires you also up size the grounding conductor when you up size the circuit conductors.

If you run 8 AWG circuit conductors from the panel to the first outlet the grounding conductor will also have to be 8AWG.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
Yes, it is fine but, it might be more economical to just put a sub panel down stream somewhere.


Roger
 
I agree with the sub panel decision and I also agree with not changing wire sizes to the end of the run. I usually do it that way. Unfortunately it is not my call on this one. I figured as long as I pulled the larger size wire at the beginning 150ft or so it would negate the voltage drop even if I went to the next size down for the next 150 ft to my , but I had never done it this way before so I didn't want to waste any money on wire in case I was wrong. Thanks for the input everyone
 

Smart $

Esteemed Member
Location
Ohio
I agree with the sub panel decision and I also agree with not changing wire sizes to the end of the run. I usually do it that way. Unfortunately it is not my call on this one. I figured as long as I pulled the larger size wire at the beginning 150ft or so it would negate the voltage drop even if I went to the next size down for the next 150 ft to my , but I had never done it this way before so I didn't want to waste any money on wire in case I was wrong. Thanks for the input everyone
It's total voltage drop that matters... not where it occurs. Largest wire at the start seems the most sensible, but for voltage drop reasons, the same length could be in the middle or on the end and it basically makes no difference.

If you run #4 for 150' and #8 for 150', you'd likely be better off just running #6 for the entire 300'.
 

PetrosA

Senior Member
It would be allowed but I would not bother changing sizes till I got to the first of the outlets, then I would adjust my wire size.

Keep in mind that 250.122 requires you also up size the grounding conductor when you up size the circuit conductors.

If you run 8 AWG circuit conductors from the panel to the first outlet the grounding conductor will also have to be 8AWG.

This would also negate the use of NM cables above 10 AWG where otherwise allowed, right?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
130714-1128 EDT

The original question is not clear.

If there are no loads on this circuit except at the far end of the 325 ft run, then it makes no sense to change wire size thru the run.

Determine the peak load current at the 325 ft point, and the maximum voltage drop that is desired at this peak load. That deterimines the maximum impedance of the circuit. You can use the impedance as the resistance and then calculatre wire size.

How do you determine a reasonable voltage drop. That depends upon the type of load, and other criteria.

Suppose the load is a motor, then you are more likely to be concerned with inrush current to the motor and the motor's startup time. My DeWalt radial arm saw for example. It does very poorly with a 12" blade, a 120 V supply, and 125 ft run of #12, and a 20 A QO breaker. It can be started once, then a considerable delay before starting to allow the QO to cool down. Try to start twice within 10 to 20 seconds and the breaker trips. Startup time is many seconds, and about 80 A. If the saw is at the end of 20 ft of #12 the startup is a fraction of a second. Thus very much less startup heating of the QO. Usually source voltage is more like 124 V.

If your nominal voltage is low 105 to 110 V, then you have greater problems than if nominal is more in the range of 120 tp 135 V.

Your loop length is 650 ft or about 2/3 of 1000 ft. At a wire temperature of 20 C (68 F) my copper tables have #4 at 0.249, and #8 at 0.628 ohms/1000 ft. Doing an equal split the the total resistance is (0.249*325/1000) + (0.628*325/1000) = 0.285 ohms. Using #6 the whole distance the resistance is 0.395*650/1000 = 0.256 ohms. The total copper weight of each approach is (126.4*325/1000) + (50.0*325/1000) = 57.3 #, and 79.5*650/1000 = 51.7 #.

In this application why would you consider the two stage approach?

Assume the loop resistance is 0.25 ohms and the load current is 100 A, then voltage drop is 25 V. Is this acceptable? Load current was not clearly defined. So now assume the load is never more than 20 A, then maximum voltage drop will be greater than 5 V by a little because the wire will be hotter than 20 C.

.
 
Last edited:

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
130714-1128 EDT

The original question is not clear.

If there are no loads on this circuit except at the far end of the 325 ft run, then it makes no sense to change wire size thru the run.
+1

If the price you pay for the wire is proportional to its copper weight, then the cheapest wiring solution (least cost in copper for the same voltage drop) for a single home run comes when the wire size is constant.
If there are other loads on the same branch, then mixing wire size might make sense if the closer load is a large one.
 

PetrosA

Senior Member
Sure would, as with most other factory-assembled cables. Put a small panel on the load end though, and sometimes you can make it work.

Seem silly that you can feed a subpanel with SER, but not a single load with upsizing for VD because the ground wire is a few sizes smaller. :rant: I can see it more with NM cables where you might have #6 or #4 with a #10 ground...
 

Smart $

Esteemed Member
Location
Ohio
Seem silly that you can feed a subpanel with SER, but not a single load with upsizing for VD because the ground wire is a few sizes smaller. :rant: I can see it more with NM cables where you might have #6 or #4 with a #10 ground...
Doing such is essentially using requirement wording as a loophole.

Doing it properly, you'd upsize the subpanel feeder for voltage drop and get into the same situation. That is, if conductors and ocpd are sized at the minimum, after the application of demand factors. But because we are not required to apply demand factors, this provides some leeway on upsizing conductors and ocpd without having to upsize the EGC, within compliant bounds of NEC intent.

This is similar to upsizing the branch circuit ocpd rating to get around the EGC upsizing requirement. However, we cannot always upsize the branch circuit ocpd rating, such as when circuit ampacity would exceed that permitted for device ratings.
 

renosteinke

Senior Member
Location
NE Arkansas
Vocabulary matters.

Once you start reducing wire sizes, you have a 'tap,' and tap rules apply. As long as that wire is big enough for the breaker rating, your tap can be infinitely long.
 

iwire

Moderator
Staff member
Location
Massachusetts
Vocabulary matters.

Once you start reducing wire sizes, you have a 'tap,' and tap rules apply. As long as that wire is big enough for the breaker rating, your tap can be infinitely long.

You are right vocabulary does matter and it is not a tap if it is still rated higher than the breaker.
 
Status
Not open for further replies.
Top