Voltage drop and pipe fill derating for more than 3 conductors

Status
Not open for further replies.
F

FullyTorqu3d

Member
Location
South Dakota
Occupation
Electrician
Wondering if anyone would have any insight on this. I was pulling five 20 amp circuits in a conduit so I pulled 10 awg for 50 percent adjustment of 10 conductors, but I needed to pull another run over 100 feet so would I have to go to 8s for voltage drop? I normally just go a size up for about every 80-100 feet but wondering how that works if I went up a size already to stuff a conduit. I don't have a lot of room to run conduit so I was trying to get as many circuits as possible in 3/4 emt

Thanks for any comments
 
It doesn't matter why you increased the size of the circuits, anytime the size is increased the voltage drop will be reduced. You need to run a voltage drop calculation based on the actual load, the conductor length and the conductor size.
 
don_resqcapt19 said:
It doesn't matter why you increased the size of the circuits, anytime the size is increased the voltage drop will be reduced. You need to run a voltage drop calculation based on the actual load, the conductor length and the conductor size.
Click to expand...
But so assuming that the voltage drop was calculated and 10 awg was appropriate for the load and length, I could still stuff a conduit in the way I described?
 
FullyTorqu3d said:
But so assuming that the voltage drop was calculated and 10 awg was appropriate for the load and length, I could still stuff a conduit in the way I described?
Click to expand...
Yes you could go up to 20-#10 CCC's in the raceway with 20 amp OCPD's.
 
infinity said:
Yes you could go up to 20-#10 CCC's in the raceway with 20 amp OCPD's.
Click to expand...
Thanks for the prompt response. This was my assumption but I assumed that you derate with over three CCC for heat so was just thinking it would impact voltage drop compensation but I may be over thinking....or under thinking; I usually fall into one of those categories when it comes to the code. I just wanted to make sure I was doing it correctly so that I can continue to max out my emt in the future when necessary. Of course, would probably be better to not go five circuits so I can fit a ground but nice to have it in the tool box when needed

Thanks
 
electrofelon said:
It sounds like you are not using MWBC's. Are these L-L or L-N loads? Single or three phase?
Click to expand...
L-N three phase. Correct, not using MWBC's; I'm installing AFCI/GFCI breakers so have to pull a neutral for everything unfortunately....pretty much how we seem to end up having to do it most of the time now.
 
FullyTorqu3d said:
Thanks for the prompt response. This was my assumption but I assumed that you derate with over three CCC for heat so was just thinking it would impact voltage drop compensation but I may be over thinking....or under thinking;
Click to expand...
Conduit fill does not impact voltage drop calculations.
 
I would expect there to be a small interaction between conduit fill and voltage drop, non-zero but negligible. Namely to my understanding wire impedance depends on operating temperature, and with higher conduit fill, the operating temperature would be higher for a given current.

Voltage drop formulas presumably use a wire impedance at a conservatively chosen temperature, so the calculated voltage drop would still be a conservative estimate. Meaning in practice this wouldn't matter.

Cheers, Wayne
 
wwhitney said:
I would expect there to be a small interaction between conduit fill and voltage drop, non-zero but negligible. Namely to my understanding wire impedance depends on operating temperature, and with higher conduit fill, the operating temperature would be higher for a given current.

Voltage drop formulas presumably use a wire impedance at a conservatively chosen temperature, so the calculated voltage drop would still be a conservative estimate. Meaning in practice this wouldn't matter.

Cheers, Wayne
Click to expand...
Of course, but it's not enough that it needs to be considered in Vd calcs.
 
wwhitney said:
I would expect there to be a small interaction between conduit fill and voltage drop, non-zero but negligible. Namely to my understanding wire impedance depends on operating temperature, and with higher conduit fill, the operating temperature would be higher for a given current.

Voltage drop formulas presumably use a wire impedance at a conservatively chosen temperature, so the calculated voltage drop would still be a conservative estimate. Meaning in practice this wouldn't matter.

Cheers, Wayne
Click to expand...
VD formulas, charts, etc. also presume nominal voltage whereas you could easily be plus or minus ~10% nominal for your actual starting point.

You could also have some drop on a service, feeder, or both before getting to your branch circuit.

Most these general purpose calculations are "good enough" most of the time. If you have really sensitive equipment to the voltage change(s) then more precision in calculations can be important to reliable operation.
 
kwired said:
VD formulas, charts, etc. also presume nominal voltage whereas you could easily be plus or minus ~10% nominal for your actual starting point.

You could also have some drop on a service, feeder, or both before getting to your branch circuit.

Most these general purpose calculations are "good enough" most of the time. If you have really sensitive equipment to the voltage change(s) then more precision in calculations can be important to reliable operation.
Click to expand...
I'll just add that the calculations are done for a nominal maximum amount of current, and voltage drop is proportional to current. If no current is flowing then there is no voltage drop.
 
Status
Not open for further replies.
Top