lowering voltage drop with parallel conductor

[xixxixxixxi]

I have a 500 foot run of 3 conductor 8 gauge copper. I only use 2 of the conductors and ground to connect my 240 volt 9.6 kv microinverter system to the main panel. My question is whether I could use the 3rd conductor to double up on one of the legs reducing voltage drop. I had people say I can't because of "imbalance" and it's probably against code, but does someone have an explanation of why it wouldn't work?

 

[sunshine_eggo]

It would work. It would cut your voltage drop to about 2/3 of what it is now.

The old and new conductor would need their own fuse/breaker, and your total current would still be limited by the single conductor.

I have no idea if this is code compliant.

 

[sunshine_eggo]

@ksmithaz1

My answer is based simply on the fact that 1000 ft (500' each way) of 8 awg will have MORE resistance than 500 ft of 8awg and 500ft of 5awg (2X 8awg in parallel).

The 2/3 was a guess.

 

[ksmithaz1]

I didn't call you on it ;-)! It makes sense. I have no idea how it's run but ...

Southwire shows 11.51% 240v@40A #8 copper 500FT, reports 20A at 5.75%, so pretty much spot on. Takes it to 8.63% so still 21v vs 27v.

Not sure about code, don't know why it would be a problem as long as you have each conductor properly fused/breaker'ed.

OTOH If you replaced the cable with 500FT of (2/0)x3+1 URD Aluminum quadraplex direct burial. $3/ft = $1500 + trenching. 40A is 2.71%, 100A is 7%. Even better, and you would have some headroom. 4/0 would be ~$4/ft ($2000), drop it under 1% for 40A, and ~7% for 150A.

So 4/0 would put you under 4% 9.5v on 80A. I honesty think that would be a better target, though 2/0 would get you down under 7v at 40A.

500FT is a pretty long run. Is it really 500ft? Either way, #8 is decidedly sub-optimal.

 

[timselectric]

It would not meet code.
Code doesn't allow parallel conductors smaller than 1/0 AWG.

 

[teal95]

Is there a reasonable reason to not do it? I can see it being a bit confusing to figure out what's going on if you weren't the installer but otherwise...

 

[timselectric]

Is there a reasonable reason to not do it? I can see it being a bit confusing to figure out what's going on if you weren't the installer but otherwise...

I wouldn't recommend it.
But if someone was going to do it, they should follow the same rules for parallel conductors of the approved size.
All conductors of the circuit paralleled. (Not just one)
All conductors of the same size. (Not different sizes)
Parallel conductors are treated as a single conductor. (Connecting to the same place on both ends, and protected by one OCP device)

 

[teal95]

Thinking about safety, not code here. Would you want them on the same breaker or on separate breakers appropriate for the wire? (ie paralleling #12, would I want a 20 amp on each wire or both hooked to a 40? Thinking about this some more I can see this getting confusing very quickly.

What does code require for breakers when paralleling is allowed?

 

[timselectric]

What does code require for breakers when paralleling is allowed?

One fuse or breaker.
You want all on or all off. You don't want a half failure.