ECN Forum Forums General Topics of the Trade General Discussion Area Voltage Drop

hi,
i have a voltage drop situation where there is a 120/240 VAC 40A circuit #6 Aluminum run to a barn. about 200 feet.

the volatage is as follows:

L1 - N = 125V
L2 - N = 80V

L1 - Grnd = 125V
L2 - Grnd = 80V

L1 - L2 = 205V

everything was working fine for over a year. the homeowner had some digging done near the line. he said no breaker was tripped but he did see the line exposed while they were digging. he says he did not see them damage the line. we dug it up and the conduit (PVC) was pulled apart at the coupling.

could the backhoe have crimped the line or broken most of the strands?

if this is not the problem what could cause it?

reading at supply panel is normal for both legs.

thanks for any replies.

frodo

I assume these voltage readings are taken with the normal load on in the barn.
Did you measure the current in each hot and in the neutral?

It looks like the wire is too small and the load is unbalanced.

For that distance with a 40 amp load you need #4 AL or #6 CU to limit voltage drop to 3%

Ed

Sounds like you checked the voltage at the panel in the barn. What readings are you getting at the panel (L1-N L2-N L1-G L2-G and L1 to L2) that is feeding this panel in the barn. That is a better place to start.

On rare occasions, backhoe operators can be amazingly and creatively delicate in their dig-in descriptions. If insulation damage exposes an aluminum conductor to soil, it will turn to white powder of virtually infinite resistance after a time. {At least it was not a 60-year-old kraft-paper-in-lead toll cable.}

Connect a hair dryer on each leg and take voltage measurements.

hi,
thanks for the reply. as i stated in the first post, the readings at the SUPPLY panel are normal. and no i did not take any readings with a load applied. none of the equipment will work now.

everything was working normal up to the point of the digging. there is adequate voltage in the L1 circuit to neutral.

the L2 is not grounded at all so there is no wire exposed to ground that i can tell.

i am assuming that the wire was damaged. i will be able to physically look at the cable tommorrow.

i will let you all know what i find. i am hoping the wire is damaged.

i do not think it is a voltage drop problem. if it were, the voltage would be inadequate in the L1 circuit. the circuits are virtually the same length.

the circuit breaker feeding the panel in the barn is a 40A. the actaul load is less than 20 A.

all readings were taken with no current being drawn. there is no point in taking current readings at thus point if you already have only 80V in one leg of a 240V system.

there is a tap that goes to a pool pump and a receptacle. these are protected by two 20A breakers at a panel near the pump. the voltage reading is normal at this point. it is approximately half way to the barn.

this is what is leading me to believe the cable was damaged. the homeowner is digging the line up where the backhoe was digging to expose the cable.

thanks for the input.

frodo

Repitan, por favor.

"If insulation damage exposes an aluminum conductor to soil, it will turn to white powder of virtually infinite resistance after a time"...

id est, "lose continuity."

hi,
if the conductor is in contact with the earth, then i would expect to see a reading to ground using my fluke.

i tested the line to ground and it is clear. how long does it take for this corrosive action between dissimilar metals to occur?

thanks

frodo

You would need a megger to read to ground. A Fluke doesn't use enough voltage to read through the connection to earth.
Don

If this feed is all in conduit (you stated PVC), why not pull it out (attach a pull rope first) and inspect the wires (yes all 200')? Then you can fix the conduit and replace the damaged wire. It must be a bad wire if you have the correct voltage at the source but not at the barn.
I had one very similar to this on an overhead feed to a barn were the SE cable noodle had corroded to the point that it had only three strand that were intact.

hi,
thanks for all the replies. i meant to get back here sooner to let you know what i found. i found that the L2 circuit had the insulation damaged on it and the aluminum turned to mush.

the homeowner had some backhoe work done...they grabbed the conduit about midway down the run...where the cables come out of the underground pipe is where the insulation was damaged...

i had to pull out the whole run and dig up everything to find the break in the line.

costly

thanks for the replies

frodo