upgrading subpanel feed. determine wire length easily?

[vartz04]

I'm trying to figure out what I need for wire and what length to buy it in. I tried pushing a fish tape thru but it won't go. I'm guessing that is because of the 90s with the access plates on them?

I guess I can go out and feed them thru the plates but is their an easier way to do this. I plan to use the old wire to pull the new thru so I'd prefer to not pull it out ahead of time.

Hopefully its run differently than how I guessed and rolled with my measuring wheel but its between 50 and 75 feet. That makes a big difference in what length spools I buy 2 100s or one 250.

 

[pattenp]

Pull the old wire out and tie a pull rope to it. Then you can measure the old piece of wire for the exact length. Of course you'll not have power to the sub for the time it takes to go get the new wire.

 

[nehog]

Well, there are tools (electronic) that will do this job for you. Basically they send a pulse down one conductor (you disconnect both ends, then short two of the conductors at the far end) and measure how long it takes that pulse to return to the source on the second wire. If you have a pulse generator, a good high quality dual trace scope, you can do it with that, or buy/rent the right tool!

With that said, what Pattenp said is what I'd do too (and I have the equipment.)

 

[Aceman]

The easiest is to pull the existing wires out with a cloth measuring tape attached to them.

Another way, pull the ground out with string taped to the end. Measure the ground. Pull the ground back in with the string.

 

[theoldwizard1]

Well, there are tools (electronic) that will do this job for you. Basically they send a pulse down one conductor (you disconnect both ends, then short two of the conductors at the far end) and measure how long it takes that pulse to return to the source on the second wire. If you have a pulse generator, a good high quality dual trace scope, you can do it with that, or buy/rent the right tool!

WOW !! Memories !!!

While I was in engineering college (a billion years ago) we had a lab class where that is exactly what we had to do. We put a "matching" terminator on the end of a random length of coax, injected the pulse and measured how long it took to get back. Then figure out the length of the cable.

We also looked at the reflection you got from unterminated (yes, there is one) and a low impedance terminator.

Don't ask me the results. Too long ago.


The modern tool is call a time domain reflectometer.

 

[matt151617]

I measured straight distance. Added 3 feet per curve, and a couple feet each for the risers. Then 5 feet per end once it got to the panels, and another 5 feet for fudge room. I ended up with about 10 feet extra, but it would have been much more difficult and expensive to splice if I was short.