I’m having some electrical work done in an outbuilding at our recently purchased home. The work involves a service upgrade from one 200 amp service to one 320 amp that will split into two 200 amp panels: one for the outbuilding and one for the main house. (My terminology/nomenclature is probably lacking.) Additionally, the electrician has recommended replacement of the buried cable leading from the meter to the main house. He says that he just doesn’t trust anything that’s as old as this cable is. It was installed in 1978, below frost line, in the PNW.
I’ve no reason to suspect that he’s trying to scam me, in fact I’ve already told him that I’m going to delay any work to the house for a future time. But it has got me to thinking about the cable. Is there any way to test such a thing, other than digging it up for an examination? Is buried cable replacement generally necesary after 32 years? Will it be necessary after 32+ years, so we might as well do it now? This is 4/0 aluminum. In the near future we will be doing a lot of upgrade to the house… physical and electrical. But the breaker panel will remain where it is, although a new one will be installed.
Thanks for any insight to this.
Replies
no reason to suspect that he's trying to scam me
You do NOW !!
1970s era THWN in conduit (which is what was already used then per WA codes - WA elec code is state administered, vs local) 4/0 Al wire is good for literally centuries if you dont dig into it with a backhoe - or have an incompetent electrician.
David Mieland (sp) is based at Friday Harbor and likely could recomend a competent person to you for electrical, the hack you have now I would not trust to put on a wire nut!
There IS a lifetime for the 5000V underground XLPE cable due to partial discharge and treeing, but that does not exist on 120/240 V lines (bsic physics)
Own house is 1971 electrical, I would not dream of replacing any of the wiring in the house, esp not by the bozo you have now.
You should check the tighness of any AL cable every decade, and make sure the mice have not eaten the nickel filled grease off the connections. <G>
BTW, yep, am an EE with over 5 years experience - and btw2, was NOT involved in polyimide insulation selections for aircraft wiring in the late 70s, early 80s <G>
BTW3 edit - WTF ya need 320 amp service for, ya got 4 acres of pot in greenhouses???
I would estimate the lifetime as 75 years, divided by the number of backhoes within a five mile radius.
We have ways...
I wouldn't automatically say the guy is trying to scam you. If he's a local contractor, he may have experience with similar installations, and be basing his recommendation on their track record. You might ask him why he thinks it ought to be replaced, and what the chances of it failing are.
There is a way to test the cable's integrity, actually. To get an idea of what condition the insulation is in, you could use a meg-ohmmeter, which places a test voltage in the wires and measures the extent to which there's voltage loss between condictors and from each conductor to earth. The wires have to be disconnected at both ends for the test, so this will be a bit of a production, with the power company involved. "Megging out" a cable is a simple, common test.
While you're at it, yo ought to consider having the grounding electrode system tested. I suspect the grounding electrode is a Ufer electrode, rebar or heavy copper wire in the foundation concrete. It could be ground rod(s), or an underground matal water pipe, or a combination of these. The quality of the connection between the electrode and the earth is essential to lightning progection for the house wiring system. There are testers that can measure the resistence of the electrode to earth. It's a simple test, and since you're upgrading the srervice, would be worth doing.
Cliff
Thanks
Many thanks to all you guys who responded. The new meter base got put in today without a hitch. I talked to the contractor about the need for new cable and he had misunderstood me. He was thinking that I was asking for a new service line... he hadn't simply said it was necessary or required. So I also misunderstood him. But now all is clear.... at least I think its clear.
Just goes to show how miscommunication can be so easy.
So I'm leaving the existing cable in the ground. If it isn't broke...
This 320 amp service is because I'm about to build a shop for all my (now stored) power machines (and it will have electric heat). The existing out-building will eventually be turned into a ceramic studio (withe electric kilns) for my wife. Then there's always the house... all electric.
Buried cables are worth some general comments here ....
First off, a cable can be tested with a 'megger' to determine if the insulation has begun to degrade. There is no need to dig it up and look at it; indeed, the digging will likely damage it.
The finer nuances of cable insulations aside, even the best cable can be damaged. This usually happens because of later digging, or because it wasn't installed right in the first place. You really do want the cable an honest 24" down, and in a bed of sand.
Yet, you still have to dig up a good cable if you want to change to a larger wire, later. If you run conduit, there's a chance you'll be able to pull new wires into the pipe, without digging. This is a real plus, especially if the line passes under concrete drives and patios.
Textbook specifications aside, I've encountered far more damaged cable than I care to remember.
you ARE the guy who buries extension cords though
yep, and some of those have been in the ground for over 25 years! <G>