Tuesday, December 19, 2006

Dealing with an old house with antique wiring

Geoff in the comments to THIS POST asks the question:
"We just moved into an older house with no ground wires to most of the outlets. Will these GFCI cords work with that?"
With a 2/3 prong adapter it should work OK. The function of a GFCI is based on current balance between the hot and neutral. A ground isn't necessary for its operation.

HOWEVER, lacking a ground, the GFCI isn't going to be able to give all the protection it is capable of when a ground is present. Ex. the case of an older power tool with a metal housing that has a 3-prong grounded plug. On that generation of tools, the ground wire is connected to the metal frame of the tool. If there is an internal problem in the tool that caused the frame to become energized (ex. hot wire insulations worn off and contacting the frame), the design relies on the ground wire to cause a short circuit that will trip a breaker or blow a fuse to clear the fault before the thing has a chance to electrocute you. So lacking the ground wire back to the panel, the frame of that tool WILL remain energized even when a GFCI is present. In this case you would have to grab the tool creating the current imbalance that trips the GFCI.

IOW, with ground present, the GFCI would have seen a current imbalance when the hot wire shorted to the frame and sent current down the ground wire -- it would have been an instant trip as soon as the situation developed. Without ground, the GFCI has to wait for something else to come along (i.e. YOU) to grab the tool and create the imbalance. You'd feel a little jolt in the few milliseconds it takes the GFCI to respond, but it won't be fatal. GFCI's trip on approx a 5 milliamp imbalance. It takes ~40ma to cause an electrocution.

Another not too hard thing you can do in a house like that is just install some GFCI breakers in the panel (if its a modernish one that GFCI breakers were made for and not a fuse box).

The GFCI breaker will then protect the whole length of the cable and all recepticles on that branch circuit. That's a good thing. [update]The bad news is if the circuit involved is a multi-wire branch circuit, the 2-pole GFCI breaker that would be needed on it is going to set you back about $100. Multiwire branches, which are quite common, are not something for an amateur to mess with. There are issues involved. Details and cautions HERE and HERE[/update]

If its an antique panel or fuse box that doesn't have breakers available, and you don't want to spring for a new service, you can have some GFCI receptacles hung near the panel and route existing branch circuits through those GFCI receptacles and let them protect all the downstream stuff. Most basements in older places don't have anywhere near enough receptacles available anyway!

When there are receptacles downstream being protected by a GFCI device (i.e. another GFCI receptacle feeding through or GFCI breaker), the National Electric Code would allow you to replace those 2-prongs with 3-prongs. You put a little sticker on the cover plates that says "GFCI protected, no equipment ground". There may still be no proper ground at the 3-prong receptacle of course, but it allows 3-prong stuff to be plugged in at least. Not ideal, but its "legal" and will pass an electrical inspection.

Depending on how an older place was wired, you may already have a viable (although unused) grounding capability at the boxes with those 2-prong receptacles. If the place was piped with EMT or rigid conduit, all the boxes would have a continuous metallic pathway back to the panel that can serve as a ground. If the place was wired with type AC cable then the cable armor can act as a valid ground path.

WARNING: not everything that looks like "armored cable" is type AC. There's a lot of older stuff out there that doesn't have this:
The Standard for Armored Cable, UL 4 requires an uninsulated bonding strip of aluminum not smaller than 16 AWG throughout the entire length of the cable
The old stuff lacking the bond strip is dangerous stuff. Its armor can act like a high resistance light bulb filament and get VERY HOT if there is a short somewhere. The resistance can be high enough that there won't be enough short circuit current to trip a breaker or blow a fuse.

That being said, a GFCI or Arc Fault breaker(all AFCI's these days implement a 30ma ground fault protection as well as their Arc Fault circuitry) can be used to protect that ancient non-bond strip armored cable and make it much safer. If there's a short circuit and the armor gets energized, a current imbalance will exist between hot and neutral and an AFCI or GFCI will trip long before the armor has a chance to torch up and burn your place down.

If a place was wired with old 2-wire non-metallic sheath cable, you're screwed. There is no possible existing ground path back to the panel with that stuff. You'd GFCI/AFCI protect it or replace it.

In the late 50's early 60's there was some really evil silvery cloth bound non-metallic sheath cable produced and used widely. I call it "crumble wire" because the sheath falls apart when you touch it ;-> I forget the brand name right now, but its hard to miss - it'll look ratty as hell and be falling apart these days. I prefer to rip it out whenever I can, but it can be rendered safe with GFCI/AFCI protection if that's too big a job.

If you have an old house with antique wiring, the best thing to do is get a qualified electrician to come in, evaluate it, and make recommendations. Someone who is familiar with all the old methods and materials and has done a lot of residential remodel work is the person you want. Most older places will be an accretion of many different materials and systems of differing generations. A commercial electrician or one who does only new construction is NOT the guy you want to do this kind of survey.

Oh, I forgot to mention - when you start GFCI and AFCI protecting things in an old place you may find the devices trip instantly when some circuits are energized.

There's going a few possible causes for this:

- some old fixtures lack a ground wire and have the neutral strapped to their metallic housings. When screwed down into a grounded metallic box, this creates a leakage path for current to flow in parallel down the ground wire. That'll result in an instant AFCI or GFCI trip.

- "back in the day", some electricians weren't too particular about what neutral they grabbed in a junction box full of different branch circuits. This will result in the GFCI/AFCI seeing current imbalances too. This is a real problem, because its possible some branch may have had its neutral wire overloaded in the past. Those cables could be cooked.

- There's serious problem and the device just found it for you. I put some AFCI's in a place with 80 year old wiring a few years ago and turning on a bathroom light caused an instant trip. I took the fixture down and all the wires inside were burned to a crisp and bare from arcing over the years. It had been trying like hell to burn the place down.

Oops, another thing to say: DO NOT put GFCI protection on a circuit that powers hard wired smoke detectors! That's a violation of the NFPA rules for installing smoke detectors. You CAN however use AFCI protection on that circuit. They don't want your fire protection system shutting down on a small 5ma current leakages.


geoff said...

Wow. Thanks for the post. I thought the answer would be "no, you moron."

The Merry Widow said...

PA- Long time! I've got my own blog and am spending a lot of time there! Der!
Anyway, I was having a dual problem, got it fixed today, but it was interesting around here!
First, Florida Flicker and Flash, my late's name for FPL, had a bad feeder line and a defective piece of equipment, well we were getting flickers, followed by 1/3 or whole house blackouts, like 20+/day! Argh!
FPL came out and the guy popped the meter box, seems that several years(20+) yrs. ago FPL changed meters...the new meters were larger than the window on the box! Leaks over the years caused 1 clamp to corrode! Soooooo, I went ahead and had a new box put in, the other was 50 yrs. old and original, plus the fact that finding a matching clamp would have proven difficult, and it would never addressed the fact that there was a mismatch!
So today, I got the job done, turned out the electrician we knew from our first church after we got back from Californistan!
No more flickers or partial blackouts!


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