I have a receptacle that seems to have been around since the house was built (all others were upgraded). It’s a simple two prong receptacle with two wires attached. I’m not sure if the wires are copper or not.. could I perhaps use a multimeter to measure their resistance to get a better idea of this?

wires for receptacle

To give a better idea of the wiring in the house here is some of the metal conduit used to carry the wiring through the older part of the house (the part I’m dealing with):


Okay so that’s the background and now the question is - it seems most conduits these days (at least at my local hardware store) expect 2 black wires and two white wires. Do I need to get a conduit that expects only one of each or can I still use the ones that expect 4 wires? Also, what’s the best way to determine what materials these wires are made of (resistance method?) and if not copper are there GFCI options out there that work with non-copper wires (I realize I would have to label this "no equipment ground")?

new receptacle

Thank you in advance for any assistance!

  • Can you post a photo looking fully into the back of the box? Oct 29, 2018 at 4:19
  • @ThreePhaseEel are you wondering if there's something I can use for a ground? The metal conduit is somewhat exposed, but someone at the local hardware store (the store itself has very similar 1930s wiring) recommended I stick with the two prong outlet unless I have access to a water pipe to ground it. This is honestly the only outlet in the house that was not updated and grounded. It's in a location with easy access to another grounded plug so it's likely the only thing we would use here is a lamp - so probably okay keeping it as is anyways.
    – Jordan
    Oct 29, 2018 at 11:52
  • 2
    That isn't metal conduit. It's armored cable. The difference is the wires can easily be removed and changed in metal conduit, and the wires are stuck in place in armored cable. if it is in fact an early metal flexible conduit, just pull the wires out and replace them with modern THHN. Oct 29, 2018 at 18:08
  • As far as needing 4 wires per receptacle, that is such a "wiring 101” question that I must advise that you hit the brakes on your work, and either bring in a pro, or it will suffice to get a decent book on home electrical and read a good bulk of it. This shouldn't be a thing you're unclear on. Like the joke about the old mechanic who hits the giant machine with a hammer and it suddenly works again, and he sends a bill for $5000 for "knowing where to hit"... Electrical isn't "attach 4 wires" it's knowing why to attach those 4 wires. Unlike the mechanic, a book will get you there. Oct 29, 2018 at 18:39
  • 3
    Best books would be specifically "how to wire your house" type books, found at Home Depot or better, your local library. House wiring has lots of idioms that general electrical theory will not prepare you for. I like "your old wiring" by David Shapiro, covers the basics very well. smile.amazon.com/Your-Old-Wiring-David-Shapiro/dp/0071357017/… Oct 29, 2018 at 21:55

2 Answers 2


First, it is extremely unlikely that a 1930’s house would have anything other than copper wiring, unless the wiring was replaced later. Aluminum wiring was only used from the mid 1960’s and later. There really aren’t any other options. In any case, you wouldn’t be able to tell by the resistance unles you had a long run to measure. The ohms/foot is so small.

To answer your second point, modern outlets only require one hot (black) and one neutral (white). The reason there are two screws on each side is that there is a metal tab connecting them. By breaking the tab, you can isolate the top outlet from the bottom. This is typically used to make one switched and one always on.

BTW, if you are replacing two-prong outlets where there is no ground wire (or equivalent) in the box, the better thing to do is to use a 3-prong GFCI outlet. This is allowed by code, so long as you mark the outlet “no ground installed”. This gives much of the safety of a grounded outlet without needing a ground.

  • Thanks @DoxyLover and yeah was hoping I could use GFCI! These are pretty heavy wires so hopefully they will fit in those connectors for the GFCI receptacle.
    – Jordan
    Oct 29, 2018 at 10:41
  • 1
    There is a chance of this being aluminum wire, in which case use a device rated CO-ALR for aluminum wires. Since the J-hooks are already on there, might as well use screw terminals. Or determine for good that it isn't aluminum, which you can do by scraping it with a knife and seeing if it's bronze color or silver. Oct 29, 2018 at 18:49

Besides copper, the only other material suitable for conductors is aluminum. In 1930 aluminum was a precious metal, as the Hall/Héroult refining process wasn't industrialized yet. Far too precious to waste on wire, which was easier done by cheap, plentiful copper. (Now it's the other way 'round lol).

This wiring looks like it may have been retrofit. If so, it may be aluminum. Scrape it with a knife to see if it comes up brassy or silvery.

That 2-prong receptacle is perfectly new, possibly 1980's more likely 21st century, noting the upward offset to leave room for an absent ground. Inside the receptacle the metal bits are identical to the grounded type.

You can retrofit ground wires

That looks like armored cable, not conduit. If you can freely pull out and replace the wires, it's conduit. Otherwise it's metal jacketed cable. Fixed metal conduit is allowable as a grounding path. That metal cable may be, depending on type; AC cable was demoted to where it's no longer allowable as a ground path. No matter, you can retrofit just a ground wire, those rules were liberalized in 2014.

You don't need to follow the original cable, but given the ease of access you have, that's exactly what I'd do. Do the same thing as the cable is doing, hop from junction box to junction box. Those steel junction boxes are exactly what I use today for that sort of work, and they don't really age unless they're exposed to wet.

The layout of this wiring really did you a favor. The splices are being made in the junction boxes, so the receptacle only takes 1 wire per side. That's a knee-saver! Just FYI, The 2 screws per side are there to optionally be used in one of two modes: a) as a handy-dandy way to splice 2 wires while also connecting them to the receptacle, or b) if you break off the tab, a way to control each socket separately. Neither applies here.

If the box is grounded, you can ground the receptacle with a "ground pigtail"*. Search the back of the junction box for a hole that's a slightly different size than the others; that would be threaded for a #10-32 ground screw (note -32). If absent, either use a grounding clip or drill and tap your own #10-32 hole (minimum size #6, thread pitch -32 or finer to bite enough metal to conduct electricity, don't use a sheetmetal screw!).

You cannot ground the receptacle via the mounting screws, but if the metal yoke bottoms out, clean metal yoke against clean box flange, that is acceptable. Typically the box is recessed shy of the drywall and that is not possible.

The ground path needs to go all the way back to the panel, but your retrofit wire only needs to reach a point with good ground that is served out of that same service panel, e.g. say if you have a water heater that is grounded.

Aluminum wire

If you believe you have aluminum wire or other old wire you just don't trust**, a 'silver bullet solution' now exists in the form of AFCI circuit breakers. They listen to the wire for any evidence of arcing, which is how most wiring problems show up. Then, they trip to shut off power. The concept of AFCI pretty much requires doing that in the circuit breaker; they make AFCI receptacles but they don't protect the wires feeding them, which defeats the purpose! If you are installing AFCI breakers, they also make AFCI/GFCI combo breakers for a few dollars more. A GFCI breaker protects every point in the circuit, however, GFCI breakers are sensitive to wiring defects such as crossed neutrals. Those are defects you want to fix anyway, but a GFCI breaker will trip until you do.

Long term anti-aluminum plan:

For small branch circuits (#8 and smaller) only use copper wire, because inspectors and homebuyers with small minds will freak if you use aluminum. Aluminum is perfectly acceptable for 50A+ feeders in #4 and above cable. If I just spoke greek, get a book.

The wiring I see in your picture would be easily replaced if aluminum; so do that. If you have aluminum wiring which is infeasible to replace, the problem with aluminum happened because they attached it to terminals that were rated for copper only. Copper terminals don't like aluminum, zinc-plated aluminum terminals play nice with everybody. So change to CO-ALR rated recepatcles and switches, and use Alumiconns instead of wire nuts for splicing. The purple wire nuts are no good. Alumiconns are actually miniature lug connectors, and like most lugs, they are made of aluminum.

* most grounds must be pigtailed anyway, note there is only 1 ground screw, that is because you are not allowed to use the "handy-dandy use the receptacle as a splice point" technique with grounds, they must be pigtailed so removing the receptacle does not break other grounds downstream. That same rule also applies to neutrals in MWBCs, which is way above your skill level right now. Upshot is if you see a neutral pigtailed "for no reason", leave it that way.

** or new wire you just don't trust, namely backstab connections. Backstabs are all the rage because they shave time when building new houses. However they are unreliable and are the #1 source of arcing problems. As a direct result, AFCI breakers are now mandatory in new construction.

  • Thanks @Harper this is very useful and educational - and yes my skill-level doesn't go beyond much what I learned in my HAM radio technicians license course, but looks like I'll be upgrading quite fast with this house.
    – Jordan
    Oct 29, 2018 at 20:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.