A 4AWG wire in residential dryer wall outlet?

Question

I was all set to replace the three-prong electrical outlet for a dryer. The plan was to change the old flush-mount with a jbox outlet and extend one of the hot wires to an adjacent jbox that holds a current relay switch to run a booster dryer vent blower when the dryer is running, per this diagram:

I bought a length of 10 gauge wire, thinking that should be sufficient since I'd never encountered anything larger than 10awg for a simple dryer outlet. After removing the cover to the existing outlet, I discovered this:

I've never seen wire this thick in a house. The outside diameter of the black wires are 0.25", which means it's what...4awg! OK, then. but I don't think I can wire a 10awg to the end of one of these behemoths, through the current relay switch and back into the outlet.

What's the correct approach for doing this right?

Edited to include response to several comments.

Thank you, everyone, for your comments. I'm collecting my replies here.

To @Ecnerwal's answer

1. Indeed, the breaker is 30A.

2. I can see very little of the cable jacket due to the conduit. From what I can see, there is no lettering. Not enough of the actual cable is exposed where I can measure it, hence my measurement that included the insulation with calipers for a rough guess. I know this isn't the best method and is short of taking everything apart.

3. The house was built in 1972, so even thought the wire is "banned" (@crip659) it was likely to code in 1972. I'm exploring to find out what it would take to retrofit this with a ground at the outlet as replacing the entire wire from breaker box to outlet is cost prohibitive. Even so, I'm getting mixed information, from both pros and DIY'ers, on just how important a ground is.

4. After searching, I can't find a 3 prong 240V/30A CO/ALR rated receptacle. Plenty of 120V/15A CO/ALR rated receptacles.

5. These connectors are new to me. They look to be a way to, essentially, splice the existing aluminum, wire to an equally rated copper wire. I don't have much room to work with and I'm hoping to minimize the amount of rework on the existing outlet.

6. I'm still with you. This makes sense so far.

7. I have the tools needed for this. Will need to research a little more to be sure I have the corrects torque specs.

8. I don't quite understand this, and don't have a good visual of what such a "disconnect" would look like. I'm familiar with disconnect boxes for things like A/C or hot tubs, but I usually have, comparatively, a lot more extra wire to work with. This would, presumably, require the removal of an appropriate length of conduit to expose more of the existing wire that can then be worked with inside a disconnect box.

A couple of other comments/questions about this situation.

First, I'm also getting mixed information about just how "dangerous" this aluminum wiring is, again, from pros and DIY'ers. Some say it's fine, others leave me in fear of the wire spontaneity combusting in the middle of the night. Is there a way to test aluminum wire to determine if it's a danger?

Second, another idea was to use a 3 prong 240V/30A extension cord such as is used for RVs and EV level 2 charging. I could then splice into the extension cord to access the hot wire I need to pass through the current relay switch. That would be a crude fix. What I would actually do is remove the female end of the extension cord, run it to a jbox with a suitable 3 prong outlet. The rest of the solution - an adjacent jbox to house the current relay switch, etc. would be according to my original plan.

Edited to include view of conduit

Answer 1

1. Make sure the breaker is 30A
2. Check the cable jacket printing (more sure) or the size of the actual conductor to assess wire gauge. Should be at least 8 as it's aluminum conductors.
3. Strongly consider (not technically required if it was legal when put in, but code finally changed in 1996 because they kept killing people) retrofitting a ground wire, hitting the 10-30 with a hammer so nobody uses the thing again, and installing a 14-30 with both ground and neutral.
4. Use a CO/ALR rated receptacle. Hmm. Could be an error. The 14-30's I can find (Including one Leviton flush-mount that does take 4 AWG, should that actually be the case) are marked with the AL-CU marking that (to my understanding) should be avoided on 120V receptacles as there were some deign issued with the early ones and the CO/ALR was the newer marking.
5. Assuming that 1 checks out, or you have corrected it if it did not, use a suitable size of Mac-Block or Polaris connector to join the hot you want to sense to the extension wire.
6. Join the extension wire and the hot and neutral from the original cable to the new receptacle. If you have implemented 3, the ground as well.
7. Check the torque on all connections, including the ones you haven't touched at the breaker. Follow manufacturer's instructions or labeling and use a torque driver. Too loose is bad, and so is too tight.
8. If the wires truly are too large for the receptacle, or possibly if the connectors in 5 are too expensive, terminate them into a disconnect (as common with air conditioners) which does not need the full working space a sub-panel does, but is a convenient way to get a couple of connectors that will take large wires so you can transition to smaller ones at a low price. You'll still need one insulated connector to keep the neutral isolated, probably, as most such disconnects are two hots and ground only.
9. Depending how large the wire gauge actually is, beware that you may need a rather large box where those wires come into the box, due to *box fill. This may be another thing that would make using a disconnect mentioned in 8 and changing to 10 AWG copper look appealing.

Answer 2

You cannot extend a dangerous old 3-prong dryer connection.

Not legal. You will need to change that to a 4-wire NEMA 14-30 type socket and cord. This is not optional given your plans. But let's talk about those plans again later because there's a better way.

The 3-wire dryer connection is definitely dangerous, which is why it was outlawed in 1965 for everything but dryers and ranges, and 1996 for those too. We tried it. We tried it to death. Science says it's a lose.

To extend this outlet, you will need to bring actual ground here, 10 AWG copper size, and it must run back to anywhere with a #10 ground back to the panel, e.g. water heater or bare Grounding Electrode Conductor. Or just back to the panel. If the steel conduit is complete all the way back to the panel, that's your ground but if so, seize the opportunity to change the wires to 10 AWG copper THHN (30 cents a foot) - life will be much easier.

That wire

Now that wire, you're measuring the outside insulation and comparing that to a table showing the diameter of the copper part. That is how you went wrong there. You have aluminum SEU cable, probably 8 AWG, possibly 6 AWG.

The aluminum wire isn't dangerous if respected by a) placing only on terminals properly rated for aluminum, and b) torquing those terminals to spec with a torque screwdriver.

If you don't shorten that conduit or pull in some slack, those wires are too short to work in a junction box. So you WILL be splicing to them. Since the wires are larger than #10 an Alumiconn won't work, so your best bet is ILSCO Mac Block Connectors or competitor (which accept aluminum wires from #14 to #6). Last choice is Polaris because of their size and cost.

Make that junction box at least a 4-11/16" steel box with a 1-gang or 2-gang mud ring depending on your 14-30 socket. That will give you the cubic inches you need for all this stuff.

The much better play, though

Think about all the trouble you're going to. The booster fan, the load monitoring system to control it, all this receptacle stuff, if only there was an easier way! And hey, what if it saved some money too?

The answer the "de-humidi-dryer" or heat pump dryer. Really, that's a thing. It's a dehumidifier inside a tumbler. Much cheaper to operate, and that dryer vent? GONE. So now you also have the savings of not pushing out massive amounts of air that you paid to heat/cool/[de]humidify, only to suck replacement air in through leaks in the building envelope and you pay again to condition it. If you have a tight house, this is almost a no-brainer because it resolves the problem of how to obtain that makeup air.

It's also easier on your clothes and doesn't wear out elastics.

Now nevermind what Technology Connections says here, some models have condensate pumps that shove the water out the washing machine's drain pipe.

And they don't take THAT much longer -- this belief is largely polluted by traveler's experience with European dryers, which do take all day because those are NOT heat pump dryers. No heat pump at all, just passively using ambient air in the laundry room as a cheap "cold side". That works about as well as you'd expect LOL.

So yeah, the whole idea of a dryer vent is completely unnecessary, and actually wastes a lot of energy shoving good conditioned air out of the house, aside from the 30 amp circuit also required.

Answer 3

in the US, when you are upgrading your service or in this case the branch circuit you must bring the branch circuit up to existing code. extending or moving the outlet is considered to be upgrading. you will need to go back to the circuit breaker and run new wire to the location that you want the plug to be at and it must follow the new codes. A 30 amp breaker is correct for your dryer and we need to match the wire size to the 30 amps with the same gauge rated for 30 amps or greater. NEC code book dated 2023, table 310.16 will give you the wire size based on the type of wire that you are going to use. a cable called NM non-metallic is okay to use, but make sure the size is equal to 30 amps or greater. the NEC code book also States that you must have four wire connections to include ground, neutral, and your two power wires. it is no longer allowed to use ground as a neutral.

Answer 4

Why mess with it? Use the laundry room light circuit to power your fan/blower, most folks leave a light on while doing laundry right, if not start doing it. 45+ years electrical troubleshooter/instrument tech, in a nuclear plant, our motto was find the safest easiest way not to get electrocuted or irradiated by doing something stupid.

Answer 5

• 4 awg is fine (oversized)
• The aluminum is fine (its seu, this is the SAME aluminum coming into your house for the service entrance... large gauge aluminum is NOT a hazard, because the hazard comes from the aluminum breaking
• wire is on 30 amp breaker (okay), makes me think they used to have larger unit there due to large wire size, but that outlet is a 30 amp outlet
• Aluminum is NOT ILLEGAL
• 3 wire dryer set-ups are NOT ILLEGAL... they are actually CORRECT. PHASE1/PHASE2/GROUND 240volts... No neutral is needed in this scenario, the bare wire is ALWAYS A GROUND coming from your electrical panel
• I would not worry about co-alr devices, i dont think they make it in that size, so just use nolox antioxidant to keep aluminum from expanding and contracting