Splicing together different gauge wire?

Question

I'd like to add this PZEM-061 digital multimeter near a 14-50 outlet I'm installing in my garage. It uses a CT sensor for the current and taps into the line for voltage. How I can safely splice the small gauge wire the meter needs into the larger wire feeding the outlet? I was planning on using 12-18 gauge for the meter, and the outlet is being fed with 6 gauge.

I'd like something that's simple and cost effective, DIYer friendly, and passes inspection. Here's what I've come up with so far:

• Tap the meter into the 14-50 outlet directly? When I search about this on EV forums, lots of people seem to be doing this. I don't think that would pass inspection with two wires under one screw, but I agree it would be the easiest. Is there a NEMA 14-50 outlet with load terminals? I haven't seen any in the home stores.
• Wire nuts pigtails? Twisting two 6#'s and an 18# seems like it would get difficult or the 18# would get damaged.
• Split bolt? Can you splice 3 wires in split bolt? I'm not a fan of the wrapping and box bulk needed to do this, but it seems the most straightforward way. 3-way Polaris blocks would avoid the wrapping, but then there's the different gauge problem.
• Distribution block? I'm looking at something like this. Pricey but seems safest. I'm not sure if I can fit that in 4 square deep junction box or if I need to find something bigger. I mean, I could go all the way up to a...
• Subpanel? I don't need a subpanel or shutoff there, but I've seen that idea suggested. The cost of the box and extra breakers just for a multi-gauge splice seems overkill.

The meter only draws minimal amps so the small gauge wire is appropriate, and it's rated for 240V. I can wire an inline fuse to the meter if there's a concern of putting such small gauge wire on a 50 amp circuit. I don't expect to to draw more than 0.1A.

How can I finish this project and make me and the inspector happy? Thanks!

Answer 1

There may be some challenges installing the meter you have selected in a code compliant permanent installation.

Regarding the connection of the meter's voltage leads, using #12 - #18 wire:

• Tap the terminal screws on the receptacle? This one is a non-starter. You'll be violating the listing of the device and the bad connection is very likely to cause serious trouble.

• Wire nut pigtails? The manufacturer's documentation for wire nuts will explicitly specify the wire combinations for which it is approved. There are large wire nuts that work with two #6's and a #12. I don't think you'll find one rated for two #6's and a #18. However be advised that making a good splice with a wire nut does take some skill.

• Split bolt? As mentioned in the comments a tap and run split bolt connection is very secure. Again you'll have to use a split bolt rated for #6 run and #12 tap, and I doubt you'd find one rated for #6 run - #18 tap. Skinning the #6 and insulating the splice again takes some skill.

• Distribution block? A distribution block or terminal block rated to handle the amperage of the circuit and with terminals rated to accept #6 and #12 conductors will work well and is easy to use. It will take up some additional space inside the enclosure. You can even find block rated to handle 65 amps with terminals that handle #6 and #18 wire. Still, I'd use #12 for other reasons (read on...)

A similar solution mentioned in the comments is a multi-tap connector like the Polaris Insul-tap. It is like a distribution block but fully insulated so it need not be mounted.

• Subpanel? This is a creative solution, and an exepensive one, it avoids some code issues but may raise others, I'd say that's a whole separate question.

Another creative solution - technically taped solder joints are still permitted in the NEC. Nobody seriously considers doing this for building wiring any more but I have to say when you come across these in old buildings, they have stood the test of time. I wouldn't suggest reviving this old method for power wiring.

Another solution would be a crimp on C-tap for #6 run / #12 tap. This makes an excellent connection, and although it has to be taped like a split bolt, it's easier because it's less lumpy. Although the C-taps are not expensive, the crimp tool that you use with them is quite expensive.

If you keep the meter voltage leads short - under 18" - and use #12 wire, I believe you can use NEC 210.20(4) Exception #1 (C) to tap the 50A circuit without adding an overcurrent protection device (breaker or fuse) to the meter leads. The meter would be an individual, non-receptacle outlet.

Exception No. 1: Tap conductors shall have an ampacity suffıcient for the load served. In addition, they shall have an ampacity of not less than 15 for circuits rated less than 40 amperes and not less than 20 for circuits rated at 40 or 50 amperes and only where these tap conductors supply any of the following loads:

(a) Individual lampholders or luminaires with taps extending not longer than 450 mm (18 in.) beyond any portion of the lampholder or luminaire.

(b) A luminaire having tap conductors as provided in 410.117.

(c) Individual outlets, other than receptacle outlets, with taps not over 450 mm (18 in.) long.

(d) Infrared lamp industrial heating appliances.

(e) Nonheating leads of deicing and snow-melting cables and mats.

Another exception

Exception No. 2: Fixture wires and flexible cords shall be permitted to be smaller than 14 AWG as permitted by 240.5.

might allow you to install a second 14-50 receptacle, and make up a flexible cord and plug for the meter voltage leads. I don't like this idea, but it might be code compliant.

Taking a step back, I might be a bit concerned that the meter in your link is listed and suitable for permanent installation.

Answer 2

I'd like something that's simple and cost effective, DIYer friendly, and passes inspection.

Wait, seriously? Okay then. Let's grab a copy of NEC...

Equipment must be approved by the Authority Having Jurisdiction.

That's the local inspector. But he doesn't run a testing lab, so he relies on Underwriter's Laboratories aka UL or other equally trusted testing labs: CSA, TUV etc.

Essentially none of the stuff sold on Alibaba/AliExpress has a UL listing or equivalent certification. That same junk pipeline also floods eBay and Amazon Marketplace. The latter is your source here. Not everything on eBay and AM is unlisted junk, but it's in the high 90’s.

So you may have a problem with this current sensor being usable. But somebody should make an equivalent sensor with proper listing.

Tap rules

This is driving me crazy here. If you had come in and said "I want to run a 20A circuit with a 12AWG backbone and come off it with 14AWG taps down to each receptacle, coz they're 15A" We woulda said "No, no, no!" But somehow, coming off a #6 branch with a #18 is okay??

Well I think BatsPlasterson has the answer, in his 210.19(A)(4) Exception No. 1(c). It's not a receptacle tap and it's short. However it requires 12 AWG wire at least for the tap. #10 might splice better with the #6, assuming the device can accept #10.

Junction Boxes and enclosing wiring

The illustrations on that Amazon page show just bringing the CT wires and individual voltage supply/sense wires right out of the junction box and onto a piece of plywood, then to the module. You can't do that.

The voltage supply/sense wires are mains voltage, and must be inside cable or conduit. The module has exposed mains screws, so it too must be inside an enclosure.

You'll get major points for a grounded metal enclosure e.g. Junction box, as that will alleviate some of the risk of the non-listed device, "let it burn" inside its steel box. Of course since they didn't use smoke-safe plastics, the fumes may yet kill you.

My recommendation here would be two boxes linked with a short 3/4" conduit. In the left box, all #6 supply wires come through, where they splice to #10? branch wires to carry voltage over to the sensor. The current transformer also goes inside this box. In the right box, you mount the measurement module.

The current transformer needs to come through that same conduit. Low voltage wiring is generally supposed to be 100% separate from mains - however that is impossible with a current transformer! So there is an exception that low voltage wiring can travel with mains if the low voltage wiring is associated with the mains wiring it's traveling with. Which this is.

Further, once LV wiring is in, it's all in - it must use mains wiring methods its entire route, i.e. be in conduit or mains-rated power cable like NM. You can't have a LV control line mingle with mains, exit mains wiring, and travel in cheap 24V thermostat cable stapled to joists. Any short in that box could put 240V on that thermostat cable and POOF! So that is why the CT must go in the conduit.

Cubic inch wise, in the left box, you'll have four #6 wires (5 cu.in. each), 1 set of clamps (5 cu.in. same as the largest wire), 1 ground (5 cu.in. same as the largest ground wire). and two #10 (2.5 cu.in. each). That equals 35 cu.in. A 4" square box won't cut it. I would reach for a 4-11/16" deep box which is about 41 c.i. Buy those at an electrical supply unless you like paying $6 for them at a big-box.

For the right box it's a question of what will physically fit. That's it. You'll have two #10s (2.5 cu.in. each) and some LV wiring so the statutory requirement won't be much.

How are you going to see this thing?

Since the entire unit must be secured inside a mains voltage cocoon of conduit and junction boxes, I fail to see how you aim to actually read the display. You can't just have an open junction box with no cover!

Perhaps this has a head unit that attaches to the main module? I couldn't tell from the pictures. You have to look at its, cough cough, UL Listing, cough cough, and the Labeling and Instructions (which you are obliged to follow, NEC 110.3, since those were the conditions under which it was tested) -- and see whether it is allowable to bring that head unit outside the mains shielding cocoon.

If not, that's a conversation you'd need to have with your AHJ. I would expect a competently made version of this product, built to be Listed, would have a metal case and a small compartment with a knockout for making the necessary connections to Code. This is the kind of thing these very cheap Cheese manufacturers tend to just skip over.