Adding a new outlet

Question

In another post I presented a project I am working on in my home. I should have gone about the question in a different way so am going to start over with a new post. I am not an electrician, but have done other wiring projects in my home without any problems.

I have a 40 amp circuit in my shop for a radial arm saw. The 8 gauge cable for this circuit runs from the circuit breaker box to the shop and ends with a dryer type outlet (sorry I don't know the proper technical term for that). I want to add another outlet of the same type between the existing outlet and the breaker panel for another machine. I plan to cut the existing cable and added a box (4" x 4" x 2") at each of those cut ends (2 boxes total). At each of those boxes then "splice" (again that is probably the wrong term - sorry) using appropriate sized wire nuts a new length of the 8 gauge cable to go to the new outlet. Those 2 new cables are then connected together at the new outlet. The junctions at the 2 boxes are being made with wire nuts that are appropriately sized for 8 gauge cable (I have tested making these connections with some scrap pieces of the 8 gauge and they are good and tight).

Am I doing anything horribly wrong here? Here is a crude drawing of what I want to do. Thanks

Here is the original post:

I just realized one other bit of information I left out. For the new outlet that I am adding I was planing to use the same 4" x 4" x 2 1/8" boxes that I used at the splices. Is that ok? Does that box need to be bigger?

Here is a couple of pictures of the existing outlet, from what Harper said (few comments down) I think that maybe this is not a proper outlet?? (I did not install this one, it was done by an electrician).

Answer 1

Can you put more than 1 240v receptacle on a circuit , YES.

Can you splice #8 wires in a 4x4 2” deep box with a 30 amp receptacle. The box fill in this case would allow it.

Box fill explained #8 wire is 3 cu in , you did not say if you had straight 240 with ground or + a neutral. I count a total of 7 conductors only count the ground 1 time. A clamp allowance of 3 only 1 clamp at largest wire size for calculating. This receptacle requires a double gang box normally a single device is 2x the wire size but a dryer receptacle requires a double device fill or 4x the wire size 21+3+12 = 36 cu inch box A 4 square by 2-1/8 inch box is only 30.3 cu inches but add a raised round device cover and add 5 cu inches so yes it can be done.

You don’t count the wire nuts to make the pigtails , I suggest non pros pre twist the 3 wires cut even then install the wire nut on large wire sizes (I make my apprentices do this not picking on you) I have seen two many failures in the past.

Note this circuit would require GFCI protection per 210.8.B unless your jurisdiction did not adopt that section of the 2017 code (Oregon did not for 240v circuits). So if you want to run normal 30 amp devices non motor loads you need to put a 30 amp breaker in. A motor load allows a larger breaker to be used but a welder and other equipment with a 30 amp receptacle no.

Got called away, will try and explain the larger breaker comment. you can use a larger breaker for motor loads when the standard breaker won’t hold on start up this doesn’t allow you to upsize the breaker to run the 2 receptacles at the same time but works out this way sometimes if you start the motor that trips the breaker first then start the second motor I am guessing this is why you have your saw on this large of a breaker. The saw fla must be below 24 amps to use a 40 amp breaker and a 30 amp receptacle (some jurisdictions require hard wire when oversizing for start up) an inspector may want proof that the saw can not start on a 30 amp breaker. Off the top of my head I believe the maximum over current allowed for devices connected with plugs is 150% of the device but will have to find that reference. Hope this info helps.

Answer 2

I'm a bit puzzled at the "dryer-type outlet" and how precise you're being on that language. There are 2 dimensions here: the type of outlet, and the ampacity.

Typewise: a true 3-prong dryer outlet, NEMA 10, is outlawed. It especially should not be used with the neutral (L-shape) being abused as ground. Neutral is not ground. A NEMA 6 is appropriate, since most machines do not need neutral. Alternately, a NEMA 14 recep is appropriate if neutral is present in the cable.

The machine can use a NEMA 14 plug even if it does not use neutral. 240V tools used to benefit from being wired with neutral in that they could use common incandescent lights for spot lights on the work. But that's moot now; 240V-tolerant LEDs are commonplace.

Ampacity-wise: a 40A breaker needs a 50A socket (there is no 40A socket type). However there's an exception for 30A-wired motors with certain specs that trip 30A breakers on startup. Ed Beal discusses this thoroughly. Especially note his comment back to me.

Fit to a "T"

I see you created 2 junction boxes up there; that's the right way to tap a cable if you don't have at least 12" of slack. Now, you do not need to run 2 cables all the way down the branch. A "T" topology is perfectly acceptable if you can bind three #8's. (you'll have to somewhere; sockets don't take 2 wires). It is perfectly acceptable to create a short cable between the boxes, and then have one single cable branch to the new receptacle. Remember to use cable clamps in all the holes. #8 cable will probably call for cable clamps that use the 3/4" knockout.

As far as box cubic inches, see Ed Beal's answer. Those boxes you have provide 30 cubic inches. You can also get 4-11/16" square boxes that give 42 cubic inches, and will be a bit more comfortable to work in. For those, buy at an electrical supply; big-box stores are overpriced. Remember box covers of course.

If you do a "T" connection and you carry neutral, I count 11 "wires" (9 conductors, all grounds, all cable clamps) which require 33 cubic inches. If you use your 4" boxes, you'll need 3 more cubic inches. So just get a blank domed cover; or a 1-gang mud ring at least 1/2" tall and a 1-gang blank cover.