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Shop Wiring: Reasonable Number of Recepticals on 20amp

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  • Shop Wiring: Reasonable Number of Recepticals on 20amp

    Wiring up my shop and am thinking that I should limit the duplex recepticals to 8 per 20 amp circuit.

    Just the usual one guy shop with typical grinders, drill press, mill drill etc....
    Everything heavier is 220v.



  • #2
    I think code says 8 but you can look it up. I would not put 8 duplexes on a circuit in a garage. Even though you may never use them all at the same time it's not a good idea. Also, you will need a GFI on each circuit according to code in a garage or outdoors.

    I think I put a GFI and 5 duplex on each of the 20A circuits in my shop.
    It's only ink and paper


    • #3
      You can use a GFI breaker to protect all outlets at once. There are some other gotchas in the code too. If any one device is going to use 80% of rated capacity for more than 30 minutes at a time then it needs it's own dedicated circuit. The 20 amp outlets must all have 20 amp receptacles, not 15 amp receptacles.

      I know that it is common in the US to wire regular 15 amp receptacles to a 20 amp circuit. That is against code. You CAN use a two way 20 amp 117 volt receptacle (NEMA 5-20R) that has a T plugin for the common blade. It will accept both 15 and 20 amp male plugs.

      You ARE allowed to use 12 guage wiring with 15 amp breakers and receptacles.
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      • #4
        From the 2008 National Electric Code:

        Table 210.24-
        15 amp circuit shall have no more than 14 taps and no less than #14 AWG (copper). = 8 duplex receptacles.
        20 amp circuit: same as above with #12 AWG wire.

        210.8 (A) Dwelling Units. All 125 volt, single phase, 15-and20-ampere receptacles installed in the locations specified in (1) through (8) shall have ground fault circuit interrupter protection for personnel:
        (2) Garages, and also accessory buildings that have a floor located at or below grade level not intended as habitable rooms and limited to storage ares, work areas, and areas of similar use.

        I didn't list 1,3-8.

        This is obviously for US installations only. Old areas are grandfathered to previous requirements (I do not have ground faults in my shop). This is intended for new installations.



        • #5
          Evan, you are mistaken. Table 210.24 allows for either 15 amp or 20 amp receptacles on a 20 amp branch circuit.

          15 amp protected branch circuits are limited to 15 amp receptacles.


          • #6
            As I read it that is what Evan said.
            It's only ink and paper


            • #7
              When I wired my shop I typically used 5 20A outputs on a string of 12 gauge wire. Each string has it's own GFI breaker and the outputs were the 20A flavor.

              Install many more than you think you will ever use and then add a few more. My shop is approx 20x20 and there are 60+ outlets not counting the 220 and direct feed lines to a couple machines. There are times I wish there were a few more!



              • #8
                Evan, you are mistaken. Table 210.24 allows for either 15 amp or 20 amp receptacles on a 20 amp branch circuit.
                The US and Canadian NEC have been harmonized so we both use the same code with only a very few exceptions. The 15 amp receptacle I am holding in my hand right now was Made in USA. It says on the back "For 15 amp branch circuit only".
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                • #9
                  That may be Evan, but the 2008 NEC in front of me says otherwise. I'm not making an opinion one way or the other. It is black and white, section 210, table 210.24 'Summary of branch circuit Requirements'.


                  • #10
                    Here's another way to look at it.

                    You would probably want to run seperate breakers for Lathe, Mill, Compressor and Bench Grinder. (must consider starting loads, rules may not apply if you have a minimill &* minilathe...) ( can be doing 2 operations at load at the same time.. still have to do it safely...) A couple outlets on the samne circuit near the door for the right angler grinder when you are welding. Then - depending on the sizre iof the shop - 2 more circuits for handheld devices. and - once again depending on the size of the shop - 2 lighting circuits - that is also a safety consideration - odds of both lighting circuits popping at the same time are low, and you can still see to figure out the problem.

                    Another thing is to plan enough outlest that you don't have to connect any "permanent" devices with an extension cord.

                    You might look at "hospital" grade receptacles if you have any cords that might crate a problem if vibratiion would cause them to work out of the wall.


                    • #11
                      My garage came stock with one duplex GFCI 15A shared with the lighting, door, and everything in the entry hallway. I added 4 20A duplex GFCI boxes each on a different breaker. I haven't tripped anything since that little upgrade. I added 2 50A 220V circuits too...

                      Map your machines out and figure out what you will be using at the same time on a given circuit. In the rest of the house, 8 duplex boxes would be overkill for most any room with the exception of the kitchen, but in the shop you might be tripping breakers on your second tool on the same drop. The real question is how many total amps you will need at any one time. You will use more if you have an air compressor, CNC mill, CNC lathe or heat treat oven, type tools where you start them up and walk away to the welder or grinder at the same time but less if you do everything one tool at a time.


                      • #12
                        I believe the code here in BC (where Evan also lives) says 12 devices per circuit. Device being an outlet, a light, or some hard-wired device such as a (bath or kitchen sized) exhaust fan. However, I would usually limit myself to 8 or so rather than push the limits. Evan's comments about de-rating and wire gauge should also be heeded. If you ever have any doubt, check your local code, because they do vary a bit and some things, especially if borderline, are at the discretion or interpretation of the inspector.

                        When I wired a shop I made sure I put enough outlets around so that there was always one close to where ever I might need one (LOTS!), and distributed that among as many circuits as it took to do it safely and per code.

                        There is a trade off between using a GFI breaker and using individual GFI outlets. One GFI breaker costs as much as maybe 8 or 10 GFI outlets, and when it goes, every outlet is turned off, not just the one that caused the problem. I personally don't like this, although I suppose you could argue that in some cases it may be safer (?). This is also another reason to make sure you have lighting on an independent circuit that is not likely to be taken down.
                        "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


                        • #13
                          You can put all of this in a different perspective as well. NFPA 70 (NEC) is a national requirement for new buildings and modifications made to existing structures (and from what I have been told here, a close Canadian standard as well). One of the biggest obstacles to NOT heeding the NEC is that should you ever sell your home or shop, you MAY be required to bring the structure up to code standards. However, if you plan to live in your place long term or have little intentions of selling or renting, you have the ability to circumvent the code just about any way you wish. Your insurance company might have issue... or they might not.

                          I have been in the electrical field for 30 odd years, and I assure you that not everything in either my house or my shop is dead nuts up to "code". Many things were done before the code even made mention of them. Even so, my shop and home are absolutely safe for occupancy. All power circuits are well grounded and all installations are solid and overcurrent protected by the book. The convenience power in my garage is not ground fault protected. It all passed the inspector non the less.

                          I rarely run more than one tool at a time from anywhere in the shop. It's just not likely that I'll turn on a grinder while, say the sander runs too. I only have two hands. My little 9" lathe is on the same circuit as my bead blast cabinet... but I simply don't run them both at once. I can't be in two places at a time! Of course if you have build parties, then by all means make allowances for many power tasks at once.

                          The point is, have some common sense with your installations. It is not necessary to use hospital grade receptacles, but I wouldn't use the 79 cent specials either. You can put eight duplexes on a circuit, but you'd be better off using less. As someone mentioned above, lay out what you COULD be doing all at once, allow for dedicated circuits for refrigerators, compressors, entertainment centers (yes I have a huge stereo/ wide screen TV/ home theater sound in my building), heaters, or battery charging stations, and install your power accordingly.

                          Just my opinion.


                          • #14
                            My wiring job is not done to code. However, it takes into account that mine is a one person shop. I only run one machine at a time and all my lights are pretty much on one circuit.

                            Keep in mind, if you try to get too fancy and put both sides of a 220 line into your shop as 110V circuits that if you are working with a circuit on one side of the room on the other side of the room it is possible to get across 220volts.

                            Most all my lighting is mounted on the ceiling and on one side of my 220 line and the other side is for all the receptical that ring the shop walls.

                            I would not advise anyone else to do it as I am as it is not code proper.
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