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I am running 3 AWG THHN in conduit as a 100 amp feeder to a subpanel. The conduit is 1-1/4” EMT in a crawlspace, transitioning to 1-1/2” PVC at a pull box to go underground to a detached garage.

I am trying to understand what product I need to install to satisfy the requirements of NEC2020 300.4(G):

300.4 Protection Against Physical Damage

(G) Fittings - Where raceways contain 4 AWG or larger insulated circuit conductors, and these conductors enter a cabinet, a box, an enclosure, or a raceway, the conductors shall be protected in accordance with any of the following:

  1. An identified fitting providing a smoothly rounded insulating surface
  2. A listed metal fitting that has smoothly rounded edges
  3. Separation from the fitting or raceway using an identified insulating material that is securely fastened in place
  4. Threaded hubs or bosses that are an integral part of a cabinet, box, enclosure, or raceway providing a smoothly rounded or flared entry for conductors

Conduit bushings constructed wholly of insulating material shall not be used to secure a fitting or raceway. The insulating fitting or insulating material shall have a temperature rating not less than the insulation temperature rating of the installed conductors.

If I am using EMT-to-box connectors like this (and analogous PVC-to-box connectors):

https://www.homedepot.com/p/Halex-1-1-4-in-Electrical-Metallic-Tube-EMT-Set-Screw-Connector-62712/100564685

Does a plastic insulating bushing, such as below, screwed on to the end of the connector satisfy the requirements of 300.4(G)?

https://www.homedepot.com/p/Halex-1-1-4-in-Rigid-Plastic-Insulated-Bushing-2-Pack-27529/100153897

Or do I need to use a connector with an insulated throat?

Comments as to what I need to do versus what I should do to protect the large conductors are also welcome.

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2 Answers 2

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You do not need an insulated throat. The EMT to box connector you've shown is the way to go. It has to be mounted into the box with the lock nut provided and then the plastic bushing get screwed on to the connector after the lock nut.

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What is that plastic bushing for? Rigid Metal Conduit is thick-wall conduit that resembles water pipe. It has certain advantages, such as only requiring 6" of cover when buried. However it's just like, y'know, water pipe. It doesn't need a connector when it comes into an enclosure, just two conduit nuts back to back. However, it was rough cut, possibly with a hacksaw (disc style tubing cutters create a terrible burr), and field-threaded. The purpose of that bushing is to protect the wire from the potentially rough end of the pipe. It has no "insulating" role.

The EMT connectors you use will be cast or drawn at a factory, and they will make them with sufficiently smooth edges. No need for a bushing.

As far as the wire...

You'd be better off using #1 aluminum for 100A or #2 aluminum for 90A (it's at a pricing sweet-spot). For large feeders like this, aluminum has proven perfectly safe. (key differences: the lugs are rated for aluminum and actually are made of aluminum, and historically installers always used a torque wrench to set lug torques, as is now Code for all terminals which specify a torque.) Copper not torqued properly is dangerous, unlike aluminum torqued properly.

Other errors to watch out for: failure to do a NEC Article 220 Load Calculation to prevent overloading the panel, getting too small a subpanel and running out of spaces, and "bonkers" oversizing of EV charging circuits.

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  • Aren't bushings required for EMT when the wires are 4 AWG and larger?
    – JACK
    Dec 8, 2022 at 20:22
  • @Harper - Reinstate Monica, the bushing manufacturers are hinting at what you're suggesting: that particular bushing comes in packaging marked as being part of their line of "rigid" fittings. And, we know that RMC is threaded NPT, which is not the same threading as these EMT connectors. So to use this bushing for EMT feels like an adaptation (abuse?) of the products intended usage.
    – aerospark
    Dec 8, 2022 at 23:55
  • @Harper - Reinstate Monica, unfortunately my AHJ bans the use of aluminum, citing a CPSC report that "homes with aluminum wiring are 55 times more likely to have fire hazard conditions than homes wired with copper." (See cpsc.gov/s3fs-public/516.pdf). I'd love to community-source a response to my AHJ that includes official (preferably government endorsed) citations as to how that issue originally happened and why it won't happen today using modern standards and techniques; e.g., CU-AL vs CO-ALR, torquing vs not, what else? Not to mention small branch circuits vs feeders/services.
    – aerospark
    Dec 9, 2022 at 0:08
  • The CPSC page refers to 15A and 20A branch circuits, not heavy feeder. If your AHJ gave you that link then they are too stupid to even talk to (or they misunderstood what you asked about). The best argument for aluminum is UL contemporarily certifying modern AL wire and CO-ALR receptacle spec, and NEC covering the aluminum wire issue with 110.14 torque drivers and requirement of AA-8000 alloy but still allowing aluminum wire in every size. iaeimagazine.org/2015/november2015/…
    – Harper - Reinstate Monica
    Dec 9, 2022 at 4:17
  • @aerospark If EMT connectors use a different thread than RMC/NPT, that's news to me. They match up to my eyes, and they definitely interchange. I don't think the bushing would do any harm on an EMT fitting, it'd just be a waste of a bushing.
    – Harper - Reinstate Monica
    Dec 9, 2022 at 4:23

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