Protecting >6 AWG Conductors at Fittings

Question

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):

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

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.

Answer 1

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.

Answer 2

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.