Junction or pull box

Question

I need help designing my conduit pull boxes and looking for advice on simplifying my pull if possible. I understand wire sizing, minimum bend radius, etc, but am not experienced running conduit. I'm planning to pull 3x4 awg and an 8 awg ground through 1" ENT (but am also considering PVC). This will run perpendicular to my joists in the basement, go up through a wall, and exit in my garage where it will transition to flush mount PVC.

To adhere to 360 degree rule and keep things simple, I know that I should have pull boxes where I have drawn red circles. Some questions:

Does it make sense to have a non-continuous run and use a junction box with pre-insulated connectors in these red circles instead of pull boxes and a continuous run?

Is it allowable for a pull box or junction box to be exposed below the edge of a joist to receive my conduit?

Would I need full boxes at these locations for a continuous pull or would an LB body be appropriate in these locations?

What type of flush mount box would you recommend to run wall mount conduit out of my wall?

Answer 1

An LB (LL, LR, etc) is a valid access point for pulling, all by itself, no need for a junction box instead.

In general, if not doing something like changing from cable to wires in conduit, a continuous run is preferable, as it has fewer junctions to potentially fail.

If the "flush mount box" you ask about is just to pass the conduit/wires from in the wall to out of the wall, I'd likely just choose another LB/LL/LR at that point rather than a full-on box. Presumably the other end of the conduit ends at your sub-panel, electric car charger, or welder, or a suitable receptacle box for those if they are plug-in (unlikely at 4 gauge.)

Incidentally, if you are sizing this for copper, compare the much lower cost of wire to the slightly higher cost for larger conduit for larger aluminum wires. Large feeder wires have never been the "problem" with aluminum wiring, and at these sizes and current prices copper is not usually a wise choice.

Answer 2

Given the large sizes of wire you are talking about (#4 and larger), there are strict rules for junction box dimensions - and they are awkwardly large.

Your best bet is to use conduit bodies instead, which are manufactured appropriate to the conduit size. An example is an LL, LB or LR type - each one provides the access cover on a different side, and you select it based on the location.

Remember conduit body access covers must remain accessible without tools. So it is not legal to cover it up with drywall or nailed-on peg board.

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While you didn't say what your project is, #4Cu is an odd size of wire. So there are two things I'd caution you of.

First, #4 copper is only good for 85 amps, and #2 aluminum is only good for 90A. See Table 310.15(B)(16). It is "folk knowledge" that those are good to 100A, but that is false and based on a wild misconception of another rule. For 100A you need #1 Al or #3 Cu.

Second, a common novice mistake is to fear aluminum. Don't. Aluminum wire is excellent for heavy feeder like this. In fact, the lugs on the panel will be made of aluminum, precisely because it is the universal donor - it plays well with both Al and CU wire. (hold that thought). What's more, we often see people polarize into their copper decision and post an "after" picture - It's a HOMeline panel, which has aluminum bus bars, for Pete's sake. Doesn't make much sense.

And lastly, the #1 panel problem seen here is "I am out of spaces in my panel". Don't economize there! Spaces are cheap. Get loads of them. Years down the road you'll want to install something else, and you won't remember the taste of the pizza you bought with the savings. And the size of a subpanel's main breaker or bus is allowed to be larger than the feeder.

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What about the aluminum horror stories? This only ever applied to small wires. In the 1970s they tried scaling down the old AA-1350 alloy (used on power lines and feeder) for 15-20A small circuits. Two big problems. #1 the terminals on receptacles were made of copper or brass - WHOOPS! And thermal expansion worked against the connection instead of for it. And #2 (my theory) torque screwdrivers were not required on small connections yet, so torques were all over the map.

Improper torque makes any connection fail. But you can see where a dissimilar-metal connection will be more sensitive to correct torque. But back then, it was common practice to torque heavy feeders, and they performed well! Even in the obsolete AA-1350 alloy.