In particular, you'll need an absolute minimum of a 6" long by 8" wide by 3" or 4" deep, NEMA 1 (indoor) pull box with a flush-mount cover, grounding kit, and 8AWG grounding jumper in order to accommodate such a tap, using dual-rated (AlxCu) insulated mechanical splicing connectors (Polaris™ or equivalent). Getting into the details of making the splice, we'll first need a big box, far larger than the junction boxes you're used to, due to NEC 314.28(A). Finally, we'll have to account for the NEC 300.14 requirement of 6" minimum of free conductor for each cable entering the box if you can't get that from slack in the existing run, you'll have to go up to a 12" long by 6" to 8" wide by 4" deep pull box and add an extra block and some 2AWG copper jumpers to connect the two, as well as a spare grounding lug such as a Brumall 6T attached to the box with a 10-32 self-tapping/self-drilling grounding screw such as the Garvin GSST. Furthermore, you will need to have the splice box be accessible now and forevermore you can paint or otherwise finish the cover to match the surroundings, but actually burying a box is no good. First off, the T-splice is a much better idea (less cable and fewer splices) than the "loop" splice, so we'd be going with your plan 1 in this case instead of your plan 2. If you do go with the splice-box plans, there are a few points you will need to keep in mind. If you really want a disconnect for the original subpanel in the new subpanel, then you can use a 100A molded case switch of the appropriate type for the new subpanel. If you do go that route, you'll want to use a subfeed lug block in the new subpanel to feed the existing panel instead of a 100A breaker since the breaker would be fairly pointless anyway due to the inability to coordinate it with the existing feeder breaker. (Aluminum cable is also lighter than copper cables with the same ampacity, which factors into the ease of pulling significantly at the larger gauges that aluminum wire is typically used for.) Going to 1/1/1/3 aluminum SER at about $1.60-$1.70/ft makes avoiding splicing a cost-effective option, though, and in that vein, I would favor plan 4 with cutting the existing feeder and rerouting it, then replacing the rest of the existing feeder run with a new run. This means that you're looking at a few hundred dollars of SER cable alone for either your plan 3 or the plan 4 you mentioned in your comment, and makes the parts required for a splice rather cost-effective in comparison. While avoiding splices, boxes, and access points sounds like a good idea, the problem with your plan is that copper 3/3/3/5 SER cable is quite expensive stuff at over $5/ft yet only a hair smaller at 0.91" diameter vs 1.08" diameter for 1/1/1/3 Al. The problem with your no-splicing plans is the cost of copper Do I need access to those junction boxes later, or can I bury them underneath drywall after inspection?.Are there T and/or splice connectors for 3ga cable that I can put into boxes in the ceiling?.Is it code compliant to Tee off the existing cable in the ceiling and feed two sub panels in parallel from one breaker in the main panel?.No cuts or splices here which is great, but 50' of extra cable running through my joists which seems wasteful. ![]()
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