I am running a 20A circuit to the garage and using a LB conduit at a point where my run comes out of the basement and goes underground. I want to run 12/2 NM from the house to this junction box and splice with lose cables that will continue the run to the garage.
This feels like Electric 101 and the box feels big enough. This is why I was surprised when doing Box Fill calculation it appears as if the junction box is not big enough.
In the photo below you see 3 12 AWG wires. Imagine a 12/2 NM comes out of the hole that goes into the house and the standard 3 splices.
- 4 12 awg conductors = 4 x 2.25 = 9 cu.in.
- Volume allowance for all grounds: 1
- 3 Clamps: 3 volume allowance.
This leads to 13 cu.in. but the box is 12. Am I double counting something? It just feels strange that a perfectly reasonable looking junction box is not sufficient for this simple use case.
Best Answer
You don't need a clamp allowance because there are no cableclamps here
There's no need for a clamp allowance here, because that is for cable clamps that sit inside a box and anchor the jacket of a cable to the box's entrance, and you don't have anything like that in this scenario. So, you have 4 2.25 in3 allowances for the 12AWG wires + a fifth for the grounds, which makes 11.25 in3, within the range of this 12 in3 marked body.