In this question, the OP has constructed a large junction box out of several extension boxes attached to a "regular" junction box:

Mazura's Ever-Expanding Box

Is there any upper limit on the size of a junction box (either as-built, or expanded)?

  • 1
    Yes, at some point it becomes a conduit.
    – Tester101
    Commented Jan 31, 2015 at 2:46
  • @Tester101 -- elaborate? Commented Jan 31, 2015 at 4:58
  • 1
    That is really awesome. I have never had to utter, "Watch your head for the junction boxes."
    – DMoore
    Commented Jan 31, 2015 at 6:32

2 Answers 2


There is no limit on SIZE - I've worked (in non-residential settings) with some you could sit in.

The practical concern with the "box and 4 extensions" shown is that the wires up at the ceiling are supposed to be able to come 6" out of the open face of the box, IIRC. From a practical working point of view it's usually better to put in a large box you can work in rather than a bunch of small extension boxes, but mostly code wants adequate volume, and adequate wire length, and does not mind a bit if you have more volume, or have made it awkward as shown.

Personally, if I need more room than an extension on a 4 or 4-11/16's box gets me I'm going to throw in a 6x6x4 or an 8x8x4 rather than construct a cluster-bleep like that. But I'll use a 4 inch and one extension quite happily, especially when it's way cheaper than a single bigger box (though I am always slightly befuddled that the extension costs more than the box does.)

Both orange and blue borgs (not a particularly good source of electrical supplies beyond the very run of the mill) stock up to 10x10x4 (400 cubic inches) at my local stores. Real electrical suppliers can get much bigger ones, but your wallet will prevent you from going there unless you actually need to.

Of course, most houses have at least one fairly large junction box, the center of which is full of circuit breakers ;-)


There is no limit to the size of electrical junction or pull boxes, nor to the number of box extensions that can be used, but section 300.14 of the National Electrical Code states that at least 150 mm (6 in.) of free conductor, measured from the point in the box where it emerges from its raceway or cable sheath, shall be left at each outlet, junction, and switch point for splices or the connection of luminaires or devices. Where the opening to an outlet, junction, or switch point is less than 200 mm (8 in.) in any dimension, each conductor shall be long enough to extend at least 75 mm (3 in.) outside the opening.

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