I have a junction box currently that I need to add wiring to.

It has the opening and internal clamps to add two more wires but this would put the box over its legal limit of wires in the box.

The box is in a location that can be gotten at without removing any part of the building.
The box is an approved junction box.
The box has internal clamps for the wiring.

So my question is do I have to replace the box to add wires or can I use another box as a cover to increase the box size.

Example: The box is 4x4x2.5 and if I use the same box as a cover it would be 4x4x5.
The box as a cover actually fits tighter then the cover that was originally on it.

Is it legal to do this?
If not are there covers that expand junction boxes?
Or is my only option to replace the box with a bigger box?

Thanks in advance for all your help.

Review this previous post for a picture of a 4x4 extension box, which is what you need to add more volumn to the existing junction box, then cover with the existing plate you have.

https://www.askmehelpdesk.com/electr...=extension+box

What you propose is not necessary, not really sure how you would attach one box to another.

To attach the one box to another you just face both open sides of the box to one another and use the exsisting screw holes to attach one to the other (wish I had a picture).

So if the extension both is legal do you think the flipping of the boxes would be legal?

Also if my box is fiberglass\plastic can I add a metal extension to it?
(I am guessing yes as long as its grounded)

No don't use another box, yes a metal box will be fine as long as it is grounded.