I have a customer with a detached garage/shed unit that wants to install a few outlets and light fixtures/recessed lighting – common residential use, meaning no heavy power tools, washers/dryers, etc.
The CLOSEST power supply to the unit is 8 AWG wire rated for 40A– which is coming from a 40A breaker in the panel. This breaker previously powered an outdoor spa. The wire is run via underground conduit out to the original spa location — which has since been removed. The conduit opening is about 6 feet away from the garage.
My initial plan was to swap the 40A breaker in the panel to 15A or 20A and pull out the 8 gauge wire entirely and swap it with 14 AWG or 12 AWG. From there, I would extend the conduit to the garage and wire up the receptacles and lights from there.
The customer wants to know WHY we can't just simply "tap into" or use the end of the existing conduit run with 8 AWG as a junction point for the 14/12AWG that will run into the garage.
Besides always wanting to matching the wire gauge EXACTLY to the breaker I have in the panel — I didn't have a good response. Obviously running NEW matching wire adds more to his cost — so I want to be sure there is a technical standard I'm meeting by pushing for doing this.
What options do I have here? Or is the customer correct?
Best Answer
You're always allowed to use bigger wire
The circuit ampacity defines the minimum wire size needed. If you have larger wire on hand, go ahead and use it.
For instance I often run single appliance circuits that demand a 15A breaker. However, I stock 10 colors of #12. I don't stock #14 at all because it's a useless and redundant wire size for me, I'd rarely use it. So the circuit gets run in #12 and breakered 15A. It's a 15A circuit because it's breakered 15A.
Good to be self-aware of the impulse to always match wire size exactly to the breaker. That is a false impulse. There are actually several reasons to consider an upsize besides the one I use: distance, conduit crowding, and yeah, use of existing wire.
The splice must be accessible
One of the rules of splicing in conduit is the splice must remain accessible. So there must remain a hatch cover which can be removed to get to that splice. Even if there was not a splice, if it is a junction box or other pulling access, it also must remain accessible.
If you wanted to bury the old junction site, you would need to reconfigure the conduit so it was straight thru, without any stops, to the garage. Then I would see if I could shift the #8 wire so instead of going from spa to panel, it goes from the garage to a point still inside the house where you could fit a junction box. Then extend the #8 from there to the service panel so the whole run is #8.
Keep it metal
If the old conduit was metal, extend in metal of the same type. IMC and Rigid conduit can be buried at 6" cover, which is pretty handy. Otherwise you will be at 18". Those metal conduits are able to serve as the ground wire. You need separate neutral and grounds, no bootlegging.