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?

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    Have you considered putting in a small subpanel in the garage using the existing #8 wire, and running the 15/20A receptacles from that? – mmathis Sep 27 '18 at 17:23
  • @mmathis I agree with your idea. Though the related question would be: Are there currently 2 hots + neutral + ground (in which case a subpanel is easy) or just 2 hots + ground (in which case you either need another wire (if the conduit has space for it) for neutral, or switch one of the hots to neutral and only have one leg in the subpanel) – manassehkatz Sep 27 '18 at 17:28
  • @mmathis - thank you for the suggestion! If cost were no object, powering a subpanel would be on the table. Right now -- the debate is probably between a few hundred for pulling new cable or not vs. triple that for a subpanel install. Can I legally (by code) splice 2 mismatching conductors of those sizes? – tresstylez Sep 27 '18 at 17:47
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    If you change the breaker to 20A, it's a 20A circuit. – Harper Sep 27 '18 at 19:13
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    Would installing a small subpanel really cost $600+? A 6 slot 100A panel costs like $20, add in another $20 for breakers, the cost of labor to wire it up seems less than pulling out the 8 gauge and replacing it with 12 gauge wire. So then you've got a subpanel for little cost, and if he ever decides to run a space heater in the garage the lights won't go out if he trips the outlet breaker. Plus with the larger wire it'll have less voltage drop if it's a long run. – Johnny Sep 27 '18 at 19:27

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.


There's nothing in code against oversizing wire (where you would change the breaker to 15-20A) or oversizing it for only part of the run. There are many folks who advise against doing so because it might confuse someone later that would change the breaker without looking at the whole circuit, but it's not a code violation at all - codes specs minimum size. More is always fine. That future person needs to be a responsible worker, or not do this type of work.

I would strongly suggest using 8 Ga for the short run into the garage, and then splitting down to to 12 or 14 depending on what size breaker you swap to - but you don't have to - it would make the sub-panel option (if there are 4 wires) easier in the future, and it also makes the point where 12 connects to 8 more easily findable by the future worker, if any.

I'm not getting "triple a few hundred" for a subpanel install based on my own experience with the price of small sub-panels, assuming you have a 4-wire 40 amp feed within 6 feet of the building to be served. It would be cheaper, in my experience, than pulling out and replacing cable, especially if the pulling out and replacing is not done DIY but by a paid worker, as your question appears to imply.

Whatever means you use to splice the wires needs to be listed for the wire sizes you splice, but that's not hard to find at all. Be sure it's wet location rated as it's going to be a junction box in an exterior conduit, but again, not hard to find at all, done all the time, just know what you are doing or hire someone who does.

  • Triple the price for the customer. :) Sure -- its cheap for me or you, but in business we pass those costs to the customer, add our markup, labor, and make our margin. So, for this particular customer -- pulling new wire is much less expensive than installing a sub. – tresstylez Sep 28 '18 at 16:36

There is nothing inherently wrong with mixing wire sizes, provided that a few rules are followed:

  1. The breaker must be sized according to the smallest wire / lowest-current-device in the circuit. Thus, if you wire the lights / receptacles with #12 wire, you have to put a 20A breaker on; you cannot leave the 40A there.

  2. All devices and connectors must be used according to their instructions, or else you violate the UL listing. This means if you have wirenuts that will accept a #8 and #14, you're good to go. If the 20A breaker accepts a #8 wire, good, otherwise pigtail it in.

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    If you wire the lights/receptacles with #8 wire, you'd still have to change the breaker, as the lights/receptacles are not going to be listed for 40A in the described scenario. – Ecnerwal Sep 27 '18 at 18:54

Since a 20 amp breaker probably won't accept #8 wire I would use a blue+gray scotch lock wire nut or the large blue ideal brand they are listed for #6&14 (14 minimum size but can take quite a bit more than that) at each end with a 20 amp breaker but use 12 gauge at each end to make the change in size totally legal and with the splice in the panel it will be obvious what was done. I agree that if it is a 4 wire a 40 amp sub makes more sense . a small 100 amp main panel is under 70$ this provides a local disconnect and if fed by the existing 40 amp circuit only a few 15 & 20 amp breakers would be needed + a ground rod well under 100$

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    Looking up the brand of breaker that's in my panel, #8 wire fits the 20 amp breaker just fine (it is the largest size that breaker is listed for, but no adapter needed.) – Ecnerwal Sep 27 '18 at 19:12
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    I was not sure if 8 would fit but it still might be a good idea to splice so someone can see the smaller wire size and not up size the breaker. I have seen where home owners upsized breakers because a larger wire was used to keep a long run below 3% voltage drop (I was called because they melted the insulation underground and it shorted what a mess). My square D QO 20 amp states only listed 10-14 copper, 20 amp cutter hammer copper 10-14. Both can handle #8 aluminum so it would fit but not listed so that might be a violation. – Ed Beal Sep 27 '18 at 20:04

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