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I am redoing my outdoor lighting and need to move 3 wall switches used for outdoor 120V lighting to a different place within the house. The issue is that the inground wire is 10/2 UF but the inside wires going to the wall switches and a timer is 12/2 on a 20A breaker. Should I keep as it is, if so what will be the pros and cons. Or should I rewire it through with 10 gauge wire on a 30A breaker. If so, what type of wall switch should be used and can I still use regular 15A lighting switches?

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    Are the existing switches 20A types and in decent shape? – ThreePhaseEel May 15 at 11:43
  • I did not check if it is 20A but still works – Michael May 15 at 12:23
  • delama is the wire gage go from 12 to 10, is any issue it can rise – Michael May 15 at 12:25
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Not sure why you got a down vote but I changed that. The problem with your electrical setup is the switches and fixtures are not rated for 30 amps in almost all cases for residential, someone may have used 10awg to limit the voltage drop. So your first job is to swap out the 30amp breaker with a 20 amp breaker. If your lighting load will work on 20 amp you may be ok. Code limits a continuous load to 80% or 16 amps on a 20 amp breaker. As far as your 40 amp timer it can be used on a 20 amp circuit and it will last longer than a 20 amp timer. Last if you do have heavy duty fixtures rated for 30 amps you could replace the 12 awg wire and all the switches with 30 amp switches, they do exists but are expensive. Prior to replacing wire and switches I would look at LED lighting, last year I replaced 4 ea 1000w MH lamps with 4 250w led floods and I now have more visible light at 1/4 the power consumption, I was also able to get an energy rebate of 80% of the cost of the lights so in reality they save enough energy that they are paying for them selves in months and I don’t have to mess with them (hopefully for 10+ years). So check your fixtures if they are standard fixtures change the breaker to the proper size of 20 amps, if they are heavy duty fixtures replace the 12awg wire with 10 awg and make sure the switches are 30 amp rated (note 15 amp snap switches are allowed on a 20 amp circuit as long as the load is not above there listed rating ).

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The key is to provide protection at the lowest level needed.

  • Wire - smallest size (highest AWG) determines necessary protection. So with 10 and 12, you breaker for 12 == 20A max.
  • Switches - lowest current rating determines necessary protection. 20A switches are more expensive than 15A switches, but they are readily available. Moving down to 15A is likely just fine if you switch from other lighting to LEDs, but then your breaker will have to go down to 15A. Oops. According to Harper, switch rating depends on loads. So as long as the switched load is lights only (because if it includes a receptacle then you need to allow for that potential load), if you are using LEDs (or other types with a real max. of 1,440 W) then you can stick to 15A switches but still have a 20A circuit with a 20A breaker.
  • Timers and other devices - a 40A timer can work on a 15A or 20A circuit just fine.
  • Lights - the total of all lights should be <= 80% of the breaker. 80% of 15A x 120V = 1,440 W. 80% of 20A x 120V = 1,920 W. That isn't much for incandescent, but (as noted by Ed Beal) is a lot for LEDs.

In the end, the lowest common denominator - likely switches in your case - determines the breaker size. The breaker size then determines the maximum lighting load.

One caution: 10 AWG is non-standard enough that anyone looking at it will hopefully not assume that the circuit is really a 30A circuit all the way end-to-end. But 12 AWG is standard enough that if you use a 15A breaker then someone looking at it later may not realize there are 15A requirements in the circuit, see the 12 AWG wire and decide to upgrade the breaker to 20A (e.g., to support some receptacles in addition to the lighting). So if you downsize to 15A then label it in the panel as a warning to not assume 12 AWG = 20 A.

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    You can also see I made the same statement about switches, as long as the load is not above the switches listed rating. I am not as eloquent as Harper but provided the same information. – Ed Beal May 16 at 14:09
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What you have there is a 20A circuit

It uses 12 AWG wire (as a minimum). You are always allowed to use larger wire.

You must keep the breaker at 20A (because you have loads on this circuit not legal on a 30A circuit) and you must use at least 12 AWG wire.

Voltage drop

The common reason to use larger wire is voltage drop. You need to go check a voltage drop calculator (lots on the web) and put in

  • 8% drop - do not use the default 3%
  • 120V if your lights are 120V
  • the distance
  • Actual amps of the load(s) at the far end - DO NOT put the breaker rating (20A)!!!!!

If there is also a receptacle on the lamp post out there, you might want to account for plug-in loads. Unless you know otherwise, assume 12 amps because that's the largest appliance they generally make with a common NEMA 5-15 plug.

The calc will state your voltage drop. If it's too much for your loads, then consider a wire size bump. For lamps only, I would not worry too much about 8%.

If the loads are lamps only, definitely don't bump unless it's a huge amperage (at that point, LED is cheaper than thick wire; cheaper on your electric bill too). If there are power receptacles also, it might be worth it.

If you have a stupendously long lighting run (1000' or more), then use multi-voltage 100-240V LED lights that are hardwired, and allow voltage to drop as much as 35% in the calculations, because the lamps don't care.

Splices must remain accessible

There's no problem splicing #10 to #12 wire. However all splices must occur in a junction box, and the junction box lid must be accessible forever without any tools at all. We often get people thinking they can make a splice in a junction box and then "mud over" the box - nope! This junction box doesn't have to be at the old switch location, but it does have to be reachable by the existing cables. You can't splice in-wall, not even with a Tyco splice kit, because it only accepts #12-14 cable.

Switch size is decided by load, not breaker

Usually, the size of things is determined by the breaker. Not with switches. If you are switching a lamp, that is certainly less than 15 amps (you'd hope! Most are under 1 amp, LEDs are often less than 0.2 amp) and a common 15A rated switch will do. If you like to power a table saw off a receptacle out there, then go with a tougher 20A switch. They make 30A switches if you feel a likelihood of needing to interrupt a 20A load on a regular basis.

A switch/lamp set would probably rate a 15A breaker. However there is an exception in code that allows 15A "outlets" (including a switch/lamp) on a 20A breaker.

The timer

"40A" is the rating of the timer, and is relevant to commercial installations where 40A lighting circuits are allowed. It will work fine with a <40A lighting load. Just make sure it's upstream of the switch; otherwise if the switch shuts off power to the timer, you will need to reset the clock everytime you do that.

  • I had a hunch - but had not recalled ever seeing anything about switch based on load not breaker. I will update my answer. I might delete my answer later as yours is more comprehensive (as is often the case). – manassehkatz-Reinstate Monica May 15 at 15:31
  • The junction box can require tools to open. Just about every box has at least 2 screws holding the cover so a screwdriver is required and that is a tool required to open the box. – Ed Beal May 16 at 14:02
  • @EdBeal I know. That's why I say the cover of the junction box must be accessible without tools. If you say "the junction box must be accessible with only the screwdriver needed to remove its cover", then people think they can overcover it with something that screws down with the same kind of screw. – Harper - Reinstate Monica May 16 at 14:14
  • It was not clear to me that’s why I made the comment. I was sure you knew this but it may confuse DYI folks. – Ed Beal May 16 at 14:22

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