running 3 way switches between buildings
Hi all. have a question about wiring a subpanel in a detatched building. Couldn’t find a good answer on prior postings or interent and have gotten mixed views from locals. I have a detatched building about 200 feet away. I have installed 1.5 inch conduit burried in the ground between the house and this building. I already bought #4 wire so my subpanel capacity will be limited by this wire size.
1st question: what is the biggest size breaker that I can saftely install with this wire size, the distance(not quite 200 feet), and the other wires that I would like to put in the same conduit(see question 2)? I am planning on running 3 #4 wires for two hots and a neutral and 1 #6 wire for grounding back to the main house panel. Can I get away with a smaller ground?
Also, there are no water pipes or anything else between the buildings but since I am running three way switches between I figured I can’t pound in a ground at the garage and have to run a ground back to the main panel. Is this correct?
2nd question: I didn’t think at the time and only installed one conduit. I would like to install two seperate three way switch circuits between the buildings. I would have a switch in the house and a switch in the detatched building for each circuit. (two circuits so 4 switches total, 2 in house and 2 in detatched building). I know that usually you run four wires for each three way circuit(hot, neutral, traveler, and ground). I was wondering if I could share the same ground wires and even the neutrals for both seperate circuits in the conduit. So any help on the number of #12 wires I need to pull to make this work would be great.
Thanks.
Replies
I'm pretty sure that curent code requires you to BOTH "pound" a ground (or two) at your building and run a ground wire back to the main building. Used to be that you could forgo the ground wire back, but that changed maybe 10-15 years ago.
90 degree C #4 copper has an ampacity of 89 amps in conduit. [Though I see other charts saying 95. Nice to see such agreement!] But note that this is generally "for not more than 3 current-carrying conductors in the raceway".
You can, in general, share grounds between circuits in a single conduit. Dunno about the ground size -- I think the code normally allows you to go one size smaller for ground, but I'm sure there are all sorts of exceptions.
But for remote 3-ways (where power is supplied from your building and the "remote" switch is back in the main building) you would not provide "hot, neutral, and traveler". Rather, you'd supply hot and two travelers. Hot goes to the common terminal of the switch, the two tavelers return back to the traveler terminals of the second switch (in your new building), and the common terminal of that goes to the light. The neutral connects to the other terminal of the light.
So for two 3-ways on the same circuit you could get away with a single "hot" (and shared ground), but you need two travelers for each switch. I count five unique wires plus the shared ground. Supplying "hot" from the main building wouldn't do any better, unfortunately -- then you'd need 4 travelers and a neutral.
The issue of whether the outbuilding should have it's own grounding rod or whether it should be bonded to ground from the main panel has been discussed here many times over the years, and it seems to waver among local ammendments to the code. Here, (BC, Canada) it's almost always bonded back to the main service panel, with animal barns being an exception. Other jurisdictions see things differently. If you're pulling a permit for the job then I'd definitely ask your inspector. It may save you a headache in the long run.
The basic ampacity for 4 ga copper THHN is 85 amps @75c
Since 2005 (NEC) you need a 4 wire feeder for the sub panel and you have needed a ground electrode at the detached building for much longer than that. The ground and neutral are separated in the sub panel.
As for your switch. if both of the lights are on the same circuit you could share the hot or the neutral, (depending on where you source the load) but you still need 2 travellers per switch.
This also bump into the problem of derating when you have 8 wires in the pipe. You can only use 70% of the 90C ampacity. That will not be a problem for your 12 or 14 gauge branch circuit wires since they are conservatavely rated in the ampacity table. The problem arises in the 4ga wires. You start that at 95a x .70 that gets you 66a and you can round up to 70a.
At 200 feet out, you might want to limit yourself to 70a anyway just for voltage drop issues.
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NOT my legal opinion ... you could use the 85a and never have a heat problem since the 4 travellers are only going to carry a couple amps but you still are dropping 10.5v on the trip.
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BUT IT ISN'T LEGAL.