workshop power & shared neutrals

[Robert Reece]

A couple of question on workshop power.

First the practical one - should power at the workbench come from a power strip on the front/underside of the workbench or should I make corded drops from overhead. Overhead is easier for me since my bench is not on a wall, but I can get power to the island if that is better.

Second - technical NEC question - are shared neutrals allowed (they weren't a few years ago) and if not why? I believe Canada allowed them and they don't seem to be having large numbers of house fires. Reason I ask is that I was thinking about running a 12/3 all over the place and wiring each half of a duplex receptacle to a separate leg. That way my vacuum and router can get plugged into the same duplex. It might be cheaper to just run 2 12ga and do every other box.

[Mike Henderson]

I'n not a NEC expert but I see a couple of problems with shared neutrals.

1. You generally have to wire the two outlets in a duplex receptacle to two breakers (one outlet to each breaker). It's easy for someone to come along after you and only turn one off before working on the box.

2. Unless the people who come along after you are very careful, someone could move a breaker so that both are on the same side of the line. In that case, the currents in the neutral line add instead of subtract and you could have a fire.

"In theory" a shared neutral will work fine. But the world doesn't work "in theory" so shared neutral is prohibited in some areas (it is around here).

The only thing you save is one wire which isn't that expensive or hard to pull when you're pulling the rest of the wires.

Mike

[Bruce Wrenn]

What you are proposing is called "multi-wire branch circuits." Both hots must be fed from a two pole breaker, with a tie bar connecting both sides. In other words, a two pole breaker. I use this system in my shop, except, I have two duplex 120 recpt's and a 220 recpt. at each location. One of the 120, and one leg of the 220 passes through a sensor in panel box which operates the DC system. I like a plug strip mounted to the bench much more than hanging pendents. Look at section 210-4 of the NEC. Cheers, Bruce

[Rod Sheridan]

It's a perfectly safe method of wiring if you use a 2 pole breaker as indicated in the post above.

My entire shop machinery is fed from one 15A 2 pole breaker.

I have 4 inch square boxes with a duplex 15A 240 receptacle in it, and a duplex 15A 120V receptacle wired as a split circuit.

The largest machines I have are 3HP for the TS and shaper, the Hammer Jointer/planer is about 3HP also.

These are located at locations for each machine, in addition a GFCI receptacle fed from one line of the circuit supplies my workbench via a drop cord.

Since I work alone, I can only use one tool or machine at a time, so it works great for me.

I also have a 15A 240V circuit that supplies my 1.5HP Oneida Cyclone.

Regards, Rod.

[Rob Russell]

Bruce, you're almost correct. Your description of a multiwire circuit is accurate and, as you note, they are currently still allowed under the NEC.

What's not accurate is the requirement to have a 2-pole breaker with tie bar. The "common disconnect" is not a general requirement for a multiwire circuit. It's a good idea - but not a requirement - again, for a multiwire circuit.

The common disconnect for both hot legs is a requirement when you have multiple circuits feeding devices on the same yoke. When you split-wire a duplex receptacle and have 1 circuit feeding 1 outlet and another circuit feeding the other outlet, then the breakers feeding both of those outlets must have a common disconnect.


So - to compare:

2 circuits feed the outlets on (1) duplex receptacle that is split-wired. A common disconnect for the 2 circuits is required by 210.4(B).
2 circuits feed (2) duplex receptacles in a 2-gang box. The receptacles are wired normally, so 1 circuit feeds 1 duplex receptace and the 2nd circuit feeds the 2nd duplex receptacle. There is no requirement for a common disconnect on the 2 circuits feeding the receptacles because the devices are not on a common yoke.


Personally, I would want a common disconnect on these 2 circuits and would wire it that way. I wouldn't want the chance of opening up the box when 1 of the circuits was still live.

There is something else to consider here. If your workshop is in a garage or unfinished basement, the 15A/20A circuits must be GFCI-protected. You can't do that on a multiwire circuit unless you put a GFCI receptacle in at every location and it would be cheaper to run (2) sets of 14/2 or 12/2. You can't use the typical technique of a GFCI receptacle protecting everything down-circuit from it on a multiwire circuit because of the shared neutral.

There is NM cable available now called 14/4 and 12/4 which have an extra neutral conductor (frequently it's white with red stripe). If you have a LOT of these circuits to put in, the */4 might be a little cheaper because you're only paying for 1 outer casing and 1 bare grounding conductor. I'd price out the difference between running */4 vs. (2) runs of */2 before I bought the */4.

[Allen Schmid]

Gentlemen,
I have been eavesdropping on this thread and find it very helpful information. A little here and a little there but some good things to take from it, certainly. Rob, I thought your description and idea for comparing the */2 and */4 wiring is a winner...have not seen the */4 wiring around...but then I have not really been looking for it either!
Robert, glad you asked the question and thanks everyone for some insight that I have now gained.
Allen

[Rick Christopherson]

What's not accurate is the requirement to have a 2-pole breaker with tie bar. The "common disconnect" is not a general requirement for a multiwire circuit. It's a good idea - but not a requirement - again, for a multiwire circuit.Nope. All multiwire branch circuits require a 2-pole breaker to simultaneously open all ungrounded conductors. Your reference in the NEC is correct, but the 2008 NEC has changed to disallow single pole breakers for any reason.

[Rob Russell]

Nope. All multiwire branch circuits require a 2-pole breaker to simultaneously open all ungrounded conductors. Your reference in the NEC is correct, but the 2008 NEC has changed to disallow single pole breakers for any reason.

Thanks for posting that. I don't have the 2008 NEC yet, so couldn't check that. I believe that is a good change in the code. It's far safer that way.

One note - not all areas have adopted the 2008 version of the NEC.


Just so y'all know what sort of wiring practices people do who don't pay attention to wiring, my wife and I bought a beach cottage this past summer. These are some of the wiring "things" I've discovered so far.

The dryer (30 amp circuit) is wired with 12/2.
There is no grounding electrode system. In other words, we have no ground rods outside. There is a small bare conductor running to the water pipe.
Most of the breakers are unlabelled in the panelboard.
One of my projects this summer will be to bring the basic electical service up to current code if for no other reasons than basic safety.

[Robert Reece]

I have a pile of 12/2 leftover after cleaning up my electrical panel and removing unused circuits and consolidating some circuits (like the lighting in the shop). I can't really justify buying 12/3 when I have all this 12/2 sitting there on the floor, so I think I will just run the 12/2. I'll be running two 20A circuits and spacing about every 6' overhead. So it shouldn't be too hard to plug in the router and vacuum at the same time. Also, the GFI issue will be a lot easier (cheaper) using 12/2.