Workshop electrical opinion

[os1kne]

I'm getting ready to move into a home where I will have a basement workshop. The shop won't be large, but I'll be starting with a blank slate. I've done a good bit of home electrical work over the years and I'm very confident of my ability to do the wiring - but I have a question.

Several years ago, I purchased 250' of 12-3 romex for a project that never happened (I've used a bit over the years, but not much). I was thinking of putting double gang electrical boxes mounted every 6 feet at 48 inches up from the floor, with a different colored receptacle for each circuit/side of the box. I'd pigtail the outlets, so the likelihood of problems caused due to a neutral being disconnected should be reduced.

Anyway, over the years I've heard people say that a setup like this where the two circuits share a common ground is a bad idea. What do you think? Obviously, I'd prefer to save some money and use some materials that I've been sitting on - but if it's a bad idea I'll bite the bullet.

Thanks!

[Tom Slick]

Sharing a ground is a non-issue, the "shared" ground would be used if you were wiring a 240v circuit so it's completely safe. Some don't like the idea of sharing a neutral but if each hot leg is on a on different bus and the the breaker handles are tied together (a 240v breaker) then a shared neutral is electrically safe. The neutral's load is split between the split phases (180* apart) and the tied handle breaker ensures both circuits are turned off.

[jdon]

As far as I can tell (as a non-electrician, but also with a lot of conscientious home wiring experience), shared neutral wiring meets code, but some discourage the practice.

The main arguments against the practice, it seems, are:

1) if the two circuits are the same phase, the current in the neutral conductor would be the sum of the two circuits, and could readily exceed the capacity of the wire. This would be the case, for example, if a tandem (piggyback) breaker is used for the two circuits, or if non-adjacent breakers are used for the two circuits.

2) the voltage between the hot wires of the two circuits is 240 volts (in fact, that's the best way to see whether you've wired it correctly) but if you were to contact both hot wires at once accidentally, you'd have 240 volts instead of 120 volts. The risk of high voltage shock would also be greater if there were a break in the neutral.

So, in my mind the arguments arise not from any inherent flaws in either the theory or practice of a shared neutral. Rather, they seem to be based on minimizing dangers due to improper implementation, i.e., the idiot factor.

The fact that you've already considered that by planning on pigtailling your neutrals puts you a step ahead, so IMHO, I'd go ahead with it if I were you. Make sure the phases of the circuits are opposite, make all connections secure, and just don't go jamming paper clips into the hot slots of the two receptacles at the same time!

BTW, in case you hadn't already considered it, you need a separate GFCI for each circuit.

[woodturner]

BTW, in case you hadn't already considered it, you need a separate GFCI for each circuit.

A GFCI will mitigate the problems with a shared neutral, to a degree. However, in a shared neutral circuit, they will typically trip constantly and won't be usable.

Electricity flows in a loop - out the hot, back on the neutral (by convention - the reality is a bit more complicated). Althought neutral and ground are connected at exactly one place in the system, typically the service panel, ground is NOT neutral and they cannot be safely interchanged. No current should flow in the ground wire.

Each receptacle needs a voltage source - the black wire - and a voltage return - the white neutral wire. NEC in general does not allow shared neutral, but there are a couple of specific exceptions.

If you can guarantee that the current draw from both hots that share the neutral will be identical, then the shared neutral is OK. If you think you might use one receptacle or the the other, might use different tools on different receptacles, or that the load the two identical tools on the separate receptacles might differ, such as two table saws where one hits a knot, do not share the neutrals.

More simply, the rule of thumb is "never share neutrals".

Another potential issue is color coding - both the wires and the cable should be properly color coded to comply with NEC. The "hot" wire should be black and the cable sheathing should be yellow for 12 AWG. The sheathing color code is a "suggestion" rather than a requirement - it's still acceptable to use the white sheathed NM-B.

[os1kne]

Thanks for the responses.

Sorry, I meant to say "sharing a common neutral". I mistyped.

I also intend to use a 240v breaker, non-GFCI - didn't think to type it.

Most likely the greatest load would come from running my BT3 on circuit A with a shop vac (or a small DC to be acquired in the future) on circuit B.

[All Thumbs]

NEC in general does not allow shared neutral, but there are a couple of specific exceptions.

...

More simply, the rule of thumb is "never share neutrals".

The parts of the NEC that pertain to shared neutrals seem to be 210.4, 300.13(B), and 408.41. Nothing in the NEC that I can find would preclude their use. Can you point to sections of the NEC that you're referring to?

[JimD]

I wouldn't do it regardless of whether the code allows it. My primary argument is that it is not necessary. I run my shop vacumn and another ~15A load off the same circuit regularly. It's about maxed out but it works fine. The closest I would come to this would be to have the two circuits off the one 220V breaker but only put outlets off one of the hot leads into each box - so every other box would tap the "other" 110V supply. I don't want 220V in my 120V outlet boxes. I know from practical experience that you can get by with a mistake with 120V. I don't want any practical experience with mistakes with 220V.

Safety plus lack of need is why I wouldn't do it.

Jim

[woodturner]

The parts of the NEC that pertain to shared neutrals seem to be 210.4, 300.13(B), and 408.41.

Which version of the code? 2011? I'll reference 2008, since that is the one available for free online, so people can easily look it up if they wish.

2008-210.4C Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

This says "don't share the neutral". However, the specific exception 2 does allow neutral sharing (common neutral in NEC terminology) if a two pole breaker is used. So, IF a two-pole breaker is used, they are technically OK per NEC - but some local municipalities (like mine) don't allow them. My point was more that it's not OK to use two single pole breakers (though it doesn't seem like I made that clear in my prior post).

From an engineering perspective, although it may be "legal", it's not a good design practice. Personally, I don't like it and I think it is false economy. Here is a link that describes some of the issues in detail
reasons to avoid shared neutrals

If GFCIs are used, as required in basements and garages, they will trip if used with a shared neutral. A GFCI "measures" the current flow in the hot and neutral conductors. If there is an imbalance of more than a few milliamps, the GFCI trips. A shared neutral results in a constant imbalance between one hot and the neutral. If the loads are perfectly balanced, there will be no current flow in the neutral, and essentially any other balance condition will result in an imbalance between the hot and neutral, since some of the current will return by the other hot. As a result, the GFCI will trip whenever anything is connected to the receptacle.

[os1kne]

I've given this a bit more thought and discovered that in my application I should use GFCI to be code-compliant. I knew that using GFCI on 2 separate 120v circuits using a shared neutral would cause problems.

So, what I'll do is use 12-2 romex to run 2 separate circuits, each with a GFCI breaker - one for each side of the shop. The shop will only be ~ 12x15, so a short extension cord will remedy any fear of overloading a breaker. It will be a one-man shop, so this should suffice.

Thanks for the help!

[cwsmith]

I guess my experience is similar. I am NOT an electrician, but have done a good share of electrical work, including putting in a completely new service (in the early 90's) from the meter box. All passed inspection with complimentary comments from the "master".

Though the NEC code allows (at least the older one that I worked too at the time, and according to recent comments from a licenced electrician) shared grounds and even two-different circuits in the same box... I don't like it and found it to be a problem. (Of course, I only know what my particular problem was and that the electrician said the box sharing was within code... was he wrong?)

I am particularly of a mind that "electrical circuits" need to be orderly and logical to the point that no mistakes can be made and that circuit analysis should be straight forward and without complication.

My little basement shop has three separate circuits, as well as one "light" that is shared with the adjacent laundry area. Two circuits are dedicated to the equipment and the third to the lighting. No grounds are shared anywhere (not any more, anyway).

I don't want a breaker trip to take out the lighting and I don't want something to go wrong in a single lighting circuit that could leave me in the dark with a running machine... hence the one light off the laundry area circuit. All circuits are 15A, 120-volt with 12 ga wire. At this point I have no need for 240-volt, or 20 amps. The biggest draw is my BT and the RAS and no two machines are ever on, other than say the shop vac.

Six years ago, I lost the lights in the kitchen in our newly purchased old home. It took me three days to find it and at one point I hired an electrician, because neither of my two relatives could find the problem either. One of the mysteries was that while the troubled circuit was seemingly dead, I was getting an 18-volt reading and at times it was enough that my "proximity" indicator could be triggered.

After the electrician spent a couple of hours he basically told me that it may take "having to open the walls" to find a hidden box or chewed wire and he suggested to keep the cost down that I just continue to explore on my own for awhile longer. (the house is very old, having been built in 1887).

I did find the problem in a miswired switch, that my wife had apparently toggled during cleaning. (Odd, that a switch in the basement would actually toggle the entire kitchen circuit!!!)

But, that extraneous voltage bothered me and I traced it back to a fairly new box that two separate circuits were sharing and that the grounds from each circuit had been crossed. The electrician had looked at this but pronounced it "within code" and so moved on.

I also found similar boxes in the shop area circuit and I rewired almost everything down there. Point is, I think, is that with things logically layed out, problems are less likely to happen and much easier to diagnose.

Of course I'm only an amateur, and I need to keep reminding myself of that,

CWS

[os1kne]

Aside from the GFCI issue and wanting to be up to code - I remembered something from when my wife and I moved into our current home 7+ years ago.

The previous owner sold the refrigerator with the house and we had a nice refrigerator from our previous home that wouldn't fit in the spot in the kitchen, so it stayed unplugged in the garage until a holiday when some extra frige space was needed. I plugged it in before bed to allow it to cool over night. I didn't know at the time, but the previous owner decided to wire a 20A 120V receptacle in the garage for 240V to run an air compressor (learned from neighbor). Looking at the receptacle, you wouldn't know. In the morning the 2yr old $1,000 refrigerator was dead. This wasn't discovered on the home inspection. I'm confident that I could make everything work well with the 12-3 romex and there would probably never be a problem - but for peace of mind, I'll keep it simple.

Thanks for helping me.

[woodturner]

I didn't know at the time, but the previous owner decided to wire a 20A 120V receptacle in the garage for 240V to run an air compressor (learned from neighbor). Looking at the receptacle, you wouldn't know.

FWIW, the "legal" plugs and receptacles for 240 VAC are different and physically won't fit in the 120 VAC receptacles. The PO must have changed the plug on the compressor, or bought it as a 120 VAC compressor and rewired it.

[All Thumbs]

Which version of the code? 2011? I'll reference 2008, since that is the one available for free online, so people can easily look it up if they wish.

This says "don't share the neutral". However, the specific exception 2 does allow neutral sharing (common neutral in NEC terminology) if a two pole breaker is used. So, IF a two-pole breaker is used, they are technically OK per NEC - but some local municipalities (like mine) don't allow them.

One last question: If AFCI's are required on all circuits with the exception of laundries, kitchens, baths, garages, and unfinished basements, and if GFI's are required on those, where could one actually use a shared neutral these days?

[os1kne]

FWIW, the "legal" plugs and receptacles for 240 VAC are different and physically won't fit in the 120 VAC receptacles. The PO must have changed the plug on the compressor, or bought it as a 120 VAC compressor and rewired it.

According to my neighbor he changed the compressor plug. My neighbor was a very handy older gentleman that passed a couple of years ago. He was good friends with the PO and advised him to buy the proper outlet and do it the right way. It was faster and cheaper to do it with parts on hand, so that's what he did.

Lesson learned was to spend an hour and test every receptacle when moving into a new home. I mentally kick myself every time that I think about it. A little bit of time and about $10 worth of tools can prevent expensive damages.

[woodturner]

One last question: If AFCI's are required on all circuits with the exception of laundries, kitchens, baths, garages, and unfinished basements, and if GFI's are required on those, where could one actually use a shared neutral these days?

In the past, they were usually they were used in kitchens, but I'm not sure there are many applications for them anymore. Maybe lighting circuits in the living and dining rooms?