running two circuits on a single piece of cable

[kzinNOSPAM99]

Hello,

I don't expect responders to be intimately familiar w/CA electrical code but is what's described below generally done?

I want to run two 120V/20A circuits into my shop. I would like every outlet to use both circuits. I've read that splitting an outlet among two circuits requires a special breaker in the panel so let's assume that I will just use two dual outlets per receptacle with the outlet on the left being on one circuit and the one on the right being on the other.

The question is can I run this using a single run of 3 wire cable running white to the neutral bar, black to one 20A breaker and red to another 20A breaker. I.e. the two circuits share a neutral and ground wire.

This seems reasonable to me since the neutrals and grounds are tied to the same potential at the panel anyway but I imagine many a house has caught fire due to things that seemed reasonable at the time.

thanks for the help ml

[toller]

That is called a multiwire circuit; it is essentially a 240v circuit used for 2 120v circuits. It is fine as long as you use a 240v breaker. 2 120v breakers are potentially dangerous because you can forget to shut one off before working on the outlet, and you can carelessly put both on the same leg of the breaker box; these careless acts are prevented by a 240v breaker.

On my house they actually did use 2 120v breakers on the same leg. Fortunately they were not heavily used circuits, so it didn't matter; but it could easily have caused a fire.

I suppose using two outlets would be safer if you are hell bent on using

120v breakers rather than a 240v; but seems klunky. Why not do it right?

[Joseph Meehan]

First problem I see is that you appear to lack some basic knowledge about what you are planning. That makes me wonder if you have the knowledge and skills to do the rest of the job. At the very least get a good book that covers this information. Newsgroups are not going to be able to provide you with all the knowledge you will need to do the job properly and safely.

Yes you can use two circuits on the cable you are looking at. Is it 12 gauge copper??? I don't recall if you can use it for what you are planning. I would want to check the code first. I'll bet someone will address that issue on the newsgroup. HOWEVER. To do what you want to do you must have each wire (red & black) on a opposite phases. This is normally assured by using a special breaker that will make sure you do that and will also assure that both sides will go dead when you turn one off. It would be very easy for someone not knowing how it was wired to turn off one circuit and test one socket to assure it was dead and then open up a box not knowing half the wires were hot.

Frankly you may want to consider a professional.

[Doug Miller]

It's called a multiwire branch circuit, or Edison circuit. I don't know if this is permitted by the CEC, but it *is* permitted under the NEC in the United States, subject to certain provisions. It's up to you to determine if this is permitted in Canada, and under what provisions.

Same issues, really. The main difference is that, with two separate receptacles, the second provision listed below is *not* a requirement of the U.S. NEC, whereas it *is* a requirement with a single duplex receptacle. The requirements may be different in Canada; and in any event, it's still a damn good idea even if the Code doesn't require it.

Yes, IF: a) the two breakers are on *opposite* legs of the 240V service, AND b) there is a common disconnecting means (i.e. trip one breaker and they both go off).

Using a single 240V (two-pole) breaker satisfies both of those requirements, and is by far the easiest and safest way to install such a circuit.

It is reasonable, and safe (if done correctly), but that has NOTHING to do with the neutrals and grounds being at the same potential in the panel. The reason it's safe is that when the two hots are on opposite legs of the service, the current in the neutral wire is the *difference* between the currents in the two hot wires. If you put the two hots on the *same* leg of the service, the current in the neutral is the *sum* of the currents in the two hot lets, and the neutral can very easily become overloaded and start a fire.

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

[zxcvbob]

Does your cable have a ground wire? You said 3 conductors, but then you mentioned black, white, and red and didn't say anything about bare or green. I assume you meant 12/3 cable, and a ground conductor is understood.

Use a 240v breaker so your black and red wire are forced to be on opposite legs and to provide a common trip and it'll work just fine.

Best regards, Bob

[kzinNOSPAM99]

I understand. Hence the need for a 240V breaker to ensure that the circuits are on opposite legs.

Ok, so wiring two circuits w/one cable is probably permissible pending check of CA code, given that I use a 240V breaker wiring the hots to each side of it to ensure they are opposite in phase and to ensure both circuits are simultaneously shut off via a single switch.

It sounds like it's simply worth running two cables so that I can have one circuit if there is ever a problem on the other.

thank you all for your responses. doing things right the first time is why i read books and ask questions

[Goedjn]

[Doug Miller]

And to make sure that *both* hot conductors are disconnected at the same time.

Right..

Maybe. If you use two separate circuits, but put one receptacle from each circuit in the *same* two-gang box, it's still a *very* good idea, from a safety standpoint, to have a common disconnect anyway even though the NEC doesn't require it. And for all I know, the CEC might require it. And if you'd be shutting down both circuits anyhow to fix a problem on one, you might as well run the multiwire circuit.

The biggest advantage to using two separate circuits is that it makes using GFCIs a whole lot simpler.

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

[kzinNOSPAM99]

agreed. now that I'm thinking about the next guy to work on this I would just alternate spaced outlet receptacles on the two circuits... having the receptacles on each circuit at a slightly different height sounds like a good idea as well.

[Goedjn]

I've got one lab that I work on that's got one row of outlets at a standard 16" above the floor, and another row just over desk height. Depending on what you want all that power for, this is a good time to think about whether you want totally separate circuts. (Maybe at ceiling height, too, if you've got high-mounted equipment.)

[nicksanspam]

On a related note, I wonder if UK-style "ring mains" are legal under CEC or NEC. The hot wire leaves the breaker, connects to a ring of outlets, THEN RETURNS to the breaker. This reduces the average wiring power loss without adding much wire, and if the wire breaks in one place, all the outlets keep working.

Nick

[Joseph Meehan]

An interesting question. I don't think so, but I sure don't know.

[Chip C]

This is legal in both Canada and the US, and is the traditional way Canadian kitchens were mandated to be wired (at 15A, not necessarily

20A) until recently. The "special breaker" is a dual-pole unit, aka 240V breaker, and you need it whether you put two duplex outlets together or one outlet with the tie bar broken on the hot side. The latter is the common, usual way to do it, and is called a "split receptacle" scheme.

The dual-pole breaker guarantees two important things: (a) the two hots will be on opposite legs, so the current cancels in the neutral instead of adds, and (b) one breaker will trip the other. I believe feature (b) is legally required only in the case of a split receptacle, not in the case of alternating duplex receptacles.

If it's all 12 gauge on a 20A breaker you can make the outlets the new (to Canada) T-slot type, but this is (I believe) not required. There is little practical advantage since no device known to mankind has a 20A plug on it. (When I asked if this was true, I got no replies, so I'll state it as bald fact and maybe someone will offer a counterexample.)

Another factino that may or may not be of interest: if the circuit has any light fixtures on it, you may use only a 15 A breaker, even if all the wiring is 12 ga. This is an oddity of Canadian code only, I believe. But a workshop circuit is a poor choice to put lighting on anyhow.

Chip C Toronto

[Chip C]

outlets,

I'm pretty darn sure that's a big No, that it's a requirement that ONE breaker be able to shut down the whole circuit.

Chip C Toronto

[nicksanspam]

It would, if the wire returns to the same breaker.

Nick

[Calvin Henry-Cotnam]

There are two advantages of running a 3-wire cable that may make you want to not consider the separate cable option...

First, in a split outlet (common in kitchens here in Canada) the top and bottom half can provide the full circuit load current if needed. If you had the situation where you needed to operate two heavy loads near each other, you could plug them both into the same outlet without the fear of tripping a breaker.

Second, if you ever found you had the need to install a 240 volt outlet (with the same current rating), you could easily replace the split 120 volt outlet with the 240 volt outlet.

[Calvin Henry-Cotnam]

The CEC probably does not require it, since the CEC (believe it or not) does not require a common disconnect for a multi-wire circuit unless it is feeding the same device (like a 120/240 volt appliance, or a split outlet).

The 1994 revision to the CEC allowed you to use two separate single-pole breakers (provided they are on opposite hots) to protect a multi-wire cable feeding two circuits -- for instance, a 14/3 cable used for the home run, then splitting to two separate 14/2 cables for each circuit.

I mention this, not from my own interpretation of the code, but I attended a new code seminar when this came out and the inspector leading the group described this quite clearly, and was asked to repeat this by a number of disbelievers, myself included.

Personally, I follow the rule that all conductors "enclosed together".

[new]

I want to do the same thing using the 12-3 wire for the long run from the circuit box then 12-2 on to each of two separate circuits. Assuming all of the 240 breaker issues are taken care of is there any problem with that either safety practical or code approval (I am in NJ).

[Doug Miller]

Don't know about Canada... but in the U.S. the NEC _explicitly_prohibits_ mixing 240V and 120V loads on a multiwire circuit.

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

[Doug Miller]

Yep, the NEC is the same. Foolishly so IMO, but they didn't ask me first. :-)

-- Regards, Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?