Electrical 3 wire question

[jmagg]

So I have an underground 12g 3 wire with ground to the garage. At the moment there is only one 20A circuit (one hot not in use) to the garage. I'd like 2 20A circuits in the garage but am unsure as to the best way. Any electricians in the house? TIA

 

[stormkroe]

buy a 2 pole 20A breaker (less than $10 in the US) and replace the single. You may have to shift things around to fit the wider breaker, and if it's a 'tandem' style, make SURE it's straddling phases or you will have 40A of120v trying to come back across that single neutral.
Buy a little 8 circuit panel for the garage end, and you're done.

 

[jmagg]

Ty. Still unsure on how not to overload the neutral in this situation . Is there a special breaker, or is straddling phases done in wiring configuration? Goog isn't telling

 

[Paperdoc]

The 2-pole breaker will do what you need. BUT there is a small complication. Most 2-pole breakers are actually two breakers in one module, and both are linked. So when one "pops", BOTH circuits get shut off, You may be quite ready to accept that and "make do" with no working circuit in the garage until the problem with the ONE circuit is fixed. BUT if you want to have two breakers completely separate so one keeps working, then you need a different arrangement. You need a small breaker panel with two bus bars for the two Hot lines, a Neutral and a Ground bus.Then you need two 20 amp breakers that you mount so that each one connects to only ONE of the Hot bus bars, and they MUST be the two DIFFERENT bus bars. Then you have two circuits on independent breakers and drawing from different Hot lines.

The Neutral thing takes care itself by design. It's not quite right to use DC terms for an AC circuit system, but it's easier than discussing phases, so I'll use those terms. You will have two 20A circuits, each fed from the bus system by separate breakers (whether they are physically separate or in one module) and each with a Neutral line back to the breaker panel. At that point, the two neutrals both are connected to the Neutral bus of the panel, and that's how they jointly connect to the Neutral line in your 3-wire 12-gauge cable from the house. Your concern is the load on that latter cable's Neutral wire. Well, Breaker # 1 is connected to Hot Line #1, and at a given moment in time it is a + Voltage source. Meanwhile breaker #2 is on Hot #2, and that Hot line at that moment is a - Voltage source. Thus the currents flowing on the Hot lines of each circuit are flowing in opposite directions, and the same goes for the respective Neutral lines. At the Neutral bus in the panel where they join, the current from one is in the opposite direction (although not necessarily the same magnitude) as the other's. So they partially cancel, and the NET (difference) current is all that must flow through the Neutral wire of your 3-wire 12-gauge cable back to the house. So the current in that Neutral wire will never exceed the current on ONE of the branch circuits, because it is always being reduced to some extent by the opposite-direction current in the other branch circuit. That is why the Neutral wire inside the 3-wire 12-gauge cable is the same size as the two Hot wires - the current on the Neutral cannot exceed the max current on either Hot line.

 

[stormkroe]

Most 2-pole breakers are actually two breakers in one module, and both are linked. So when one "pops", BOTH circuits get shut off, You may be quite ready to accept that and "make do" with no working circuit in the garage until the problem with the ONE circuit is fixed. BUT if you want to have two breakers completely separate so one keeps working, then you need a different arrangement.

I don't have my NEC with me, but the CEC does not allow the use of 2 separate breakers to supply feeders consisting of multi-phase hot conductors with shared neutral.

14-302 Construction of circuit breakers (see Appendix B)
Where circuit breakers are provided for the protection of apparatus or ungrounded conductors, or both, they
shall open the circuit in all ungrounded conductors by the manual operation of a single handle and by the action
of overcurrent, except
(b) in branch circuits derived from a 3-wire grounded neutral system, two single-pole manually operable circuit
breakers shall be permitted to be used instead of a 2-pole circuit breaker, provided that
(i) their handles are interlocked with a device as provided by the manufacturer so that all ungrounded
conductors will be opened by the manual operation of any handle;

 

[jmagg]

Got it. TYVM

 

[NL5]

I don't have my NEC with me, but the CEC does not allow the use of 2 separate breakers to supply feeders consisting of multi-phase hot conductors with shared neutral.

Same for NEC.

 

[colonel]

So I have an underground 12g 3 wire with ground to the garage. At the moment there is only one 20A circuit (one hot not in use) to the garage. I'd like 2 20A circuits in the garage but am unsure as to the best way. Any electricians in the house? TIA

you missed information when you said 3 wires, it means black, white, ground? How far is the garage from the main box? How deep is the UF wire in the ground? I will get a subpanel there, you never know maybe in the future you will use heavy machines who might draw a lot of power. I did my Shed with a UF 10g 4 wires, 100 meters from the box with a subpanel and I followed the code for inspection.

 

[Greenman]

you missed information when you said 3 wires, it means black, white, ground? How far is the garage from the main box? How deep is the UF wire in the ground? I will get a subpanel there, you never know maybe in the future you will use heavy machines who might draw a lot of power. I did my Shed with a UF 10g 4 wires, 100 meters from the box with a subpanel and I followed the code for inspection.

Three wire in residential generally means black, red, white, the ground isn't counted as a conductor. Wire designation reflects that. The wire you used in your shed would be referred to as 10/3.

 

[colonel]

Yes, You are correct. 10/3 since my shed was 100 feet from the main panel, I calculated the drop. I was in the line with 8/3, but I haven't gotten yet any issue on my circuits.