I'm re-wiring the kitchen. I took a 10/3 wire and dedicated it to the microwave and refrigerator (It used to be a built-in oven dedicated circuit). Now I would like to add one wall electrical outlet to this line. Can I tap into the 10/3 in the attic (junction box), and use 12/3 wire running to the outlet from the 10/3, or do I need to stick to 10/3 wire all the way to the wall outlet? Also, is either a 20amp or 30 amp breaker OK on this circuit? Thanks.

 

Tractor,

Ok, I am going to give you this from a practical issue versus a CODE issue because I see WAY to many NEC wannabe's in here and I actually teach the NEC....However, we need to explain a practical issue.

1.) As you begin to add things to this ( Common Neutral ) circuit you have to take into account the unbalanced load you may place on the 10/3 wire in question here. The Microwave in general should have it's own 20A circuit so you placing that on the system and now adding a Fridge which can pull between ohhh....lets say 6-9 amps if the compressor kicks in but more over the actual draw each appliance you are wanting to put on the system.....

Now......we talk alot about fellow electricians saying not all things are on at the same time and so on...but in this case the fridge comes on without your control and the Microwave will most certainly be used with the fridge on at many times....

2.) Ok...as for splicing on 12 ( 20A ) wire to 10 (30A ) wire......well we do it all the time for voltage drop and so on to maintain the highest level of voltage we can....in this case you are not trying to do that....

"3.) Now lets say your fridge is going......someone is plugging a toaster in that recept you are wanting to add and using the microwave......sets say for popcorn...had to come up with something....ok...now....technically you have overloaded the rating of the supply line's neutral rating.... much less we dont know which item you placed in the recept and which applicance it is on with"

Overloading: When I speak to a layman who I do not wish to go into unbalanced issues or so on....it is ALWAYS better in a forum with many who will not understand to explain....I do not expect them to know Ohms Law and so on....so easy to tell them...sure it will only take the unbalanced load...but I have SEEN the mistakes a improperly wired circuit can cause....if you tell them to be aware of it they are always safer....when dealing with circuits like this we always instruct as a SAFETY thing..not CODE thing...to never load the neutral past 70% of the combined load capacity.... I am more worried about the RECEP statement on the same circuit....and this is why i asked what we intended to PLUG into the recep....since the question was not use 10 AWG..it was 12 AWG...and on one he has a Fridge and the other a Microwave....NOW.....adding a RECP to that for a counter top lets say...has no one refered to this.....the neutral is a after thought I threw in...

Statement:
"As for the breaker....if you place a 30A on the circuit yet use 12 at the end you are not rating the protection correctly....now if you place it on a SAFE 20A breaker.....guess what you may if you do # 3 above...you may have problems. Obviously, the 20A breaker is the SAFER move in this situation..."

P.S....Yes it is code......but overall it is a SAFETY issue to why it is CODE

Reason: has NOTHING to do with getting into the RECP...simply answering the question he posed based on the title...can you mix guages of wire, I answered it without being overly expressive.

We do add things ( outside the spectrum of the CODE ) all the time in service work and service call issues as an Electrician...but we always have to consider the potential load and value of the load and make a SAFE choice in the operation....

but to answer the BASIC question..yes you can splice 12 AWG to the 10AWG but you need to protect the wire atthe 12 AWG rating ( 20 A ) to truly be safe.......I am sure others will disagree...but hey....LIFE MOVES ON !


NOTE : I stated "Ok, I am going to give you this from a practical issue versus a CODE issue because I see WAY to many NEC wannabe's in here and I actually teach the NEC....However, we need to explain a practical issue."

 

If the microwave is on one side of the shared neutral circuit and the fridge is on the other the maximum neutral current will be when only one of them is on. If the microwave is running then the fridge kicks in the neutral current actually will drop.

Actually he has no choice, he must use a 20A breaker even if he runs 10ga all the way to the receptacle, since the receptacles will need to be 15A or 20A for the microwave and fridge.

Bottom line is that yes you can mix wire gauges as long as the breaker is sized to protect the smallest wire on the circuit.

 

Real 10/3 or 12/3 NM-B cable is a black/red/white/bare cable. But some people mistakenly use these terms to refer to a black/white/bare cable. Before going on, I would like tractor to clarify what kind of cable he's using.

But in any case, as has already been pointed out, the breaker absolutely must be no more than 20 amps.

 

Yep.....I did not get into the specifics of the 20A......as it pertains to the recep being used.......just the sheer fact the wire he is asking about needs to be protected at rating....20A.....I wont even venture into his using a 10/3 on the recep.......


As for the neutral drop........thats fine BUT I am refering to the OVERALL....with his addition of the recep and no idea of what he intends to plug into it and the fact he is doing a extended " Common Neutral " and other unknown variables ( ie: whats attached to the recep...we dont know yet )......Just dont like the idea of it....sorry.

Yes John is right also.....might be confusing 10/3 with 10/2 w/ ground.....important difference...lol

 

ElectricalMan, as for the common neutral, mikewu99 is absolutely correct. The neutral current will drop with any load on both circuits simultaneously. The neutral carries only the load imbalance. It will NEVER exceed the rating of either of the breakers, or wire, in the multi-wire circuit, as long as it is installed properly.



It's not a matter of just safer. It is code.
Also as mikewu99 said, he has no choice but to use a 20 amp breaker on ANY general use receptacles. A 30 is not even an option code wise. But you already knew that.

 

OK, first, I hear loud and clear ONLY a 20 amp breaker. That's fine with me, and I actually understanding the reasoning due to 12/2 with grd and the microwave and refrigerator.
The reason I want to add the recepticle outlet is to provide power to a phone machine. These days you need to plug in the phone for power, and not just a phone line. True, some day someone might plug in that toaster and draw the 6 - 9 amps, and that's what I should plan for when wiring this project.
Here's a bit more: The original 10/3 wire had two hot (black lines) and one white line, so I green-taped one of the black lines on both ends (for identification) and converted it into my ground wire. This gives me the only grounded circuit in the whole house, since the original wiring is only 12/2 (no ground) throughout.
I'm pretty sure this is OK, but I'll check back in case one of you thinks this isn't a good idea.
Many thanks for all the input.

 

hmmm.....rip me why dont you for keeping it BASIC for the guy and not getting over technical....I did not go into the recep or anything because it is Oh also........just inspected a job where a common was done on a kitchen...and ended up being wired wrong and applied 240 to areas and burnt up some things....don't kill the messanger...I just do not like layman doing Common Neutrals.....sorry.

Anyway....I think my post was intended to explain his basic question...can you MIX wires...sure you can...but I dont prefer to QUOTE code as stated in the first line of the post....which I edited to explain.

The entire INTENT of the NEC CODE is safety.....ie: SAFER !

My main concern here is he was asking to switch to 12 wire....and then you DO have a potential neutral issue IF the recep is added and have no idea what is being plugged into that recept....I dont care how much it drops the CODE will still require the minimum rating for the neutral which governs the POSSIBLE draw could be....

I was more or less refering to him switching from 10 to 12 as the original post questioned....i stated you could....but it is SAFER which it is...now it is CODE to size the OCPD for the smaller wire....But i said at the top of the post I would not say CODE...lol...I would keep it practical and keep it that way....SAFER statements...lol.....

I guess from now on I will just say CODE on everything and not try to keep it practical...lol......my concern over the neutral has nothing to do with OHMS law and reduction of unbalanced load......it is more per what we teach in the safety aspect not what if aspect....in that we in our area....Virginia apprentice programs....to never load a common past 70% of the maximum potential draw.....not textbook because we are aware of the unbalanced load is reduced....but what if he plugs something into that recep after it was reduced to 12 AWG and ohh....lets say ( and we have no idea what it might be ) something that has triple harmonics.....or becomes a linear vs. non-linear loads and thus the neutral becomes a carry conductor in regards to amps........and blah...blah..blah......

Just becoming the nature in teaching to have a guideline to the neutral when doing such circuits at least in VA.....might not agree but it keeps it SAFER and SAFER is CODE.