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-   -   Multi-Wire Branch Safety check Please!! (http://www.diychatroom.com/f18/multi-wire-branch-safety-check-please-20831/)

kuhiomall 05-09-2008 07:56 PM

Multi-Wire Branch Safety check Please!!
 
I just moved into a new house (40 years).
I noticed during Kitchen remodel, I saw a 12-3 wire (Red/Black/White/Ground) feeding two circuits,
Circuit #1 - Dishwasher, three recessed lights and receptacle (15amp)
Circuit #2 - Garbage disposal and receptacle (15amp)

I went and checked the breaker panel, and there are two 20 breakers on different bus, the circuit #1 is on 1st breaker (20amp) and the circuit #2 was on 3rd breaker (20amp).

I heard that there might be a 240 volt risk on multi-wire branch if ground is disconnected, but having two breaker on different bus can cancelled out the current coming back to the breaker panel.

Is this true?? Is my house safe?
Thanks in advance.

Speedy Petey 05-09-2008 08:01 PM

You have the idea but are a bit confused about the workings of a MWBC.

The "ground" is not a concern. The NEUTRAL is.

The breakers of a MWBC MUST be on different legs of the service. This is why the neutral only carries the imbalance of the current between the two hots.
If the two hots were on the same leg then the neutral would carry the sum of the current, or double of what it is supposed to.

The issue is if the neutral becomes broken or loose. It should NOT in normal use. The fact of it being disconnected should not be an issue because you should be tuning off breaker before working on these, or ANY, circuits.

kencaz 05-09-2008 09:00 PM

I would say your fine as long as your on different legs as Speedy stated... You can check this at the panel with a voltage tester. You should have 240v between the black and the red conductors. If not they are on the same leg and you'll have to move the breakers.

Also, 2008 code states that multiwire branch circuits be on common trip breakers or be tied together so that all non-grounded conductors within the branch circuit can be disconnected at the same time.

I would say if your panel has the room get a double pole 20A breaker for the circuit. This is all around the safest way.

KC

joed 05-09-2008 10:35 PM

If the receptacles on the two circuits are on the counter then you have a code violation.

Termite 05-09-2008 11:03 PM

Quote:

Originally Posted by joed (Post 122176)
If the receptacles on the two circuits are on the counter then you have a code violation.

He said it is a 40 year old house, so current code doesn't apply unless he re-wires it or remodels. But yes, nowadays you must have two circuits serving countertop receptacles.

kuhiomall 05-10-2008 01:19 AM

Thanks a lot!! I just checked the voltage of two hots and it was 240. :thumbsup: So I am good on that. Unfortunately I don't have an extra room for 2 pole common trip breakers.:mad:
Regarding counter top receptacles, I have one dedicated circuit for one counter top receptacle, but one receptacle is with kitchen circuit and two receptacles I mentioned in my original post.

One more question, Can I combine D/W and Garbage Disposal in one circuit and use the other circuit for countertop receptacles?? If yes, then I will have two dedicated countertop receptacle circuits...
My D/W max watt is 1400 with heat dry and garbage is about 6-8 amp (i am guessing).

handifoot 05-12-2008 12:22 AM

You know I've been reading past posts regarding MWBC as I'm considering saving on some wire for new general use circuits to my basement. It's important for me to have a full understanding of why I'm doing what I'm supposed to be doing. So I get the concept of a balanced neutral that only caries the difference between the two circuits as long as they are on separate phases. The part I'm still having trouble with is how do we create an overload situation if the neutral is interrupted? It would seem in my mind that if the neutral were interrupted that both circuits would just stop functioning as in a regular single circuit. Could anyone please take the time to explain what I'm missing here?
Oh and Joba, if you're reading this and your self esteem needs a boost, go ahead and take your best shot, I can handle it.

kuhiomall 05-15-2008 02:36 PM

My understanding was the overload happens only if you have two circuits in the same leg.

joed 05-15-2008 07:42 PM

Quote:

Originally Posted by kuhiomall (Post 123383)
My understanding was the overload happens only if you have two circuits in the same leg.

That can cause the neutral be overloaded. An open netral in a proper MWBC causes a different type of problem.

kuhiomall 05-16-2008 01:59 AM

joed, what kind of problems? Can you be more detail? Thanks.

joed 05-16-2008 09:34 AM

When the neutral opens on a MWBC the loads are put in series. The voltage divides according to the loads. If the loads are equal then 120 volts goes across each load. If the loads are unequal the voltage does not split equal.
As an example if you had two loads of 1000W and 100W then the voltage would split. The current would be 4.6 amp through both loads(1100watts/240volts). That would make the voltage on the 1000 watt load 217 volts and the 100 watt load 21.7. Imagine if your TV or VCR was the 1000 watt load.

Silk 05-16-2008 10:10 AM

1 Attachment(s)
Slight problem with your math Joed, you can't just add up the wattages to get total circuit amperage because the wattage is dependant upon the applied voltage with will change when you open the neutral. You must find total circuit resistance, and then you can find the total circuit amperage which would be appr. 1.5 amps in your example.

The 100 watt load will be dropping the most voltage (appr. 216 volts), which would mean the small loads like computers and plasma TV's will take the hit and not the large loads like space heaters.

kencaz 05-16-2008 09:08 PM

Quote:

Originally Posted by kuhiomall (Post 123497)
joed, what kind of problems? Can you be more detail? Thanks.


I would say there is certainly a danger with these circuits, however, it's not easy to create that situation. Being that you have to have a device on each circuit, both devices have to be switched ON, and then you would have to open the neutral in the right place, (most likely at the panel).

If you had 2 duplex outlets each 20 feet appart, one hot feeding each and sharing a neutral. If you broke the neutral in between the outlets, the first device would work fine but the second would not work. You would not have any over voltage because the path for the current would be cut.

To me, MWBC's are safe and I have never had a problem with them... They save tons of time and money, especially if your running 208/120 service in your biz.

KC


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