Definition mwbc

[retirede]

So, the 12” depth direct burial is not allowed to a garage, as a garage needs two circuits. A garage would need to be 6” in rigid, 18” in conduit, or 24” direct burial.
You could do 12” direct burial for a shed or workshop, though, just not a mwbc.

P.S. how many get frustrated with auto correct when typing electrical acronyms? It obviously doesn’t like terms like gfci, mwbc, afci, egc, and such.
I spent an inordinate amount of time switching all those terms back to what I had.
Rant over

On most devices, you can add terms to your dictionary and/or turn off autocorrect.

 

[wwhitney]

MWBCs have a dual nature and it seems to me that they may be treated as one circuit or two circuits as is convenient. I would think that applies to Table 300.5 as well. I see no problem with this treatment of a 12/3 circuit:

- Treat it as (2) 120V circuits for Table 300.5 requirements.
- Treat it as (1) MWBC for Article 225 requirements.
- Treat it as (2) 120V circuits for Article 210 requirements.

Cheers, Wayne

 

[Dennis Alwon]

In this case the 240 volt circuit would negate your ability to use the reduced burial depth of 12" with GFCI portection.

One can argue that it is 2- 120v circuits not 240V

 

[infinity]

One can argue that it is 2- 120v circuits not 240V

For the purpose of burial depth IMO you'll lose that argument every time.

 

[wwhitney]

For the purpose of burial depth IMO you'll lose that argument every time.

Why? Is there some reason that 300.5 restricts the dual nature of MWBCs more than other sections?

And as a related question, say you have 12/4 UF (if that exists), and you use it to carry (2) 2-wire 120V circuits, where the ungrounded conductors are on opposite legs. Does that qualify for Column 4 treatment in Table 300.5? If so, why would a 12/3 MWBC require a greater level of protection than that 12/4?

Cheers, Wayne

 

[LarryFine]

Could you run a GFCI protected 120V feeder at the shallow depth, and then have a panel at the garage to give the required circuits?

Yes, but the sub-panel triggers electrode requirements.

 

[infinity]

Why? Is there some reason that 300.5 restricts the dual nature of MWBCs more than other sections?

And as a related question, say you have 12/4 UF (if that exists), and you use it to carry (2) 2-wire 120V circuits, where the ungrounded conductors are on opposite legs. Does that qualify for Column 4 treatment in Table 300.5? If so, why would a 12/3 MWBC require a greater level of protection than that 12/4?

Cheers, Wayne

The MWBC exceeds the 120 volt limit because it's 240 volts.

 

[wwhitney]

The MWBC exceeds the 120 volt limit because it's 240 volts.

Yes, if you consider it as an MWBC. But 210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." So I choose to consider it as two circuits. Then each circuit is 120V, and the two circuits comply with Column 4.

Cheers, Wayne

 

[infinity]

Yes, if you consider it as an MWBC. But 210.4(A) says "A multiwire circuit shall be permitted to be considered as multiple circuits." So I choose to consider it as two circuits. Then each circuit is 120V, and the two circuits comply with Column 4.

Cheers, Wayne

Among other things, if it's two circuits then it cannot be used to supply a detached structure.

 

[wwhitney]

Among other things, if it's two circuits then it cannot be used to supply a detached structure.

Choosing the one circuit / two circuit interpretation differently for different code sections is standard. That's what allows an MWBC to serve a detached structure (one circuit) while still complying with rules about different loads being on different circuits (two circuits). See my first post in this thread.

Cheers, Wayne

 

[Srv52761]

Could you run a GFCI protected 120V feeder at the shallow depth, and then have a panel at the garage to give the required circuits?

Jon

Code says circuit.
Feeder has a lot of different rules. Feeder needs to have its own ground system at a detached building.

 

[Dennis Alwon]

What Wayne is saying is to have 2 circuits in a 14/4 or 12/4 cable on the same phase. One cable but still 2 circuits... I think this needs some attention. NC amended it to include 240v circuits even up to 50 amps... I would have left it at 20 amps....

 

[wwhitney]

What Wayne is saying is to have 2 circuits in a 14/4 or 12/4 cable on the same phase.

To be clear, that wasn't a proposed solution. That was a thought experiment, and specifically involved having the two ungrounded conductors on opposite legs or phases. Then if that complies with Column 4, why shouldn't an MWBC? In terms of appropriate safety level given the hazard level, not in terms of code wording.

Cheers, Wayne

 

[infinity]

300.5(A) is very clear, it applies to the voltage carried within the cable. If it's 240 volts then column 4 doesn't apply. A 4 wire cable with both circuits on the same phase would apply but in this scenario it (2 circuits) couldn't be run to the detached structure.

300.5 Underground Installations.
(A) Minimum Cover Requirements. Direct-buried cable, conduit, or other raceways shall be installed to meet the minimum cover requirements of Table 300.5.

 

[wwhitney]

That sentence doesn't say anything about the voltage rating of the direct buried cable, or of the totality of the circuits it carries. The heading of Column 4 is "Residential Branch Circuits Rated 120 Volts or Less with GFCI Protection and Maximum Overcurrent Protection of 20 Amperes". So if your cable is only carrying 120V branch circuits, it doesn't matter what the voltage rating is between the circuits.

Cheers, Wayne

 

[infinity]

We can agree to disagree via the Article 100 definition of voltage. The voltage between two of the conductors within the cable is 240 volts. What you're saying is that a 2-wire, 240 volt circuit is not permitted to follow Column 4 but a 3-wire,120/240 volt MWBC is. That makes no sense to me.

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

 

[wwhitney]

We can agree to disagree via the Article 100 definition of voltage

I think the question for applying the definition of "Voltage (of a circuit)" in this context is, for an MWBC L1-L2-N supplying Load1 L1-N and Load2 L2-N, must you consider the circuit L1 - Load1 - Load2 -L2, which is 240V and would exclude the use of Column 4? Or is it sufficient to consider only the circuits L1-Load1-N and L2-Load2-N? I read 210.4(A) as permitting the latter.

Now, if we had a definition for Voltage (of a cable or conduit) that referred to maximum voltage between conductors in the cable or conduit, and if the Column 4 heading used language referencing that definition, then I would 100% agree with you. If that is the code writers intention, then some better word smithing is required.

Cheers, Wayne

 

[LarryFine]

In my opinion, this says it very clearly:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.

 

[wwhitney]

The question is the scoping of "circuit concerned." 210.4(A) says we may consider the MWBC as two circuits. Which means we are only concerned with 120V circuits. Just like in my 12/4 example, two 120V circuits. Both cases comply with Column 4 as written.

Cheers, Wayne

 

[LarryFine]

The question is the scoping of "circuit concerned." 210.4(A) says we may consider the MWBC as two circuits. Which means we are only concerned with 120V circuits. Just like in my 12/4 example, two 120V circuits. Both cases comply with Column 4 as written.

Which brings us back to only one circuit vs a feeder.