Trying to understand multiwire branch circuits

[ctaylo360860]

I am running home-runs for a commercial building, and I am thinking it will be easier to run multiwire branch circuits.
I plan on running 3/4 fmc from panel to12”x12” x6” j box in the ceiling. Each Fmc will contain 3 multi wire branch circuits. 6 circuits total.
I plan on using 2 pole 20 amp circuit breakers for each multi wire branch circuit. Each mwbc will contain two ungrounded conductors and one grounded giving me a total of 10 #12 conductors in each 3/4” fmc raceway.
They should have a 21a correction factor after adjustment factors.

As I understand it, as long as each each mwbc is protected by a 20 amp circuit breaker, and fed from A and B phase, the current on the neutral Should not exceed 20 amps.
Example
Circuit 5 has a load of 10 amps and circuit 7 has a load of 8 amps. The neutral conductor will have an ampacity of 2 amps.


Am I understanding this correctly, or no?
Do the loads have to be balanced?

Picture is exaample of how the circuits will be fed from circuit breaker, what the circuit is providing power to, and each raceway for mwbc’s

Again I appreciate all the help in advance!

 

[tthh]

It is just generally a bad idea to use multi-wire branch circuits nowadays since you will, in most cases, need to use a GFCI, AFCI or GFCI/AFCI combo breaker. These breakers are expensive, harder to source, or don't exist at all, depending on the panel and especially if you need a tandem size breaker.

 

[don_resqcapt19]

First question. You said a commercial system so is the building voltage supply single phase 120/240, or three phase 208Y/120?

 

[ctaylo360860]

First question. You said a commercial system so is the building voltage supply single phase 120/240, or three phase 208Y/120?

Single phase 120/240

 

[don_resqcapt19]

It is just generally a bad idea to use multi-wire branch circuits nowadays since you will, in most cases, need to use a GFCI, AFCI or GFCI/AFCI combo breaker. These breakers are expensive, harder to source, or don't exist at all, depending on the panel and especially if you need a tandem size breaker.

He specified commercial so there are no AFCI requirements, and two pole GFCIs are common if the circuits require GFCI protection.
Multiwire circuits save both material and energy, however because of the common disconnect rule, the project specifications will often prohibit them.

 

[ctaylo360860]

It is just generally a bad idea to use multi-wire branch circuits nowadays since you will, in most cases, need to use a GFCI, AFCI or GFCI/AFCI combo breaker. These breakers are expensive, harder to source, or don't exist at all, depending on the panel and especially if you need a tandem size breaker.

The only circuit I should need to protect will be mw and a coffee maker within 6 ft of s sink. I should be able to use a 2 pole 20 amp gfci circuit breaker to protect them. Building is a scale house/office for grain company. So shouldn’t need any arc fault protection.

 

[don_resqcapt19]

Single phase 120/240

You only have six current carrying conductors per raceway so the ampacity adjustment factor is 80%. No problems

 

[ctaylo360860]

He specified commercial so there are no AFCI requirements, and two pole GFCIs are common if the circuits require GFCI protection.
Multiwire circuits save both material and energy, however because of the common disconnect rule, the project specifications will often prohibit them.

No plans and no building code,

He specified commercial so there are no AFCI requirements, and two pole GFCIs are common if the circuits require GFCI protection.
Multiwire circuits save both material and energy, however because of the common disconnect rule, the project specifications will often prohibit them.

is the common disconnect rule dictated by Nec? I am in the middle of no where. There are no building codes or amendments I’m aware of. No prints or design either.

That was the one disadvantage I saw. Example is if I had to work on a the lighting circuit I’d have to shut of one 2 pole breaker shutting off all the the lights inside of the building…

 

[ctaylo360860]

You only have six current carrying conductors per raceway so the ampacity adjustment factor is 80%. No problems

So my neutrals would not be considered current carrying conductor even if the load isn’t balanced per se like in my example?

 

[roger]

So my neutrals would not be considered current carrying conductor even if the load isn’t balanced per se like in my example?

Correct

 

[infinity]

So my neutrals would not be considered current carrying conductor even if the load isn’t balanced per se like in my example?

This may help:

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current
carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the
ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between
the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100
definition) then the neutral would count as a CCC.
1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two
ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does
not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting,
adjustable-speed drive systems, and similar equipment may be nonlinear loads.

 

[augie47]

Just a reminder... NEC 200.4 requires you identify or group your neutrals when there are multiple neutrals in one conduit.

(B) Multiple Circuits. Where more than one neutral conduc or associated with different circuits is in an enclosure, grounded circuit conductors of each circuit shall be identified or grouped to correspond with the ungrounded circuit conductor(s) by wire markers, cable ties, or similar means in at least one location within the enclosure.

 

[electrofelon]

No plans and no building code,

is the common disconnect rule dictated by Nec? I am in the middle of no where. There are no building codes or amendments I’m aware of. No prints or design either.

That was the one disadvantage I saw. Example is if I had to work on a the lighting circuit I’d have to shut of one 2 pole breaker shutting off all the the lights inside of the building…

I hate the common disconnect requirements for MWBC's, as it is a dangerous rule. I usually use single pole breakers with handle ties, but I just use the snap on cover and skip the rod so I can remove easily for repairs and troubleshooting. Honestly if it's a job that isn't getting inspected, I don't even do that.

 

[ptonsparky]

Biggest issue is the common trip with 2 pole breakers. Your scale house manager will not like two circuits going down at a time.

 

[jim dungar]

Biggest issue is the common trip with 2 pole breakers. Your scale house manager will not like two circuits going down at a time.

If these are being switched often, why not just run a second neutral.

About the only advantage for a MWBC is to save money for the installer. As a user and troubleshooter I dislike them intensely, as they probably cost me more than was initially saved.

 

[charlie b]

As I understand it, as long as each each mwbc is protected by a 20 amp circuit breaker, and fed from A and B phase, the current on the neutral Should not exceed 20 amps.

Absolutely true. The highest the neutral current could ever get would happen if phase A (or B) had no load and the other had a 20 amp load. That would give the neutral a load of 20 amps.

 

[charlie b]

Keep in mind that 3 MWBCs comprised of (A, B, N) (A, B, N) and (A, B, N) will give you 6 current-carrying conductors. But if you use (A,N) (A,N) (A,N) (B,N) (B,N) and (B,N), you would have 12 CCCs. That will alter your ampacity values, possibly requiring larger conductors.

 

[LarryFine]

How far are you running these flexes? What will be the wiring method after they split?

 

[James L]

Each Fmc will contain 3 multi wire branch circuits. 6 circuits total.

This may only help a tiny bit, but if you have 3 multiwire branch circuits, you don't have 6 circuits. You have 3 circuits.

It's not "multi-circuit wire" it's "multi-wire circuit"

2 hots + the neutral is one circuit because the balanced portion is hot-to-hot and the unbalanced portion is hot-to-neutral

Just a technical tidbit, though it was well understood what you meant.

 

[roger]

This may only help a tiny bit, but if you have 3 multiwire branch circuits, you don't have 6 circuits. You have 3 circuits.

Exactly.