Multiwire Branch Circuit and wire sizing

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kbruin

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Denver, CO, USA
I am designing a multi circuit light panel and circuits for a room and would like to check I am interpreting NEC appropriately for the application.

Scenario:
-9x 115v circuits (single pole 20A breakers)
-Each circuit feeds a single duplex receptacle
-Receptacles are fed from a UL 508A industrial control panel
-Panel feed is 120v/208v, 3 phase, 4 wire wye
-A single 3/4" liquid tight non metallic conduit runs from the panel and feeds the 9 receptacles
-Use #12 THHN wire for all conductors (hot, neutral, ground)
-3 sets of 4 wires (phase 1, phase 2, phase 3, neutral) will be used
-Each receptacle will have 2x 660w LED fixture plugged in.

NEC Considerations:
-Per 310.15(B)(5)(a) I am not counting the neutrals in each set of 4 wires giving 9 current carrying conductors
-Per 310.15(B)(3)(a) I am derating the #12 THHN by 70% and from table 310.15(B)(16) the 30 Amp rating becomes 21 Amps (protected by 20 Amp single pole breakers)
-Per Annex table C.5 I am allowed 16 conductors in 3/4" in liquidtight flexible nonmetallic conduit (there are 12 in this scenario)

Questions:
A) Am I applying the neutral exception appropriately?
B) For these multiwire circuits do I need to use either a 3 (or even 4) pole breaker or single poles with handle ties?
C) As the lights are unpluggable, would we run into an adverse or unsafe situation where, assuming the circuits are energized, the first set of 2 lights are plugged in and turned on while the other 2 sets of 2 lights are unplugged (due to the shared neutral)?

Alternative:
I can avoid having to do any 'funny business' if I were to run 18 wires (hot and neutral for each circuit), rate as if they are all current carrying (derate 50%), use #10 wire, and 1-1/4" conduit. As you can see the alternative is going to be significantly more expensive. Any and all thoughts on how to tackle the design requirements here would be appreciated. Let me know if more information regarding the scenario is needed to give an exact answer.
 
My first thought would be that with your feeding LED fixtures there will be harmonics involved and your neutrals will count as current-carrying.
 
Running mwbc's will require a three-pole breaker, or handle ties on each set of three single pole breakers.

Unplugging various light fixtures presents no problem to the circuits. The only concern would be plugging in more than permitted by Code.
 
Yah augie47, I'm asking the light manufacturer for specs on the mean well LED driver. The type of load is certainly my one of my principal concerns for counting the neutral.

Thanks Smart $, I figured as much with the handle ties. As for the unplugging, just to be clear:

Only one set of 2 lights plugged in:
Phase A - 1320
Phase B - 0
Phase C - 0

Only two sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 0

All sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 1320

I felt like in the last scenario all the loads are the same (balanced?), but in scenario 2, without anything on C phase, would there be too much current on the neutral?
 
kbruin said:
Yah augie47, I'm asking the light manufacturer for specs on the mean well LED driver. The type of load is certainly my one of my principal concerns for counting the neutral.

Thanks Smart $, I figured as much with the handle ties. As for the unplugging, just to be clear:

Only one set of 2 lights plugged in:
Phase A - 1320
Phase B - 0
Phase C - 0

Only two sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 0

All sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 1320

I felt like in the last scenario all the loads are the same (balanced?), but in scenario 2, without anything on C phase, would there be too much current on the neutral?
Click to expand...

If the loads are linear the set in bold will have approximately the same amount of current on the neutral as the phase conductors.
 
kbruin said:
Yah augie47, I'm asking the light manufacturer for specs on the mean well LED driver. The type of load is certainly my one of my principal concerns for counting the neutral.

Thanks Smart $, I figured as much with the handle ties. As for the unplugging, just to be clear:

Only one set of 2 lights plugged in:
Phase A - 1320
Phase B - 0
Phase C - 0

Only two sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 0

All sets of 2 lights plugged in:
Phase A - 1320
Phase B - 1320
Phase C - 1320

I felt like in the last scenario all the loads are the same (balanced?), but in scenario 2, without anything on C phase, would there be too much current on the neutral?
Click to expand...

If the loads turn out to be substantially linear, the neutral current for three situations will be:

one set
1320

two sets
1320 (but at a different phase angle)

three sets
0
 
Thanks Rob, that was what I suspected. In that bolded situation where you have 1320w on A, B, and N, don't you still only have 3 current carrying conductors (presumably current on phase C is zero)? No matter which of the 3 scenarios I gave, where 1, 2, or 3 sets of lights are plugged in, I feel like A) you never have more than 3 current carrying conductors and B) the current on the neutral is either equal to or less the phase conductor.

Either way, let's assume for a moment that due to either the harmonic issues of an LED or because I need to account for the neutrals in my 310.15(B) calc and thus derate the wire by 50% as I will have 12 wires in each conduit. As I mentioned in the original post, if I change my wire size to #10 to account for the 50% derating:
-Is this an acceptable 4 wire multiwire branch circuit configuration? Or do I need to provide separate neutrals?
-Any reason I'd have to size the neutral larger than the phase conductors in the 4 wire multiwire branch circuit configuration?

Appreciate the thoughts everyone. If I do have to go with larger wire size, larger conduit, and possibly more neutrals, I'll have to consider if going to 3 conduit run (and no derating) vs 1 will be cheaper for the install.
 
kbruin said:
... original post, if I change my wire size to #10 to account for the 50% derating:
-Is this an acceptable 4 wire multiwire branch circuit configuration? Or do I need to provide separate neutrals?
-Any reason I'd have to size the neutral larger than the phase conductors in the 4 wire multiwire branch circuit configuration?

Appreciate the thoughts everyone. If I do have to go with larger wire size, larger conduit, and possibly more neutrals, I'll have to consider if going to 3 conduit run (and no derating) vs 1 will be cheaper for the install.
Click to expand...

If you go to individual neutrals you will add three (not just two) to the current carrying conductor count for derating purposes. (Assuming linear loads.)

Theoretically (and it would not ever go this far in practical applications) if the load current were 100% third harmonic the neutral current would be exactly three times the ungrounded line (phase conductor) current. For lower levels of triplen (factor of 3) harmonic current you work your way down from there all the way to zero.
This increase in the neutral current above the phase current has actually been found to cause overheating in single same size neutrals in the built in 3 phase MWBC wiring of office partitions when the load was mainly older (low distortion power factor) computers.
Not quite the same anymore, but modular office makers have also added additional neutrals.
 
kbruin said:
I am designing a multi circuit light panel and circuits for a room and would like to check I am interpreting NEC appropriately for the application.

Scenario:
-9x 115v circuits (single pole 20A breakers)
-Each circuit feeds a single duplex receptacle
-Receptacles are fed from a UL 508A industrial control panel
-Panel feed is 120v/208v, 3 phase, 4 wire wye
-A single 3/4" liquid tight non metallic conduit runs from the panel and feeds the 9 receptacles
-Use #12 THHN wire for all conductors (hot, neutral, ground)
-3 sets of 4 wires (phase 1, phase 2, phase 3, neutral) will be used
-Each receptacle will have 2x 660w LED fixture plugged in.

NEC Considerations:
-Per 310.15(B)(5)(a) I am not counting the neutrals in each set of 4 wires giving 9 current carrying conductors
-Per 310.15(B)(3)(a) I am derating the #12 THHN by 70% and from table 310.15(B)(16) the 30 Amp rating becomes 21 Amps (protected by 20 Amp single pole breakers)
-Per Annex table C.5 I am allowed 16 conductors in 3/4" in liquidtight flexible nonmetallic conduit (there are 12 in this scenario)

Questions:
A) Am I applying the neutral exception appropriately?
B) For these multiwire circuits do I need to use either a 3 (or even 4) pole breaker or single poles with handle ties?
C) As the lights are unpluggable, would we run into an adverse or unsafe situation where, assuming the circuits are energized, the first set of 2 lights are plugged in and turned on while the other 2 sets of 2 lights are unplugged (due to the shared neutral)?

Alternative:
I can avoid having to do any 'funny business' if I were to run 18 wires (hot and neutral for each circuit), rate as if they are all current carrying (derate 50%), use #10 wire, and 1-1/4" conduit. As you can see the alternative is going to be significantly more expensive. Any and all thoughts on how to tackle the design requirements here would be appreciated. Let me know if more information regarding the scenario is needed to give an exact answer.
Click to expand...

Mr. KBruin,

Your thread is very well worded and carefully prepared. My kudos to you on this since so many are very incomplete. I do suggest you hand sketch your design with pencil and use cellphone to picture and share. Too many words which bring ambiguity.
Please wade through my detailed comments to the end. We seem to have a difference re the deratings which would alter the wire and conduit size. Kindly let me know if you agree or not and post next iteration to your design.

A) Am I applying the neutral exception appropriately?

This translates to if the load is linear or non-linear. I will opt to say it is the latter since LED drivers are thus. So you have 3 times 4 CCCs (=12 CCCs) and derating is required accordingly.


B) For these multiwire circuits do I need to use either a 3 (or even 4) pole breaker or single poles with handle ties?

You will use "common trip" 3-pole breakers.

C) As the lights are unpluggable, would we run into an adverse or unsafe situation where, assuming the circuits are energized, the first set of 2 lights are plugged in and turned on while the other 2 sets of 2 lights are unplugged (due to the shared neutral)?

See answer B. No issue since "common trip" being used.

And........
You will also run a single #12 ground (green) wire daisy chained to each j-box or use EMT as ground conductor.
All of the neutrals at each j-box will be pigtailed per code.
You will need grouping of each "boat" (4 wire A,B,C,N set) identified with a wire tie and a label on the panel.
You will save on cost and be fully compliant with code using MWBC but it must be designed correctly.

Further direct notes and comments on your posting.......my inputs are in boldface

Scenario:
-9x 115v circuits (single pole 20A breakers) ........3 3-pole common trip breakers (although see below re the 20A)
-Each circuit feeds a single duplex receptacle........No....I think you mean each boat feeds 3 receptacles
-Receptacles are fed from a UL 508A industrial control panel......ok since one single panel per code is being used
-Panel feed is 120v/208v, 3 phase, 4 wire wye.......ok, also known as a "boat" (I do not know origin of the name)
-A single 3/4" liquid tight non metallic conduit runs from the panel and feeds the 9 receptacles.......ok but not if derating below has to change
-Use #12 THHN wire for all conductors (hot, neutral, ground).......You need to see below where we differ on derating!!!
-3 sets of 4 wires (phase 1, phase 2, phase 3, neutral) will be used......ok (aka 3 boats)
-Each receptacle will have 2x 660w LED fixture plugged in......ok but this is a non-linear load LED driver so neutral is a CCC

NEC Considerations:
-Per 310.15(B)(5)(a) I am not counting the neutrals in each set of 4 wires giving 9 current carrying conductors.....Not ok, it is 12 CCCs see above
-Per 310.15(B)(3)(a) I am derating the #12 THHN by 70% and from table 310.15(B)(16) the 30 Amp rating becomes 21 Amps (protected by 20 Amp single pole breakers).......no, IMHO, it should be 50% not 70% so the 30A becomes 15A so you may have to go to rework the MWBC using #10 AWG
-Per Annex table C.5 I am allowed 16 conductors in 3/4" in liquidtight flexible nonmetallic conduit (there are 12 in this scenario).....need to revisit this based on previous info
 
I would not lump all LED drivers into the category of non-linear loads.
The first drivers were just resistors in conjunction with an AC to DC converter that could be as simple as a bridge rectifier and maybe a capacitor.
The resistors made for a linear load once you got past the threshold voltage, which leads to an overall non-linearity, but not 100% harmonics.
Later drivers used instead current limiting circuits, which still left room for non-linearity at the input.

But there are also LED drivers which are deliberately constructed to minimize the distortion power factor and could well be treated as not "substantially non-linear", the hard to quantify threshold set by the NEC.

Try to find the power factor information for the actual LED drivers (or completed LED luminaires) to be used.
 
GoldDigger said:
I would not lump all LED drivers into the category of non-linear loads.
The first drivers were just resistors in conjunction with an AC to DC converter that could be as simple as a bridge rectifier and maybe a capacitor.
The resistors made for a linear load once you got past the threshold voltage, which leads to an overall non-linearity, but not 100% harmonics.
Later drivers used instead current limiting circuits, which still left room for non-linearity at the input.

But there are also LED drivers which are deliberately constructed to minimize the distortion power factor and could well be treated as not "substantially non-linear", the hard to quantify threshold set by the NEC.

Try to find the power factor information for the actual LED drivers (or completed LED luminaires) to be used.
Click to expand...

Thank you for clarifying this.

Cutsheets for devices are fairly silent on specifying if they present a linear or non-linear load. How does one decipher the info???
 
dionysius said:
C) As the lights are unpluggable, would we run into an adverse or unsafe situation where, assuming the circuits are energized, the first set of 2 lights are plugged in and turned on while the other 2 sets of 2 lights are unplugged (due to the shared neutral)?

See answer B. No issue since "common trip" being used.
Click to expand...

Can you explain why you feel using a common trip breaker has any impact at all on the question asked? I don't see it.

Are you aware common trip breakers are NOT required they are permitted. Approved handle ties on single pole breakers are an option as well.
 
iwire said:
Can you explain why you feel using a common trip breaker has any impact at all on the question asked? I don't see it.

Are you aware common trip breakers are NOT required they are permitted. Approved handle ties on single pole breakers are an option as well.
Click to expand...

I mis-read his question. The situation he described with A plugged in but B and C unplugged will cause current return on the shared neutral equal to the current on the ungrounded phase A. This will not be a problem since AWG of N = AWG of A. This I deduce from from the phasor diagram if I read it correctly.
 
dionysius said:
Mr. KBruin,

Your thread is very well worded and carefully prepared. My kudos to you on this since so many are very incomplete. I do suggest you hand sketch your design with pencil and use cellphone to picture and share. Too many words which bring ambiguity.
Please wade through my detailed comments to the end. We seem to have a difference re the deratings which would alter the wire and conduit size. Kindly let me know if you agree or not and post next iteration to your design.

A) Am I applying the neutral exception appropriately?

This translates to if the load is linear or non-linear. I will opt to say it is the latter since LED drivers are thus. So you have 3 times 4 CCCs (=12 CCCs) and derating is required accordingly.


B) For these multiwire circuits do I need to use either a 3 (or even 4) pole breaker or single poles with handle ties?

You will use "common trip" 3-pole breakers.

C) As the lights are unpluggable, would we run into an adverse or unsafe situation where, assuming the circuits are energized, the first set of 2 lights are plugged in and turned on while the other 2 sets of 2 lights are unplugged (due to the shared neutral)?

See answer B. No issue since "common trip" being used.

And........
You will also run a single #12 ground (green) wire daisy chained to each j-box or use EMT as ground conductor.
All of the neutrals at each j-box will be pigtailed per code.
You will need grouping of each "boat" (4 wire A,B,C,N set) identified with a wire tie and a label on the panel.
You will save on cost and be fully compliant with code using MWBC but it must be designed correctly.

Further direct notes and comments on your posting.......my inputs are in boldface

Scenario:
-9x 115v circuits (single pole 20A breakers) ........3 3-pole common trip breakers (although see below re the 20A)
-Each circuit feeds a single duplex receptacle........No....I think you mean each boat feeds 3 receptacles
-Receptacles are fed from a UL 508A industrial control panel......ok since one single panel per code is being used
-Panel feed is 120v/208v, 3 phase, 4 wire wye.......ok, also known as a "boat" (I do not know origin of the name)
-A single 3/4" liquid tight non metallic conduit runs from the panel and feeds the 9 receptacles.......ok but not if derating below has to change
-Use #12 THHN wire for all conductors (hot, neutral, ground).......You need to see below where we differ on derating!!!
-3 sets of 4 wires (phase 1, phase 2, phase 3, neutral) will be used......ok (aka 3 boats)
-Each receptacle will have 2x 660w LED fixture plugged in......ok but this is a non-linear load LED driver so neutral is a CCC

NEC Considerations:
-Per 310.15(B)(5)(a) I am not counting the neutrals in each set of 4 wires giving 9 current carrying conductors.....Not ok, it is 12 CCCs see above
-Per 310.15(B)(3)(a) I am derating the #12 THHN by 70% and from table 310.15(B)(16) the 30 Amp rating becomes 21 Amps (protected by 20 Amp single pole breakers).......no, IMHO, it should be 50% not 70% so the 30A becomes 15A so you may have to go to rework the MWBC using #10 AWG
-Per Annex table C.5 I am allowed 16 conductors in 3/4" in liquidtight flexible nonmetallic conduit (there are 12 in this scenario).....need to revisit this based on previous info
Click to expand...

Thanks for your very thorough response. For all interested this is the driver for the LED fixture Datasheet Mean Well HLG-320H-36A. Some of the highlights for its input if you don't want to open the link.
-Power Factor: PF>0.98/115VAC at full load
-Frequency Range: 47 ~ 63 Hz
-Total Harmonic distortion: THD<20% when output loading >= 50% at 115VAC/230VAC input and output loading >=75% at 277% input (I will be running at 115VAC)
-Inrush Current: Cold Start 70A at 230VAC

I'll get a drawing up momentarily.
 
Here is a diagram of the install. The 660w LED fixtures are basically 4'x4' each arranged in a tight 3x3 grid. They will be installed in a rack system so there are two vertical levels (this is an overhead view) with each LED fixture plugging into a duplex receptacle. As I stated in my original post, there are 9 individual circuits being fed from a central industrial control panel.

Based on the discussion so far:
-I can wire with 3x 'boats' of 4 wires at #12 AWG IF the LED load is not substantially 'non-linear'. If the LED fixtures do cause a problem, the neutral becomes a CCC and causes everything to go up in size (wire and conduit).
-It doesn't appear that I would need to run individual neutrals regardless of the type of load from the LED, I'm focused on derating exclusively (understanding I am properly installing the MWBC with common trip or handle ties and correct labeling).
-If the LEDs are actually a problem, and that forces me to a #10, I could always split this design up into 3 conduit runs instead of 1. Anyone have a sense which would be more cost effective, 3 smaller runs with 1x #12 AWG 4 wire setup in each or 1 large run with 3x #10 AWG 4 wire setups?

Overhead View.jpg
 
Just a gut reaction but I would consider the loads to be linear and not worry about it.
 
infinity said:
Just a gut reaction but I would consider the loads to be linear and not worry about it.
Click to expand...

I agree but for reference what are the specified items and the limits on these parameters that declare linearity. If an inspector was extremely difficult and disagreed the rework would be brutal and there is no court of appeals.
 
dionysius said:
I agree but for reference what are the specified items and the limits on these parameters that declare linearity. If an inspector was extremely difficult and disagreed the rework would be brutal and there is no court of appeals.
Click to expand...

Been doing this 30 plus years and it does not happen in my area
 
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