When do you upsize shared neutrals?

When do you upsize shared neutrals?

  • All MWB circuits (receptacle & lighting)

    Votes: 0 0.0%

  • Ballasted lighting circuits only (for harmonics)

    Votes: 1 7.7%

  • Depends on actual loads

    Votes: 12 92.3%
  • Total voters
    13
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donw

Senior Member
Location
Arizona
Do you specify that shared neutrals of multi-wire branch circuits are to be upsized...
1. All shared neutrals (lighting and receptacle)
2. Ballasted lighting only (due to harmonics)
3. Based on the actual loads
 
Ive seen cubicles with two 2 wire circuits #12 thhn cu and their respective neutral at #10 thhn cu prewired from the power pole. Usually 4 circuits with 2 shared neutrals. This is for a office cubicle load: computers, task light, electric calculator... the ever problamatic space heater.
 
The 08 code book has a fine print note on shared neutrals. No specific rules (thus an FPN) but it does want installers to be aware that MWBC's with non-linear loads may require beefing the neutral wire up because of increased neutral current from third order harmonics. Not a problem in most resi wiring.
 
That FPN has been in the code for many code cycles. The only time it could ever be a problem in a dwelling unit is if the supply was two phases and the neutral conductor from a 3 phase 208Y/120 system. The harmonics do not add on single phase systems.
 
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