Why are Double Tap Neutrals Not Allowed in a Main Load Panel?

MrM1MrM1 Posts: 277Registered Users ✭✭✭
In a main load center,  why is it unacceptable to double tap the neutral if the bus bar is otherwise full.  I know it has been spelled out in the NEC since 2002 as unacceptable.  But it is also not required to go back and change if it was done in earlier homes if the lugs of the bus bar are designed for 2 wires.

I did some digging and the only reason I can find for it not being allowed  (from publications by Schneider etc) is that (1) in the case of diagnostic when a circuit needs to be isolated, in the loosening of the two different circuit's neutrals under the same lug, if the other circuit (not the one being isolated) is under load could present a safety hazard, and may establish an overvoltage condition.   And (2) if the double tapped neutrals came loose it could cause over heating and/or arching.  But again if the lug is made for 2 conductors .... ??

But who would want to work on any circuit underload in a main load panel while it is energized. Or even work in an energized main load panel for that matter - except maybe to tighten the main breaker lugs.  I know I will not touch a neutral bus bar unless the main breaker for that panel is off.  Why not just turn off the main breaker, disconnect the neutral needed to diagnose and reconnect the other neutral before turning the main back on? Seems this whole ruling in the NEC was written to protect workers who should have been taught differently. Wouldn't  it make more sense to teach a different technique than to make a rule for every possibility.

What am I not seeing?  What other dangers could there be if the lug of the bus bar is designed for 2 conductors?  Not that I'm going to do it,  I am simply trying to understand.  I am having to redo a sub panel at my house that has a bonded ground / neutral bus bar (I am running a new ground from the main panel)  and also found some neutral double taps.
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Comments

  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    The rational behind rules is to protect both personal and property from potentially hazardous circumstances, in the case of electricans it is common to work in an energized distribution panel, this is where the isolation of a single conductor in a double tap presents potential issues. Secondly a terminal designed for a single conductor may not provide adequate connection to two conductors due to displacement, potentially generating heat, which could result in fire. Electrical code rules are constantly updated to address misinterpretation of existing rules or to define changes in practice, the rules and changes are primarily intended for new installations, however when retrofitting an existing installation, such as a panel upgrade the new rules will apply.  
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  • MrM1MrM1 Posts: 277Registered Users ✭✭✭
    Thx
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
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