Neutral Current - Danger (8-1-99)
Dangerous Neutral Current
I ran into an interesting situation today. One of our contract electricians was removing an old panel to replace it with a new one. He killed the power to the panel and started removing wires. His experience told him to place an amprobe around the neutral wire to see if it was loaded. This saved him from possible electrocution.
The nuetral of this 480v/ 277v panel was indeed drawing current (7 amps). Of course if he had lifted
the wire expecting it to be dead it would have had a potential of 277v and if he got between it and
ground it would have shocked him. Someone had previously installed a circuit from another panel and
picked up a neutral from the panel our contractor was removing. I'm sure both panel neutrals occupied
the same j-box. Is there a Code rule to prevent this?
If not, I may make a formal request for a change in the next code. I think that where neutrals from
different panels are located in the same j-box or control panel there should be panel numbers on the
wire to prevent this dangerous condition. I know the rule for differentiation of neutrals of different
voltages but I've never seen one for this situation. Both neutrals are from the same transformer but
different panels. Just thought you might find this interesting and wanted your input.
Comment No. 1
The story about the neutral from another panel is not as rare as you would think. I have been an electrician in Arizona for 26 years, a lot of that time I have worked in the service departments of some of the top contractors in the state. I have encountered this same situation over and over again. The miss placed neutral poses several hazards
- The danger of electrocution as mentioned in the previous letter.
- There is also the issue of unbalanced loads inducing current in the conduit system. I have seen many times where when taking a system apart arcs have occurred from one piece of conduit to the next, if the induced current is high enough it can cause electrocution, or at least a nasty shock.
- On one occasion I investigated a fire in a storage room and found that it started in a conduit that ran from a switch on the wall to a 4' fluorescent fixture on the ceiling. The person who had installed the light had picked up a feed from an outlet below the switch, then ran the switchleg up the wall by itself to the fixture, then had ran the neutral to a fixture in the next room that was served by another panel. The switchleg had gotten so hot that it melted its insulation back and shorted to, then through the conduit causing the fire.
I'm not sure that we can totally remove stupidity by creating new rules, but we do need to try to educate electricians as to the dangers of these types of practice.Mike White, Mwhite2690@aol.com
Comment No. 2
Mike, while changing out an old service this past week we found almost
the same conditions as related in this story.
Jim Brewer, firstname.lastname@example.org
Comment No. 3
I believe that the code does address this issue. I'm not in front of a book right now, but will be sure to look when I am. What I recall is that all neutral wires must terminate in the same panel that the hot wires terminate in. This sounds like an illegal installation to me.
Comment No. 4
Guys! The NEC already prohibits that practice. Section 300-3 (b) states that ' "All conductors of the
same circuit, the grounded conductor and all equipment grounding conductors shall be contained within
the same raceway, cable tray, trench, cable, or cord." That means picking up a neutral from another
panel is already illegal. Anyone doing that is guilty of: (1) ignorance of the code (2) poorly trained
and supervised (3) probably adding on to a poorly arranged and marked system that even has the conduit
from the different panels connected together by someone guilty of both (1 & 2).
SO if the practice that leads to the hazard is a code violation and evidence of poor work practices already; more code will note prevent the error. Careful training, discussions like this one, and careful checking before disassembling of neutral connections CAN help.
By the way: the result question centered on a case where the electrician checked the neutral current before removing the feeder neutral in the panel. That is not a foolproof method (or even fool-resistant). The miss-wired load could easily be a water cooler, portable heater, refrigerator, or other intermittent load. Heck neutral current, find none, remove the connection, two seconds later the load is energized and you have a live wire hanging in the bundle that is hanging out around your face. And who of us is normally afraid of a neutral wire, in a disconnected panel, hanging out completely disconnected from the buss? So: WATCH OUT!
Lynn Adams, email@example.com
Mike Holtís Comment
The 1999 NEC rules for this installation are:
Section 200-6(d) Grounded Conductors of Different Systems. Where conductors of different systems are installed in the same raceway, cable, box, auxiliary gutter, or other type of enclosure, one system grounded conductor, if required, shall have an outer covering conforming to Section 200-6(a) or 200-6(b). Each other system grounded conductor shall have an outer covering of white with a readily distinguishable different colored stripe (not green) running along the insulation, or other and different means of identification as allowed by Section 200-6(a) or (b) that will distinguish each system grounded conductor.
Section 210-4 General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.
- Identification of Ungrounded Conductors. Where more than one nominal voltage system exists in
a building, each ungrounded conductor of a multiwire branch circuit, where accessible, shall be
identified by phase and system. This means of identification shall be permitted to be by separate
color coding, marking tape, tagging, or other approved means and shall be permanently posted at
each branch-circuit panelboard.
There is no rule that prohibits the mixing neutrals (grounded conductors) from different systems in the same enclosure, which permitted this problem. I also agree that no rule will prevent poorly trained persons from screwing up.
This Newsletter, courtesy of Mike Holt, 1-888 NEC CODE