A client asked me my off the record opinion about how urgent it is to correct double lugged neutrals.
All I could say is I have confidence that there is a good reason for the code/standard change, because changes are usually reactive as opposed to proactive.
It seems like it must be a fire concern to some extent, and I plan on continuing to call it out either way… but how dangerous is that, really? What is the most likely dangerous scenario and why exactly?
Over time one or the other wire gets loose. Most of the clamping force is on one wire. The other wire gets hot. It creates scorch marks. Lights or whatever flicker.
Will it cause a fire?
I’m not promising it won’t.
Hot neutral, makes no difference (except in the MWBC)
As far as I understand, it has to do with isolating circuits. If you loosen the screw to remove a neutral of a circuit you are working on, you may also be disconnecting a neutral for a still live circuit. Here is a great write up…
Nice, good points. Thanks guys.
This new section should ensure that grounded (neutral) conductors terminate within the panelboard to an individual terminal. This has been a UL requirement (UL Std. 67 – Panelboard Standard ) for some time, and the addition to the NEC is intended to bring this information to the installers. Technically, this is covered by 110.3(B), which requires all equipment to be installed in accordance with the manufacturer’s installation instructions and markings, but nobody knew it existed. UL Std. 67 (Panelboard Standard) permits up to three 10 AWG equipment grounding conductors to terminate on a single terminal, if the terminal is marked for this purpose. Figure 408-3
The intent of this requirement is to ensure that the grounded (neutral) conductor of a multiwire branch circuit is not momentarily disconnected, which could result in the destruction of electrical equipment and fires from overvoltage.
408.21 Grounded Conductor Terminations. Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.
Thanks to Mike Holt.
“If two grounded ( neutrals ) conductors are in the same terminal, and someone removes one of the neutrals, the other neutral may unintentionally be removed as well. If this happens on a multiwire circuit, it could result in excessive line-to-neutral voltage on one of the circuits…”
Thanks to Paul Abernathy
It never has been permitted technically by UL67 and under NEC 110.3(B).
It was clarified in NEC 408.21 In 2005, this reference was moved to 408.41
Even Mike Holt was confused by this issue.
Holt’s comment:
Switchboards and Panelboards
408.21 Grounded Conductor Terminations
Intent: This new section should ensure that grounded (neutral) conductors terminate within the panelboard to an individual terminal. This has been a UL requirement (UL Std. 67 – Panelboard Standard) for some time, and the addition to the NEC is intended to bring this information to the installers. Technically, this is covered by 110.3(B), which requires all equipment to be installed in accordance with the manufacturer’s installation instructions and markings, but nobody knew it existed. UL Std. 67 (Panelboard Standard) permits up to three 10 AWG equipment grounding conductors to terminate on a single terminal, if the terminal is marked for this purpose. Figure 408-3
The intent of this requirement is to ensure that the grounded (neutral) conductor of a multiwire branch circuit is not momentarily disconnected, which could result in the destruction of electrical equipment and fires from overvoltage.
The grounded conductor is a current carrying conductor. The grounded conductor is energized but has no potential (Voltage) between it and ground. If the connection is lost, there is a potential between the grounded conductor (which is no longer grounded) and ground. Anyone coming into contact with the white wire elsewhere in the house could be electrocuted.
The other major problem is that if the two grounded conductors are in contact with each other but have a loose connection to the bus bar, 240V could be impressed on 120V loads (if they are on opposite legs). That’s a common cause of electrical fires.
My answer to the client would have been the same as yours… my answer to the thread title? There are probably millions of instances of doubled tapped neutrals in the US and all the houses aren’t burning down because of them. I had a bunch in my last house (circa 1979) and never did anything about it. The ground/neutral bar was crowded and there really wasn’t an easy option. I’d check them every few years to be sure they were tight and amazingly survived there for 16 years unscathed. As HIs we’re definitely at the front (scared) edge of things. As far as risk, what’s on a person’s dinner plate is more likely to kill them. But yes… I write them up as an HI when I see them.
IMO if the two neutrals are the same size, the same material, and properly torqued then there is little chance of a problem if left undisturbed. Having said that they still need to be in a single hole in the neutral bus.
This is my explanation I use in my reports:
Multiple neutral conductors in a single termination create a significant problem when the circuit needs to be isolated. In order to isolate the circuit, the branch breaker is turned off and the neutral is disconnected by removing it from the terminal. If the terminal is shared with another circuit, the connection on the other (still energized) circuit will be loosened as well. Loosening of the second neutral (loss of neutral) under load is a safety hazard, and may establish an overvoltage condition on lighting and appliances if the neutral is part of a 120/240 Vac multi-wire branch circuit. Also, the neutral assemblies are not evaluated with multiple neutral conductors in the same terminal.
When there is two neutral wires in one hole on the neutral bar of an electrical panel several things may happen. They relate to safety and should be corrected. One is that the wires may be from two different circuits and that the loading of these two neutral wires may fluctuate causing the wires to loosen which can cause arching which could lead to a fire hazard.
I won’t add to all the answers as I think you guys have it covered. However I will say that I once missed this deficiency on a new build and when the home was inspected at the one year mark, it was called out by another inspector. The homeowner called me for clarification. This can easily happen to anyone of us as often there are so many wires and/or breakers hiding visual access to portions of the bus bars.
You’re the authority on this one, Robert. It is my understanding that the ungrounded neutral carries the excess voltage in a circuit back to the transformer. Why then is there not a conflict at the neutral bus bar if the power being returned by the neutrals (from devices) have been fed by opposing legs of power which are 180° out of phase with each other?
You answered your own question. Current flow is also 180 degrees out of phase. This means current flow in the neutral never exceeds supply from either leg (they each share the neutral at separate times).
Not so fast. It is this 180•-out-of-phase relationship between the two power legs which give rise to the 240-volt potential. At what point do each of the two opposing sine waves become duplicative of the other? I really don’t see them following the same circuit and path back to the transformer while still out of phase with each other.
You obviously don’t have the math or logic skills to be satisfied then. In the first case you are talking about a center tapped transformer. (essentially two transformers one common secondary lead)
In the 240v case you are talking about a single secondary winding where all voltage and current share the same phase. There is no neutral or grounded conductor.
I think that you meant excess current or in a better term the unbalanced current between the two hot legs because there is no excess.
For example Leg A has 10 amps @ 120 volts and Leg B has 5 amps @ 120 volts then the neutral current is the difference between the two or 5 amps.
I am speaking of the excess current that flows through the outlet and working device, back through the neutral wire to the bus bar and then to the transformer.
David can you explain in a little more detail why you mean by excess current? Is the example below what do you perceive as excess current?
For example Leg A has 10 amps @ 120 volts and Leg B has 5 amps @ 120 volts then the neutral current is the difference between the two or 5 amps.
Is this not what happens to the current in excess of that which a (120V) tool, or a fan motor, or a heating element will use for work?
There is no excess current. If the 120 volt tool draws 5 amps then 5 amps flows in both the hot leg and the neutral. In a two wire circuit the current is equal in both conductors.
No, I am referring to only one leg of current connected to one load.
