Neutral/ground separation in subpanels

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  #1  
Old 08-27-13, 09:08 AM
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Neutral/ground separation in subpanels

Separated from I don't quite "get" my inspectors junction box suggestion

Originally Posted by Zorfdt
Subpanels require the ground and neutral busses/wires to be separated...
...the important piece is that the neutrals from these circuits need to land on the neutral bar and NOT the ground bar, since now electrically, they are different.
Can you elaborate on this? I understand that it needs to be done, I'd like to understand why it's important.
 

Last edited by Nashkat1; 08-27-13 at 09:51 AM.
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Old 08-27-13, 10:17 AM
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It's an NEC requirement. Some pictures to help explain it.

As to why, I'll take a stab at it.

Your main panel is the only place that the ground and neutral are connected. Everywhere else they need to be kept separate. Here's an example of a potential safety issue if that's not the case.

Picture an 'old' subpanel run with 3 wires (hot, hot, neutral/ground). Everything works just fine because neutral and ground are at the same potential, so as long as your appliances are connected as expected, all is good.

Your toaster oven is on and you're putting dinner into your electric oven, holding the metal door handle. Over the past few months/years, the neutral connection on the 3-wire feed has been corroding a bit, the connection has been loosening, and now the higher current draw causes that neutral to loosen and lose connection.

Now the 30A of current from the oven and 10A from the toaster no longer have a path via the neutral wire... but you seem to be a decent conductor, so the electricity follows the path through the ground wire (which is connected securely to the neutral), through the oven, through you, to ground.

Granted, it's an unlikely chain of events, but I'm sure it's happened as have similar scenarios. As people learned the increased safety factor, code has moved from 3-wire subpanels to 4-wire subpanels in all residential situations.
 
  #3  
Old 08-27-13, 10:58 AM
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Your electrical service - every electrical service - has, or should have, a Grounding Electrode Conductor established at the point where it enters the building - the place where the first overcurrent protection device for the service is located.

If your service starts with a meter base which is connected with unfused conductors to a main distribution which has a main breaker or fuse that interrupts all of the power coming in, then the GEC is established in that panel.

The purpose of the GEC is to protect the service, its components inside the structure, the structure itself and its occupants from "high-voltage transients." A surge created by a lightening strike is one example.

The GEC is created by bonding the incoming grounded conductor - the utility neutral - to one or more low-impedance paths to earth ground. The cold water inlet is the oldest choice and often the preferable first choice. Others include the gas supply pipe and one or more driven ground rods.

In addition, at this point, all of the branch circuit neutral and grounding conductors - the neutral wire that is part of each 120V circuit and the ground wire, or Equipment Grounding Conductor, that is run with every circuit - are also bonded to the GEC, along with the enclosure that the panel is in. The result is that you the main drain for the service (the utility neutral), the emergency overflow drain (the water pipe, gas pipe and any ground rods), the internal drains (the branch circuit neutrals) and the internal emergency drains (the grounding conductors), plus the container (the panel enclosure) all bonded together and diverting any excess, loose voltage out of the structure and into the ground.

From that point on, in all the wiring done inside the structure, all of the grounding conductors are bonded together, regardless of what circuit they were pulled with. They're not part of any circuit; only the current-carrying conductors are. In addition, every metal electrical enclosure - receptacle outlet, switch outlet, lighting outlet, junction box, or subpanel or disconnect enclosure - is bonded to the grounding conductors. They're all tied into the emergency drain. All of the neutrals are kept separate from that. They're only connected to devices that need a neutral, on the circuit that they're part of. They're never connected to any metal enclosure. That keeps the regular drain - the neutrals - separate from the emergency drain.

There are two reasons for doing this. One is that a high-voltage transient that originates outside the structure will only have one low-impedance path to earth, the GEC, which is outside. There isn't a better path that includes any of the internal wiring. The other is that a high-voltage transient that originates inside - a short in an appliance or in the wiring - will also only have only one low-impedance path to earth, through the EGC wiring. If that wiring is interconnected everywhere with itself, and never connected to any neutral, then that transient or surge will be carried outside and into the earth without having a way to get to, and damage, any other connected load, or start a fire.

Zorfdt types faster than I do. I like his life-safety take on it.
 
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