250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).(1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the overhead service conductors, service drop, underground service conductors, or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.
This means that the grounded (neutral) from the service must be connected to ground, and that the connection can be made by bonding the neutral bus bar to the grounding electrode.
(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.
This means that the grounded (neutral) conductors should only be grounded at the main service disconnnect.
If the main service panel happens to be the same place that the grounded (neutral) conductor is bonded to the grounding electrode, then there is no problem mixing grounds and neutrals on the same bus bar (as long as there is an appropriate number of conductors terminated under each lug). If the two bus bars are not connected; as would be the case anywhere other than the main disconnect (exceptions exist), then you cannot mix them.
Notice how the grounded, and grounding bus bars are connected in the main service panel. This means that; electrically speaking, they can be considered a single bus bar. Which means that both grounded (neutral), and equipment grounding conductors can be terminated on either bus bar.
In the subpanel, the bus bars are kept separate. So grounded (neutral), and equipment grounding conductors cannot be mixed.
When should the ground and neutral wires be separated at a subpanel? It depends.
First, what’s a subpanel? In my words, a subpanel is an electrical panel wired downstream from the Service Equipment, which is more commonly known as the main panel. For a lengthy discussion on the definition of a subpanel, check out Bruce Barker’s 2009 article in the ASHI Reporter on subpanels.
Next, what’ the deal with connecting grounds and neutrals together? In my words, if grounds and neutrals are connected together at a subpanel, they won’t have separate paths back to the service equipment. This means you’ll have current on the grounding conductor, which can be bad news for anyone working on the circuit. For a detailed discussion of this, check out Charles Buell’s blog post and video on bonding grounds and neutrals together at subpanels.
Pre-2008
Up until the 2008 version of the National Electric Code, there were two ways to wire a subpanel.
The first was with a four-wire feed; two hots, a neutral, and a ground. Grounds and neutrals were isolated to provide separate paths back to the panel.
Another way to wire a subpanel was with a three-wire feed; two hots and a neutral, with grounds and neutrals connected together at the subpanel. In this case, the grounds and neutrals have to be connected together. There were several rules for this method, however. This was only allowed at detached buildings, and the detached building had to have its own grounding electrode system. Additionally, there couldn’t be any continuous metallic paths bonded to the grounding system in each building.
2008 and after
Starting with the 2008 National Electric Code, the only acceptable way to wire a subpanel is with a four-wire feed. Two hots, one ground, and one neutral wire. The grounds and neutrals must be isolated. The two illustrations below, courtesy of the fine folks at CodeCheck (copyright © 2018), illustrate the difference between a service panel and a subpanel. Click on either for a larger version.
In short, it’s not always an improper installation if the grounds and neutrals are connected together at a subpanel. It depends on when it was installed and what else is going on.
Author: Reuben Saltzman, Structure Tech Home Inspections
I know the electrical code requires the ground and neutral bars in a subpanel to be separate, and for the subpanel to have its own ground rod. I don’t understand why, though. Don’t the grounds and neutrals all end up going to the same place anyway?
—Andy Engel, Roxbury, Conn.
and neutrals must be isolated from one another. Here, they connect to different bars in the subpanel.
Cliff Popejoy, a licensed electrical contractor in Sacramento, Calif., replies: Let’s start by looking at what the neutral and the ground do in a circuit. Whether it’s a feeder circuit supplying a subpanel, or a branch circuit supplying a receptacle outlet, the neutral conductor is the return path for electrical current. Any time a circuit supplies power to a light bulb, tool, or other device, electrical energy flows from the source along a wire (usually black or red in a 120v system) to the thing that uses the energy (the load), and then the electrons return to the source on the neutral wire.
A ground wire, which is better called an “equipment-grounding wire,” is there to provide a path from any metal parts of an electrical device that could possibly become energized and pose a shock hazard back to the breaker panel that supplies the circuit, and only carries current if there’s a ground fault. These occur when a hot wire—or neutral wire carrying current because the load is on—touches some metal part of the device due to a loose wire or other deficiency. The grounding conductor provides a safe path for the wayward electricity to flow back to the panel to trip the breaker and kill the power. Without the grounding wire, that misdirected electricity could shock you.
At the main service panel, the neutral and grounding wires connect together and to a grounding electrode, such as a metal ground rod, which is there to handle unusual pulses of energy, such as a lightning strike. This is the only point at which the neutral connects to ground. If the neutral and grounding wires are connected together anywhere else, the return current that is meant to flow on the neutral will flow back to the panel on both the neutral and ground. This is dangerous for several reasons; most importantly, if there’s a poor connection or break in the grounding wire and the neutral wire, the parts of the grounding system on the far side of the break (from the panel) will be energized and present a shock hazard. This is a big deal, because any exposed metal part of a fixture, tool, or appliance may shock or electrocute you—and breaks or poor connections happen more often than you’d think.
The National Electrical Code (NEC) requirement for separated neutrals and grounding wires in a subpanel and separate neutral and grounding conductors back to the main panel, when both panels are in the same building, dates to the 1999 revision. The requirement for separation of neutral and grounding conductors in and to a subpanel in a separate structure first appeared in the 2008 NEC. Does that make a system with a subpanel with combined neutral and grounding connections unsafe? No, although maintaining separation makes for a safer installation.
Photo: Matthew Millham
From Fine Homebuilding #294
Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.