Grounding and bonding

Why it matters

Grounding and bonding are the parts of an electrical system that only matter when something goes wrong, which is exactly why they are easy to get wrong. A missing bond can leave a metal water pipe or appliance case energised. A wrong neutral-to-ground connection can put current on the metal parts people touch. Article 250 is one of the longest and most tested articles in the NEC, and the concepts behind it make the rest of the article readable.

Awareness level only. Grounding and bonding work is done on real systems only under a licensed electrician's supervision with employer training. Never open a panel or touch service equipment to "look" at the bonding.

Two different jobs

The words sound alike but describe different jobs.

Grounding means connecting to the earth. Article 250 says this is done to limit voltages caused by lightning, line surges or unintentional contact with higher-voltage lines, and to stabilise the system voltage to earth during normal operation. A grounding electrode system (ground rods, a concrete-encased electrode, a metal water pipe, and so on) connects the system to the earth through the grounding electrode conductor.

Bonding means connecting metal parts together so they form a low-impedance path back to the source. Its job is to carry fault current large enough, and quickly enough, to trip the breaker or blow the fuse, and to keep all the metal a person might touch at the same voltage.

The key point: the earth is a poor conductor, and it is not what clears a fault. If a hot conductor touches a metal box and the only path is through a ground rod and the earth, the current may be far too small to trip a 20 A breaker, and the box stays energised. It is the effective ground-fault current path (equipment grounding conductors, metal raceways, bonding jumpers, back to the neutral at the service) that clears faults.

How the pieces fit together at a service

At a typical dwelling service:

1. The utility delivers a grounded conductor (neutral) with the ungrounded (hot) conductors.
2. In the service equipment, the main bonding jumper connects the neutral bar to the equipment grounding bar and the enclosure. This is the one point where neutral and ground are joined.
3. The grounding electrode conductor runs from that point to the grounding electrode system.
4. From there on, the equipment grounding conductors (EGCs) run with every circuit to bond boxes, appliance frames and receptacle ground pins.
5. Downstream of the service, for example at a subpanel, neutrals and grounds are kept separate. Joining them again would put normal neutral current on the EGCs and metal raceways.

Worked example: why the bond trips the breaker

A 120 V circuit on a 20 A breaker feeds a metal-cased appliance. A loose hot wire touches the case.

• With a good EGC: the fault path (hot conductor, case, EGC, back to the service, neutral, transformer) has very low impedance, say 0.2 Ω total. The current is about 120 / 0.2 = 600 A, and the breaker trips almost instantly.
• With only a ground rod (no EGC): the path goes through the earth, which might be 25 Ω or more between the rod and the utility's ground. The current is about 120 / 25 = 4.8 A. The breaker never trips, and the case sits at a dangerous voltage until someone touches it.

That difference is the reason the NEC requires an effective ground-fault current path.

Sizing (where to look)

You will meet two sizing tables in Article 250:

• The grounding electrode conductor is sized from the size of the service-entrance conductors.
• The equipment grounding conductor is sized from the rating of the overcurrent device protecting the circuit (Table 250.122), and is increased if the ungrounded conductors are increased, for example for voltage drop.

Look up the values in your code book; do not memorise them from videos.

Common mistakes

• Believing the ground rod clears faults.
• Bonding neutral and ground in a subpanel.
• Calling the neutral "the ground".
• Relying on a metal water pipe alone as the only electrode.
• Forgetting to bond metal piping systems and structural metal that may become energised.
• Sizing the EGC from the conductor size instead of the overcurrent device.

Quick recap

Grounding connects the system to earth to handle lightning, surges and stabilise voltage. Bonding ties metal parts together into a low-impedance path so faults trip the breaker. Neutral and ground join once, at the service, through the main bonding jumper. EGCs run with every circuit and are sized from the overcurrent device. Check Article 250 in your adopted edition (NFPA 70; checked on 2026-10-23).