Rule 21 of 40 · Chapter III — Grounding, Bonding, and Protection
Do not use the neutral as ground
Why this rule exists
The neutral (grounded conductor) and the equipment ground are bonded together at exactly one point, the service, and must stay separate everywhere else. The neutral carries return current; the ground carries none until a fault. Tie them together downstream, or use the neutral to ground a device, and normal return current flows on the grounding system and every bonded enclosure. Metal parts that should be at zero volts sit at a potential, and a broken neutral can raise them dangerously. It also confuses GFCIs and invites shock and fire. One bond, one place.
In practice
Land neutrals on the neutral bar and grounds on the ground bar; in subpanels, keep the two bars isolated by removing the bonding screw or strap so they are separate. Never pigtail a receptacle's ground to its neutral to fake a ground on an old circuit; use GFCI protection instead. When you find a downstream neutral-ground bond, correct it. Check that multiwire circuits share a neutral properly sized for the combined return, and that the neutral is never opened while conductors are live.
When it doesn't apply
The main service and separately derived systems (generators, transformers) each get exactly one neutral-ground bond at their source; that bond is required, not a violation. The rule is one bond per system at the source; the error is additional bonds downstream. If you are unsure whether a point should be bonded, trace it back to its source.