A gray wire can be the grounded conductor on an NEC project and a phase conductor under IEC practice. Color shows what the installer intended, so test before you touch. Contracts often say “the NEC or the local code, whichever is stricter.” That works where both books measure the same thing the same way. It fails for earthing: a TT system and an NEC solidly grounded system are different designs, so you choose one and meet all of it. This series anchors to NEC 2023.
| Question | NEC 2023 | IEC practice |
|---|---|---|
| Gray conductor | grounded conductor (200.6) | can be a phase, energized |
| Blue conductor | a common phase color by custom, not by the Code | neutral |
| Green / green-yellow | equipment grounding conductor (250.119) | protective earth (green-and-yellow) |
| System grounding | solidly grounded is common. Ungrounded (250.21) and impedance grounded (250.36) are also permitted | named arrangements: TN-S, TN-C-S, TT, IT |
| Breaker | ampere rating and interrupting rating, and the curve comes with the device | IEC 60898-1 curve letter B, C or D sets the magnetic trip |
| Conductor sizing | correct and adjust from the conductor’s own temperature rating, capped at the termination rating | installation method, then correction factors |
A 16 A type C breaker is a 16 A breaker with a defined magnetic band, not a 20 A breaker by another name.
Grounded conductors 6 AWG or smaller are identified by “a continuous white outer finish,” “a continuous gray outer finish,” or “three continuous white or gray stripes along the conductor’s entire length on other than green insulation.” Under 200.6(B), 4 AWG or larger can also be marked white or gray at its terminations at the time of installation.
Wire-type equipment grounding conductors “shall be permitted to be bare, covered, or insulated,” and an insulated one has “a continuous outer finish that is either green or green with one or more yellow stripes.” Green-and-yellow is the closest the two books come to agreeing. Check the drawings anyway.
New in 2023: “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.” 250.142(B) says the same for equipment. A site transformer (a separately derived system) gets its own bond, under 250.30.
Some ac systems of 50 to 1000 V are permitted to be grounded but not required to be, such as systems that supply only industrial furnaces, or control-circuit transformers where only qualified persons service them and continuity of control power is required. 250.36 permits impedance grounded 3-phase systems of 480 to 1000 V with qualified persons, ground detectors, and no line-to-neutral loads.
“The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with 110.14(C).” A 90°C conductor can start from its 90°C column, and the terminals still set the ceiling.
The letter sets the magnetic trip as a multiple of the rating: B is 3–5×, C is 5–10×, D is 10–20×. IEC 60364-5-52 sizes cable from the installation method. The same cable can size differently in each book, and each answer can be correct in its own system.
Where both books measure the same thing the same way, take the tighter number.
Earthing is where that rule fails. In a TT system the fault current returns through the earth, and that path usually has too much resistance to trip a breaker quickly, so TT practice relies on residual current devices. The NEC arrangement returns the fault through a bonded metal path to the service, so the breaker can open. Pick one design and meet every requirement of it.
Write the decision down: which provision you took, and why. On a project where nobody inspects, that record is your review trail.
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