DC conductor identification — L+, L− and M under IEC 60445

The guide on core colours covers the fixed colour code of NEN-EN-IEC 60445 for AC installations: brown/black/grey for the phases, blue for the neutral, green/yellow for PE. The same standard also governs the identification of direct current (DC) conductors — but in a fundamentally different way, one that is becoming increasingly relevant with the growing practice of PV, battery storage and DC microgrids.

Letters instead of a fixed colour

For DC systems, IEC 60445 prescribes alphanumeric identification rather than a fixed colour code:

This is a fundamentally different starting point from AC: whereas brown/black/grey for the phases is a fixed, universally recognised colour convention, there is no comparable universal colour prescribed by IEC 60445 itself for DC polarity. The standard puts the emphasis on letter marking at terminals, connection points and documentation — not on a colour code of the core insulation itself.

Why cable colour alone is never sufficient

In practice, cable manufacturers often do use a colour convention for DC applications (for example red for positive, black for negative on PV strings), and the guide on EN 50618 PV cables mentions this practice. The crucial point: this is manufacturer and project convention, not a colour system universally mandated by IEC 60445 itself the way it is for AC phases. That has direct practical consequences:

• Different manufacturers or cable batches may follow a different colour convention — cable colour alone is therefore not a reliable proof of polarity when combining material from different sources.
• After re-terminating or re-fitting connectors (see also the guide on mixing MC4 connectors from different PV manufacturers) a cable can end up with the "wrong" colour for its actual polarity if the rewiring was not done carefully.
• Older or improvised DC wiring, for example in a DC microgrid or battery installation, does not always follow a consistent colour convention throughout the whole run.

The practical consequence: always measure polarity with a multimeter before making a DC connection, particularly at the final connections to inverters, chargers or battery management systems — never rely solely on cable colour, even if it looks convincingly "standard".

Application in PV, battery storage and DC microgrids

Correct and consistent DC identification is particularly relevant to:

• PV strings: reversed polarity when connecting to the inverter can damage the inverter — see also the related points in the guide on DC arc-fault detection regarding the risks of poor DC connections.
• Battery energy storage systems (BESS): the guide on DC protection and cable sizing for BESS already emphasises the importance of correct DC cable sizing; correct polarity identification is a precondition that precedes it.
• DC microgrids in buildings: the guide on DC microgrids and earthing choice covers the earthing strategy of the bus conductors (L+, L−, M) — the identification requirement in this article is the additional, practical wiring side of that same bus structure.

Common mistakes

1. Relying solely on cable colour for polarity determination without a multimeter check, particularly when combining material from different manufacturers or batches.
2. Assuming IEC 60445 prescribes a universal DC colour code as it does for AC — the standard works for DC primarily with the letter symbols L+/L−/M, not with a mandatory colour.
3. Not re-checking polarity when re-fitting connectors — a cable that has been cut and fitted with a new connector can accidentally be re-wired with reversed polarity.
4. Confusing the mid-point conductor "M" with a protective conductor (PE) — M identifies a live mid-point of a DC system, not an earth connection.

Related

• Core colours NL/EN/RU/UA/PL
• PV DC cable — EN 50618 (H1Z2Z2-K)
• BESS — DC protection and cable sizing
• DC microgrid — earthing choice