AC vs. DC: How Current Type Affects Cable Design

Electricity travels in two main forms: Alternating Current (AC) and Direct Current (DC). AC constantly reverses direction (50 or 60 times per second), while DC flows steadily in one direction. You might think a cable is just a cable, but the type of current dramatically influences how that cable must be designed. From the conductor itself to insulation and shielding, engineers make very different choices for AC and DC systems. This article explains why.

1. The Fundamental Difference: Steady vs. Pulsing

In a DC cable, electrons move at a constant, unidirectional speed. The current density is uniform across the conductor’s cross‑section. In an AC cable, electrons oscillate back and forth. This changing magnetic field creates skin effect and proximity effect – phenomena that do not exist in DC.

These effects force AC current to flow mainly near the conductor’s surface, not through its entire cross‑section. That fundamentally changes how the conductor must be built.

2. Skin Effect: Why AC Cables Need Thinner Strands

Skin effect is the tendency of high‑frequency AC to crowd towards the outer surface of a conductor. At 50/60 Hz, the effect is modest but not negligible, especially for large conductors.

For DC, you can use a solid, thick copper bar. For AC, a solid bar would waste inner material (carrying little current). Instead, AC cables use stranded conductors – many thin, individually insulated strands. This increases the surface area and reduces skin effect losses. Very large AC cables may use Milliken construction (strands insulated and transposed) to further mitigate skin effect.

DC cables, by contrast, can happily use solid or coarsely stranded conductors with no penalty.

3. Proximity Effect: When Cables Crowd Together

When AC cables run close together, the magnetic fields of adjacent conductors push current to the far side of each conductor – an effect called proximity effect. This increases resistance and heating, especially in tightly packed cables.

In DC systems, proximity effect does not exist because the magnetic field is steady.

To combat proximity effect, AC cable designers:

• Maintain spacing between cables.

• Use transposition of phases in three‑phase systems.

• Choose conductor stranding patterns that minimise mutual inductance.

For high‑current AC busbars, they are often hollow or split into multiple thin laminations – a design never needed for DC.

4. Insulation Stress: DC vs. AC

Insulation must withstand voltage without breaking down. But AC and DC stress insulation differently.

• AC: The voltage cycles from positive to negative peak. Insulation is stressed in both directions. Dielectric losses (heating of the insulation) occur, especially at higher frequencies or with polar materials (e.g., paper, some polymers).

• DC: The voltage is steady. There are no dielectric losses from polarity reversal. However, space charges can build up inside the insulation over time, potentially causing local field enhancement.

For high‑voltage DC (HVDC) cables, insulation is often made of cross‑linked polyethylene (XLPE) or mass‑impregnated paper – materials chosen for low space charge accumulation and high DC dielectric strength. AC cables use similar materials but must also consider loss tangent (tan δ), which is irrelevant for DC.

Interestingly, a cable rated for AC may have a higher DC voltage rating (typically 1.5 to 2× the AC RMS rating) because peak AC voltage already includes a safety margin. But that is not a simple rule; the insulation must be qualified for the specific DC stress.

5. Magnetic Fields and Shielding

AC cables generate a time‑varying magnetic field. This field can:

• Induce currents in nearby metalwork (heating, losses).

• Interfere with adjacent signal cables (electromagnetic interference – EMI).

To control this, AC cables often require screening (e.g., copper tape or wire braid) to contain the field. Three‑phase AC cables are often armoured with non‑magnetic materials (aluminium) to avoid eddy current heating.

DC cables produce a static magnetic field. It does not induce currents in stationary objects and causes little interference. Therefore, DC cables generally need no magnetic shielding. However, they can affect magnetic compasses or sensitive instruments if run in close proximity.

6. Armour and Metallic Sheaths

For AC cables, metallic armour or sheaths must be treated carefully:

• Steel wire armour (SWA) is fine for single‑core AC only if the armour is non‑magnetic (aluminium) or the cable is three‑core so the magnetic fields cancel. With single‑core AC, steel armour would overheat due to eddy currents.

• Aluminium wire armour (AWA) is preferred for single‑core AC.

For DC cables, steel armour works perfectly – no eddy currents, no heating. This simplifies and reduces the cost of DC cables for railway, solar farm, or HVDC applications.

7. Losses and Efficiency

For long‑distance transmission, DC has lower losses because there are no skin or proximity effects, and no reactive power flow. That is why HVDC is preferred for submarine cables and very long overhead lines – despite the higher cost of converter stations.

8. Practical Examples

Example 1: Household wiring (230 V AC)
Cables are stranded to reduce skin effect. They are not armoured (steel would be fine because three‑phase circuits cancel fields, but single‑phase circuits still cause some heating). Insulation is PVC or XLPE, rated for AC voltage.

Example 2: Solar farm DC string cables (1500 V DC)
Cables use fine stranding (for flexibility, not skin effect). No shield is needed. Steel wire armour can be used for burial without heating concerns. Insulation is DC‑rated XLPE.

Example 3: Railway DC traction (750 V / 1500 V DC)
Cables often use steel armour for mechanical protection. Conductors may be solid or coarse stranded. No magnetic shielding is required.

9. The Rise of HVDC and What It Means for Cable Design

HVDC is growing rapidly (offshore wind, interconnectors). These cables must handle very high voltages (up to 600 kV). Special design features include:

• Mass‑impregnated paper or XLPE insulation optimised for DC stress and space charge control.

• Return conductors (metallic return or earth return) – often integrated.

• Segmented conductors to reduce bending forces during laying.

• Double armour for deep‑water protection.

Many of these designs differ significantly from AC cables of the same voltage class.

AC and DC may both flow through copper, but the cable around that copper must be engineered very differently. AC forces designers to fight skin and proximity effects, manage magnetic fields, and carefully select armour materials. DC frees them from those burdens but introduces space charge challenges in insulation.

Understanding the difference helps engineers choose the right cable for the job – and helps the rest of us appreciate why a wind farm cable looks different from a railway cable, even if both carry “electricity.” The next time you see a fat, stranded AC power cable or a solid, steel‑armoured DC cable, you will know: the current type shaped every layer inside.

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