[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A 400 V three-phase sub-main feeds a 100 kW panel at cos φ 0.9 over a 120 m route on a perforated cable tray. The contractor proposes aluminium XLPE cable instead of copper to cut cost.

• How much larger does the aluminium cross-section need to be?
• Is there a simple ratio, or must current-carrying capacity and voltage drop be recalculated?
• What else changes with aluminium: short-circuit check, terminations, minimum sizes?

A worked comparison with IEC 60364-5-52 and a note on NEC practice would help.

[MEP Center Engineering]

There is no fixed conversion ratio: size the aluminium cable from scratch with the same three checks as copper (current-carrying capacity after derating, voltage drop, short-circuit withstand). In practice aluminium ends up one to two standard sizes larger, roughly 1.4 to 1.7 times the copper cross-section.

Why aluminium is larger

• Resistivity at 20 °C: copper 0.0172, aluminium 0.0283 Ω·mm²/m, a ratio of about 1.64. For equal resistance (equal voltage drop) aluminium needs about 1.6 times the area.
• For equal current-carrying capacity the ratio is smaller, because a larger conductor dissipates heat better. Tabulated aluminium ampacity is about 78 % of copper of the same size.
• Short-circuit factor k (IEC 60364-5-54, XLPE): copper 143, aluminium 94, so the minimum size from S ≥ √(I²t)/k is about 1.5 times larger.

Worked example (IEC 60364-5-52, method E, XLPE multicore)

1. Design current: I_B = 100 000 / (√3 × 400 × 0.9) = 160.4 A. Protective device I_n = 160 A.
2. Derating: ambient 40 °C (0.91) × two circuits touching on perforated tray (0.88) = 0.80.
3. Required tabulated current: I_t ≥ 160 / 0.80 = 200 A.
4. From the method E table: copper 50 mm² is too small, copper 70 mm² passes; aluminium 70 mm² is too small, aluminium 95 mm² passes (about 182 A after derating, ≥ 160 A).
5. Voltage drop, ΔU = √3 × I_B × L × (R cos φ + X sin φ), with R at 90 °C and X ≈ 0.08 Ω/km:

Cable	R at 90 °C (Ω/km)	ΔU (V)	ΔU (%)
Cu 70 mm²	0.342	11.4	2.85
Al 95 mm²	0.410	13.5	3.37
Al 120 mm²	0.324	10.9	2.72

If your voltage-drop budget for this sub-main is 3 %, copper 70 mm² passes, aluminium 95 mm² fails and aluminium 120 mm² is the answer. Here voltage drop, not ampacity, governs the aluminium size. On long routes this is the usual result, and it is why a fixed ratio misleads.

Short-circuit and other checks

• Confirm S ≥ √(I²t)/k with the let-through energy of the actual MCCB, using k = 94 for aluminium XLPE.
• The protective conductor of an aluminium cable has a higher impedance, so recalculate fault-loop impedance and disconnection time.
• Aluminium is normally used only from 16 mm² upwards for fixed power circuits.
• Terminations: use bimetallic (Al/Cu) lugs or terminals rated for aluminium, contact grease, and the manufacturer’s torque. Re-torque checks belong in the commissioning plan.
• NEC practice: use AA-8000 series alloy conductors, take ampacity from the 75 °C column (NEC 110.14(C)), and make sure the equipment terminals are marked AL/CU.

Common mistakes

• Converting with a single “one size up” rule and skipping the voltage-drop check.
• Checking the cable against I_B and forgetting the I_n ≤ I_z rule for the breaker.
• Gland, lug and busbar connections sized for copper. A larger aluminium conductor may not fit the breaker terminals without spreaders. See Cable Gland Selection and Cable Tray Sizing: Fill, Spacing and Derating.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

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