Cable Size Calculator Guide

For a 5 kW, 400 V DC PV array over 50 m (one-way), the calculator recommends 6 mm² copper cable. This is based on a design current of ~13.9 A (I = P / V = 5000 W / 400 V), factoring in 2% allowable voltage drop (8 V). Using the standard voltage drop formula ΔV = (2 × ρ × L × I) / A (where ρ = 0.0172 Ω·mm²/m for Cu), 6 mm² yields ~1.8% drop — within IEC 62548 and NEC 690.71 limits. Always verify against local codes: NEC requires ampacity derating per Table 310.16 and ambient temperature correction (e.g., 0.82 at 40°C). Aluminum is not recommended for DC PV due to oxidation and higher resistance; stick with stranded, XLPE-insulated, sunlight-resistant Cu cables (e.g., PV Wire or USE-2 rated).

The calculator uses one-way length because DC voltage drop depends on the total conductor path from source to load — which includes both positive and negative conductors. Since each carries full current in series, the effective resistive path is 2 × L (L = one-way distance). Internally, the tool applies the standard DC voltage drop formula: ΔV = 2 × ρ × L × I / A. Specifying 'one-way' avoids user error (e.g., double-counting) and aligns with IEC 60287-1-1 and IEEE 1547-2018 guidance. Confusing this with AC single-phase (which also uses 2×L) or three-phase (which uses √3×L) is common — but for unidirectional DC PV strings, the 2×L convention is universal. Always measure physical routing distance, including conduit bends and vertical drops, not just straight-line distance.

No — the core calculation assumes standard conditions: 20°C ambient, 75°C-rated copper conductors, and free-air installation. It computes minimum cross-section for voltage drop and ampacity only, without automatic derating. Per IEC 62548 §7.3.2 and NEC Article 310.15(B)(1), you must manually apply correction factors: e.g., 0.82 for 40°C ambient (NEC Table 310.16), or 0.71 for bundled cables (>3 conductors). PV-specific cables (e.g., UL 4703 PV Wire) have higher thermal ratings (90°C wet/105°C dry) — enabling smaller sizes than generic THWN-2. Always verify final ampacity against conductor rating, not just voltage drop, as per NEC 690.8(A)(1) and IEC 62548 Annex B tables.

Aluminum is generally not recommended for DC PV arrays below 1000 V. Per UL 4703 and IEC 62548 §7.3.1, aluminum suffers from higher resistivity (≈1.6× copper), greater thermal expansion, and galvanic corrosion when joined to copper terminals — leading to increased voltage drop, hotspots, and fire risk. While NEC permits Al for feeders ≥1/0 AWG (33.6 mm²), PV string circuits typically require smaller gauges where Al’s creep and oxidation issues dominate. Field failures (e.g., NEC 690.31(E) violations) are well-documented. If used, require dual-rated lugs (AL/CU), antioxidant paste, and torque verification per manufacturer specs. For reliability and compliance, specify stranded tinned-copper PV Wire (UL 4703) or USE-2 — especially for rooftop or high-UV environments.

Accuracy degrades beyond 100 m due to unmodeled variables: skin effect (minimal at DC but relevant for high-frequency ripple), contact resistance at connectors, and non-uniform temperature gradients. The calculator uses idealized DC resistance (ρ = 0.0172 Ω·mm²/m @ 20°C), ignoring real-world losses from MC4 connectors (typically 0.5–2 mΩ each) and crimp quality. For >100 m runs, IEC 62548 §7.3.2 mandates field measurement or advanced modeling (e.g., ETAP or PVsyst with distributed resistance). Also, voltage drop tolerance tightens: EN 50530 recommends ≤1.5% for large plants to preserve MPPT efficiency. Always add 10–15% safety margin and validate with clamp-meter current + voltmeter measurements under STC conditions before commissioning.

No — this tool sizes cables solely for continuous current carrying capacity and voltage drop, not fault protection. Per IEC 62548 §7.3.3 and NEC 690.8(A)(2), DC cables must withstand maximum available fault current (e.g., from parallel strings) for the duration of OCPD clearing time. For example, a 10-string array may require cables rated for 1.56× Isc × Nstrings (NEC 690.8(A)(3)). Voltage drop calculators don’t model short-circuit thermal withstand (I²t). You must separately verify conductor ampacity against fault duty using standards like IEC 60947-2 or IEEE C37.010. Always coordinate with your DC breaker/fuse selection — undersized cables can melt before protection operates, violating NEC 690.9(B) and IEC 62548 §8.2.

mm² measures cross-sectional area directly; AWG is a logarithmic gauge scale where smaller numbers = larger area. Conversion isn’t linear: 6 mm² ≈ 10 AWG (5.26 mm²), while 10 mm² ≈ 8 AWG (8.37 mm²). Use ISO 13606 or ASTM B258 for exact equivalents — never approximate. PV applications favor metric (mm²) per IEC standards; NEC accepts AWG but requires explicit listing (e.g., ‘10 AWG PV Wire’). Critical point: AWG tables assume solid copper — stranded PV Wire has ~5% higher resistance due to lay length, so 6 mm² stranded may perform like 5.7 mm² solid. Always size using actual conductor area from datasheets, not nominal AWG. UL 4703 lists minimum areas (e.g., 6 mm² min for 30 A), making mm² the safer, code-aligned unit for engineering calculations.