Cable Size Calculator

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.