Cable Voltage Drop - Maximum Run Length by Wire Size

Cross-section, mm²	Load current, A
Cross-section, mm²	2	4	6	10	16	20	25	32	40	50	63
Allowed voltage drop 3%
1.5	148	74	49	30	18	—	—	—	—	—	—
2.5	246	123	82	49	31	25	20	—	—	—	—
4	394	197	131	79	49	39	32	25	—	—	—
6	—	296	197	118	74	59	47	37	30	—	—
10	—	—	329	197	123	99	79	62	49	39	31
16	—	—	526	315	197	158	126	99	79	63	50
Allowed voltage drop 5%
1.5	246	123	82	49	31	—	—	—	—	—	—
2.5	411	205	137	82	51	41	33	—	—	—	—
4	657	329	219	131	82	66	53	41	—	—	—
6	—	493	329	197	123	99	79	62	49	—	—
10	—	—	548	329	205	164	131	103	82	66	52
16	—	—	876	526	329	263	210	164	131	105	83

How to use

Find the row with the conductor cross-section and the column with the load current - the cell at the intersection is the longest run that keeps the voltage drop within the chosen limit. 3% is the usual limit for lighting, 5% is the maximum for other loads. Values are for the one-way run length.

Formula

The length is calculated as L = ΔU · S / (2 · I · ρ), where ΔU is the allowed drop in volts (3% or 5% of 230 V), S is the conductor cross-section, I is the current and ρ = 0.0175 Ω·mm²/m is the resistivity of copper. The factor of 2 accounts for both conductors of the run (out and back).

Important

The table is for copper conductors and a single-phase 230 V supply. The final allowed run length is the smaller of two limits: voltage drop (this table) and breaker tripping on a short circuit (maximum cable length table). The calculation assumes cable of true nominal cross-section; undersized or copper-clad conductors have higher resistance and shorter real limits. Reference material - the actual design should be done by a qualified electrician.