Utility　tunnels　serve　as　critical　infrastructure　for　urban　energy　and　electricity　transport　systems,　housing　multi-layered　cable　arrangements　that　pose　significant　fire　hazards.　This　study　investigates　the　effects　of　cable　arrangement　and　layer　spacing　on　the　combustion　parameters　and　flame　evolution　of　double-layer　cable　fires,　emphasising　their　implications　for　energy　infrastructure　safety.　Using　a　reduced-scale　utility　tunnel　model,　we　conducted　fire　experiments　to　measure　the　heat　release　rate　(HRR),　oxygen　consumption,　and　fuel　energy　of　cable　combustion.　The　key　parameters　evaluated　were　the　cable　fuel　energy,　surface　temperature,　pyrolysis　efficiency,　and　flame　morphology.　Our　findings　indicate　that　both　total　combustion　time　and　HRR　decrease　with　increasing　layer　spacing.　Notably,　the　pyrolysis　efficiency　of　double-layer　cable　fires　can　exceed　100　%,　reaching　up　to　110　%　at　a　spacing　of　1　cm　due　to　the　mutual　fuel　effect　created　by　the　close　proximity　of　the　layers.　This　effect　enhances　the　fire　intensity,　illustrating　how　the　layer　spacing　critically　influences　the　combustion　dynamics.　When　the　layer　spacing　exceeds　the　critical　threshold,　the　lower　cable　cannot　ignite　the　upper　cable,　resulting　in　a　single-layer　fire　scenario.　Additionally,　the　flame　characteristics　are　positively　correlated　with　increased　HRR　and　reduced　layer　spacing.　We　analytically　delineated　the　critical　layer　spacing　into　three　distinct　regions　and　proposed　novel　mathematical　models　to　quantify　the　relationships　between　pyrolysis　efficiency,　flame　height,　and　layer　spacing.　These　insights　are　vital　for　the　design　of　safer　cable　layouts　and　fire　prevention　strategies　in　urban　utility　tunnels,　thereby　enhancing　the　operational　safety　of　energy　transport　systems.　This　study　provides　essential　knowledge　for　improving　fire　protection　measures　in　energy　infrastructures.　©　2025　The　Authors