R-Pro › Field cases › Air conditioning
AC New Install — Indoor/Outdoor Communication Cable (Shield, EMI)
Likely causes
Step 1 — Choose communication cable (indoor↔outdoor 14/4, VDC pulse; VRF requires shield)
Check: Standard: 14/4 stranded (2 power + 1 comm + 1 ground, 300V). Single-zone may use unshielded. VRF/M-series/branch box = shielded twisted pair required (per Mitsubishi). VDC pulse comm can share conductor with AC power (supported models only).
What to do: Communication shield ground = single point only (both ends → ground loop, EMI). Typically grounded at outdoor unit. Keep communication and power in separate conduits when possible (per Mitsubishi VRF manual). Near inverter board with EMI exposure → use twisted pair + ferrite core.
Communication error = most common is cable defect. E-code (comm error) is the most frequent fault during commissioning — check wiring/shield ground. Branch box / BS box (simultaneous heat/cool VRF) = mandatory termination resistor on separate comm line.
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