An 800 m HART loop checked against the host system's total capacitance limit — and how much further the same cable could actually run before capacitance becomes the constraint.
| Loop type | HART |
| Cable run length | 800 m |
| Cable capacitance | 150 pF/m (typical shielded twisted-pair instrument cable) |
| Host/barrier maximum system capacitance | 200 nF |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Total cable capacitance vs. host limit | ≤ 200 nF | 120 nF | ✓ PASS |
| Conductor size for an 800 m run | General guidance | #24 AWG typically adequate | ✓ PASS |
Key insight: Capacitance limits and voltage-budget limits are two separate, independent constraints on HART/4-20mA loop length — this calculator checks capacitance, but the loop's voltage budget (transmitter headroom, barrier drop, wire resistance) is a completely separate check handled by the 4-20mA Current Loop calculator, and a real installation needs both to pass, not just one.
Every input in this example is editable in the live calculator — free, no signup.
Open Control / Instrumentation Cable Sizing calculator →HART superimposes a small AC communication signal on the 4-20mA DC loop current. Excess cable capacitance attenuates and distorts that AC signal (acting like a low-pass filter across the line), which can corrupt or block HART communication even though the underlying 4-20mA analog signal keeps working fine — this is why host systems and safety barriers publish a maximum total system capacitance rating specifically for HART compatibility.
Only up to a point — this guidance reflects common industry practice, not a single universal standard figure, and #24 AWG's higher resistance per meter becomes a voltage-budget problem on longer runs well before it becomes a capacitance problem. Beyond roughly 1500 m, #20 AWG or larger is the usual recommendation, and any run's actual voltage budget should always be confirmed with the transmitter and barrier manufacturers' own documentation.