For a network with five devices totaling LEN 14 on a 30-meter heavy cable backbone powered from one end, what is the estimated voltage drop?

For a network with five devices totaling LEN 14 on a 30-meter heavy cable backbone powered from one end, what is the estimated voltage drop?

• A. 0.25 Volts
• B. 0.50 Volts
• C. 1.0 Volts
• D. 2.0 Volts

Answer: (B)

Voltage drop on a NMEA 2000 trunk is found from V_drop ≈ I_bus × R_loop, where I_bus is the current drawn by all devices on the backbone and R_loop is the resistance of the power path through both conductors. First, total current on the backbone comes from the devices’ LENs. In practice problems for NMEA 2000, each LEN is treated as about 30 mA of current. With a total LEN of 14, the backbone current is approximately 14 × 0.03 A = 0.42 A. Next, determine the loop resistance for a 30-meter trunk. The current travels out the 30 m and back the return path, so the loop length is 60 m. For a heavy trunk, this two-conductor loop has a combined resistance of about 0.02 Ω per meter, giving a loop resistance around 60 m × 0.02 Ω/m = 1.2 Ω. Now multiply to find the drop: 0.42 A × 1.2 Ω ≈ 0.504 V, about 0.5 V. So the estimated voltage drop is roughly 0.50 V.

Voltage drop on a NMEA 2000 trunk is found from V_drop ≈ I_bus × R_loop, where I_bus is the current drawn by all devices on the backbone and R_loop is the resistance of the power path through both conductors.

First, total current on the backbone comes from the devices’ LENs. In practice problems for NMEA 2000, each LEN is treated as about 30 mA of current. With a total LEN of 14, the backbone current is approximately 14 × 0.03 A = 0.42 A.

Next, determine the loop resistance for a 30-meter trunk. The current travels out the 30 m and back the return path, so the loop length is 60 m. For a heavy trunk, this two-conductor loop has a combined resistance of about 0.02 Ω per meter, giving a loop resistance around 60 m × 0.02 Ω/m = 1.2 Ω.

Now multiply to find the drop: 0.42 A × 1.2 Ω ≈ 0.504 V, about 0.5 V.

So the estimated voltage drop is roughly 0.50 V.