Retail Tenant Improvement: Conductor Derating and Voltage Drop
An electrical contractor is wiring a retail tenant improvement in a one-story building. Nine 20A, 120V branch circuits for display lighting are installed in a single 1-inch EMT run from a panel to a junction box 120 ft away. The conductors are 12 AWG THHN. The ceiling space is not an air plenum, but the ambient temperature at the raceway is measured at 42°C (108°F) because of adjacent heating equipment. Each circuit is a continuous load of 16A. The contractor has installed a 20A breaker for each circuit and wants to use the existing 12 AWG because '12 AWG is good for 20A.' The inspector notes that the conduit contains nine current-carrying conductors and that the run is long. The tenant's lighting is sensitive to voltage drop and has been flickering. The inspector must decide whether to accept the installation or require a change.
1. Perform the ampacity adjustment for the 12 AWG THHN conductors. Is the circuit compliant for the 16A continuous load?
Read the worked answer
12 AWG THHN at 90°C has a base ampacity of 30A. Table 310.15(C)(1) requires a 70% adjustment for 9 current-carrying conductors in a raceway: 30A x 0.70 = 21A. The ambient correction at 42°C for a 90°C conductor is 0.87: 21A x 0.87 = 18.27A.
The load is 16A continuous. The adjusted conductor ampacity of 18.27A is greater than the 16A load, so the conductor is adequate for the load. The 20A breaker is permitted for 12 AWG under 240.4(D), and 210.20(A) requires the breaker to be at least 125% of the continuous load: 16A x 1.25 = 20A.
So the ampacity is acceptable, but the margin is small. The inspector should still check voltage drop because the circuit is long and the lighting is flickering.
2. Calculate the voltage drop and decide whether the 12 AWG should be accepted. What would you require?
Read the worked answer
12 AWG copper has about 1.93 ohms per 1,000 ft. For a 120 ft one-way run, the round-trip length is 240 ft: 1.93 x 0.240 = 0.463 ohm. At 16A, the drop is 16 x 0.463 = 7.4V, or 7.4/120 = 6.2%.
That exceeds the 3% branch-circuit recommendation in the informational notes to 210.19(A). The NEC generally does not mandate 3% voltage drop, but the flickering and the long run make it a practical problem. Require 10 AWG THHN, which has about 1.21 ohms per 1,000 ft: 1.21 x 0.240 = 0.290 ohm; 16A x 0.290 = 4.64V, or 3.9%. If that is still too high for the lighting, reduce the number of conductors per raceway, split the run, install a local panel, or use a 277V lighting circuit where the luminaires permit it. The 12 AWG ampacity is technically adequate, but the design should not be accepted without addressing voltage drop.
3. Could the contractor use a 15A breaker and 14 AWG THHN for the 16A load? Explain.
Read the worked answer
No. A 16A continuous load requires a branch circuit rated at least 16A x 125% = 20A under 210.20(A). A 15A breaker is too small. 14 AWG is also limited to 15A protection by 240.4(D).
Even before derating, 14 AWG THHN at 90°C has a base ampacity of 25A. After the 70% adjustment for 9 current-carrying conductors and the 0.87 ambient correction, the ampacity is 25 x 0.70 x 0.87 = 15.2A, which is below the 16A load. So 14 AWG fails both the load requirement and the overcurrent protection limit.
The minimum conductor size for the ampacity calculation is 12 AWG, but the voltage-drop problem may require 10 AWG. The contractor cannot solve the derating problem by reducing the breaker and conductor size.