EMT Fill and Ampacity With Mixed Conductors
A 1-inch EMT homerun in a commercial tenant space carries twelve current-carrying THHN copper conductors: eight 12 AWG and four 10 AWG. The circuits are 120/208 V, 20 A, with 75°C terminals. The conduit is run through a 35°C ambient space, and the electrician wants to use 20 A breakers on all circuits. The EMT is 1-inch trade size with an internal area of 0.864 in². The 12 AWG THHN area is 0.0133 in² and the 10 AWG THHN area is 0.0211 in². The electrician also plans to pull a single 8 AWG equipment grounding conductor in the same conduit. The AHJ questions both fill and ampacity. No derating was applied. The conductors are copper. The 8 AWG THHN area is 0.0366 in².
1. Calculate the conduit fill, including the equipment grounding conductor, and determine whether the 1-inch EMT complies.
Read the worked answer
Total conductor area = (8 × 0.0133) + (4 × 0.0211) + 0.0366 = 0.1064 + 0.0844 + 0.0366 = 0.2274 in². For more than two conductors in a raceway, the maximum fill is 40% of the EMT internal area: 0.40 × 0.864 = 0.3456 in².
Because 0.2274 in² is less than 0.3456 in², the 1-inch EMT complies with the fill requirement. The equipment grounding conductor counts for conduit fill even though it is not a current-carrying conductor.
2. Calculate the adjusted ampacity of the 12 AWG and 10 AWG conductors and determine whether 20 A breakers are permitted.
Read the worked answer
Twelve current-carrying conductors require a 50% adjustment factor from Table 310.15(C)(1). The 35°C ambient correction for 90°C conductors is 0.96.
12 AWG THHN: 90°C ampacity 30 A × 0.96 × 0.50 = 14.4 A. 10 AWG THHN: 90°C ampacity 40 A × 0.96 × 0.50 = 19.2 A. The 75°C terminal limits are 20 A for 12 AWG and 30 A for 10 AWG, but the adjusted ampacities are lower and control.
A 20 A breaker is not permitted on either size with twelve current-carrying conductors. The 12 AWG is limited to about 14.4 A and the 10 AWG to about 19.2 A. Options are to reduce the number of current-carrying conductors so the adjustment is 70% or better, or to increase conductor size. With nine current-carrying conductors, the 12 AWG adjusted ampacity becomes 30 × 0.96 × 0.70 = 20.16 A, which would support a 20 A breaker.
3. Does the equipment grounding conductor count for conduit fill and for ampacity adjustment?
Read the worked answer
The equipment grounding conductor counts for conduit fill because all conductor areas must be added when sizing a raceway. It does not count as a current-carrying conductor for ampacity adjustment, so the adjustment is based on the twelve circuit conductors, not thirteen. Its presence still affects fill, and its size must comply with Table 250.122 for the 20 A branch circuits.