Eight Current-Carrying Conductors in One Rooftop Conduit
An HVAC replacement adds a rooftop unit on a 45 ft run of 3/4 in. EMT from a 200 A panel to a rooftop disconnect. The crew pulls four branch circuits through the single conduit: two 20 A, 120 V receptacle circuits (12 AWG THHN), one 20 A, 240 V circuit for a condensate pump (12 AWG THHN), and one 30 A, 240 V circuit for the unit (10 AWG THHN). All conductors are copper and all terminations are rated 75 °C. The conduit is supported 3 in. above the roof deck in direct sun, where the measured ambient at 3 p.m. is 104 °F (40 °C). The crew plans to land the three 20 A circuits on 20 A breakers and is confident the 3/4 in. conduit 'has plenty of room.' The jurisdiction enforces the currently adopted NEC.
1. Check conduit fill. Is the 3/4 in. EMT adequate, and is fill the real problem?
Read the worked answer
Conductor areas (Chapter 9, Table 5, THHN):
- 12 AWG THHN = 0.0133 in² × 6 conductors = 0.0798 in²
- 10 AWG THHN = 0.0211 in² × 2 conductors = 0.0422 in²
- Total = 0.1220 in²
Allowable fill: 3/4 in. EMT, over two conductors, 40% = 0.213 in².
The pull uses about 57% of the permitted fill. Fill is not the problem - the installation is well within Chapter 9, Table 1. The failure, if any, will come from ampacity adjustment, which the crew has not considered.
2. Determine the adjusted ampacity of each conductor and state which breakers, if any, are not permitted.
Read the worked answer
Current-carrying conductors: each 2-wire circuit contributes 2 (the grounded conductor of a 2-wire circuit counts). 4 circuits × 2 = 8 CCC.
Adjustment - 310.15(C)(1): 8 CCC = 70%. Ambient correction - Table 310.15(B)(1), 90 °C column, 40 °C: 0.91.
12 AWG THHN: 30 A × 0.91 × 0.70 = 19.1 A 10 AWG THHN: 40 A × 0.91 × 0.70 = 25.5 A
(Note: the rooftop temperature adder in Table 310.15(B)(3)(c) applies to conduits less than 7/8 in. above the roof; at 3 in. it does not apply, but the measured 40 °C ambient still does.)
Consequences:
- The two 20 A receptacle circuits are violations. 19.1 A is below the 20 A breaker rating, and 240.4(B) cannot rescue them because it does not apply to multioutlet branch circuits supplying receptacle outlets for cord-and-plug loads. 240.4(D) sets a 20 A ceiling for 12 AWG; it does not permit a breaker above the conductor's ampacity.
- The condensate pump circuit is arguably acceptable. It is not a multioutlet receptacle circuit, 19.1 A is not a standard OCPD rating, and the next higher standard size is 20 A, so 240.4(B) applies.
- The 30 A breaker on the 10 AWG is acceptable through 240.4(B): 25.5 A does not correspond to a standard rating and the next size up is 30 A, which is also the 240.4(D) ceiling for 10 AWG.
3. Give two compliant fixes and explain why they work.
Read the worked answer
Option 1 - Split the pull into two conduits. Four CCC per conduit gives an 80% adjustment: 30 A × 0.91 × 0.80 = 21.8 A for 12 AWG, which now exceeds the 20 A breaker. The 10 AWG in its own raceway: 40 × 0.91 × 0.80 = 29.1 A, still acceptable on a 30 A breaker under 240.4(B).
Option 2 - Upsize the 12 AWG to 10 AWG. 40 A × 0.91 × 0.70 = 25.5 A, comfortably above 20 A. The 75 °C termination limit for 10 AWG (35 A) is not reached, and the conduit fill stays acceptable (six 10 AWG plus two 10 AWG = 8 × 0.0211 = 0.169 in², still under the 0.213 in² allowance).
Either fix must also preserve the equipment grounding conductor for each circuit and keep the rooftop disconnect and the panel terminations rated for the conductors used. What the crew must not do is simply install a 15 A breaker to 'make the math work' on the two receptacle circuits - that would defeat the circuit design rather than correct the installation.