Adding a 36 A Continuous Load to an Already Derated Raceway
A journeyman is adding a 208 V, single-phase resistance heat bank to an existing commercial tenant space. The nameplate shows 36 A continuous. The only available raceway is a 1 in. EMT that already carries three THHN copper current-carrying conductors; the new circuit adds two more in the same 18 in. section, for five current-carrying conductors. The ceiling plenum where the EMT runs is measured at 40 °C (104 °F) at the worst case. The panelboard has 75 °C terminations and a spare 40 A single-pole breaker, and the shop has 8 AWG and 6 AWG THHN on the truck. The foreman says, '8 AWG is good for 50 A, so a 40 A breaker is fine - just pull it.' The heat bank must run unattended for six hours at a time, and the owner wants it energized Friday. The journeyman has to decide whether to follow the foreman or push back, and needs the calculation to make the case.
1. Calculate the adjusted ampacity of 8 AWG THHN in this raceway and decide whether the foreman's plan is code-compliant for a 36 A continuous load.
Read the worked answer
Start with the 90 °C ampacity because THHN insulation is rated 90 °C: 8 AWG copper = 55 A (Table 310.16).
Apply the two adjustment factors required by 310.15:
- Ambient 40 °C: 0.91 for 90 °C conductors (Table 310.15(B)(1)).
- Five current-carrying conductors in one raceway: 0.80 (310.15(B)(3)(a), 4–6 conductors).
55 A × 0.91 × 0.80 = 40.0 A.
That is not the whole answer. The terminations are 75 °C, so the 75 °C column value for 8 AWG (50 A) also caps the usable ampacity; the lower of the derated value and the termination value governs, so this circuit is limited to 40 A.
The load is 36 A continuous. Under 210.19(A)(1) and 210.20(A), conductors and the overcurrent device must be sized at 125% of the continuous load: 36 × 1.25 = 45 A. A 40 A breaker is below 45 A, and the adjusted conductor ampacity of 40 A is also below 45 A. The foreman's plan fails on both the overcurrent device and the conductor. It is not a 50 A conductor in this installation; it is a 40 A conductor with a 45 A requirement.
2. Select the minimum conductor and overcurrent device from the 8 AWG and 6 AWG THHN on the truck, showing each step.
Read the worked answer
Required ampacity = 125% of 36 A continuous = 45 A.
6 AWG THHN, 90 °C column = 75 A. Apply 0.91 × 0.80 = 0.728; 75 × 0.728 = 54.6 A. The 75 °C termination value for 6 AWG is 65 A, so the governing ampacity is 54.6 A, which exceeds 45 A.
Choose a 45 A breaker (a standard size under 240.6). A 50 A breaker is also within the conductor's capability only if the continuous load does not exceed 50 × 0.80 = 40 A; 36 A is under that, so 50 A would work, but 45 A is the tight, correct choice at 125%.
8 AWG is out: its adjusted ampacity is 40.0 A, below the 45 A requirement. Do not 'round up' to the 50 A 75 °C column or to the 55 A 90 °C column; the ambient and bundle factors reduce the usable ampacity below the load requirement.
The spare 40 A breaker is not usable for this continuous load, regardless of conductor size.
3. If the panelboard terminations were rated 60 °C rather than 75 °C, would the answer change?
Read the worked answer
The numbers change, but the conclusion does not.
At 60 °C, 8 AWG copper is 40 A (Table 310.16). That is below the 45 A requirement even before any derating, so 8 AWG is still unacceptable.
At 60 °C, 6 AWG copper is 55 A. The derated 90 °C ampacity is 54.6 A, so the lower value is 54.6 A, still above 45 A. 6 AWG with a 45 A breaker remains acceptable.
The rule to state in the answer: circuit ampacity is the lowest of (a) the derated 90 °C ampacity, (b) the ampacity from the termination temperature column, and (c) any equipment listing limitation. If the calculated value fell between the 60 °C termination ampacity and the derated 90 °C value, the 60 °C number would govern and a larger conductor would be required.