Multiwire Branch Circuits in a Crowded Ceiling Run
A retail tenant space is fed from a 208Y/120 V, three-phase, four-wire panelboard. Two three-phase, four-wire multiwire branch circuits (six ungrounded conductors and two neutral conductors) plus one equipment grounding conductor are to be installed in a single run of 1 in. EMT about 90 ft across a ceiling. The circuits supply receptacle outlets and LED drivers and are protected by 20 A breakers. The measured ambient at the top of the ceiling during summer is 118°F. The contractor proposes 12 AWG THHN copper with 20 A breakers and argues that the neutrals need not be counted as current-carrying conductors because the loads are balanced.
1. How many current-carrying conductors must be counted in this raceway, and why?
Read the worked answer
Ungrounded conductors: 6 - all six carry load current and are counted.
Neutrals: 2 - and under 310.15(E) they generally do count here.
The neutral of a four-wire, three-phase wye circuit is normally excluded from the count because the fundamental currents cancel. But that exclusion does not apply where the load is a major portion of nonlinear loads - LED drivers, electronic power supplies, and similar equipment produce triplen harmonics that add in the neutral instead of canceling. A retail lighting/receptacle tenant build-out with LED drivers is exactly that case, so both neutrals are counted.
Count used: 8 current-carrying conductors.
- 310.15(C)(1) adjustment for 7–9 conductors = 70%.
- The EGC is never counted.
Why it matters here: the argument that "the loads are balanced, so the neutral does not count" is a common shortcut that ignores the harmonic caveat. Interestingly, this project fails either way - see the next task - but the correct count is still 8, and the documentation should show it.
2. Calculate the adjusted ampacity of 12 AWG THHN in this installation and determine whether the proposal works.
Read the worked answer
Step 1 - Base ampacity, 12 AWG THHN
90°C column = 30 A; 75°C column = 25 A (termination limit).
Step 2 - Ambient correction, 310.15(B)(1)
118°F = 48°C → 46–50°C band. For a 90°C conductor: factor 0.82.
Step 3 - Conductor adjustment, 310.15(C)(1)
8 current-carrying conductors → 0.70.
Step 4 - Adjusted ampacity
30 A × 0.82 × 0.70 = 17.2 A
That is below the 20 A branch-circuit rating → noncompliant. The conductor is no longer protected for its load; the adjusted ampacity must be at least the branch-circuit rating (and at least the load).
Sanity check if the neutrals were (incorrectly) excluded:
30 A × 0.82 × 0.80 (6 conductors) = 19.7 A → still below 20 A.
So the installation fails regardless of how the neutrals are counted - but the count must still be documented correctly for the record and for future load increases.
Termination check: even at 17.2 A, the 75°C limit of 25 A is not the governing problem; the adjusted ampacity is.
3. What is the minimum compliant fix, and does the 1 in. EMT need to be replaced? Show the fill check.
Read the worked answer
Conductor fix: upsize to 10 AWG Cu THHN/THWN-2
- 10 AWG THHN, 90°C column = 40 A
- Ambient 0.82 × adjustment 0.70 → 40 × 0.82 × 0.70 = 23.0 A ≥ 20 A ✔
- 75°C termination column for 10 AWG = 35 A, well above 20 A ✔
- 240.4(D) permits a 20 A device on 10 AWG ✔
If the neutrals were not counted, 10 AWG would give 26.2 A - still compliant, so 10 AWG is the robust answer either way.
Fill check (Chapter 9, Table 1 and Table 4)
- 10 AWG THHN area ≈ 0.0211 sq in (Chapter 9, Table 5).
- 8 current-carrying 10 AWG = 8 × 0.0211 = 0.1688 sq in.
- EGC per 250.122 for a 20 A circuit = 12 AWG (0.0133 sq in), total ≈ 0.1821 sq in.
- 1 in. EMT at 40% (over 2 conductors) = 0.346 sq in.
0.1821 < 0.346 → the existing 1 in. EMT is more than adequate; even 3/4 in. EMT (40% = 0.220 sq in) would work. Do not upsize the raceway - upsize the conductors.
Related requirements to verify in the field:
- 210.4(B): each multiwire branch circuit needs a means to simultaneously disconnect all ungrounded conductors (handle ties or a 3-pole breaker).
- 300.15 / 314.28: pull points and box sizing for the 90 ft run.
- Voltage drop advisory: at 20 A over 90 ft, 10 AWG gives roughly 4.5 V on a 120 V circuit (≈3.7%); 8 AWG brings it near 2.3%. Consider 8 AWG for the longest home runs even though 10 AWG is code-minimum.