No, you do not need to upsize the wire for the four 20A 120V circuits in a single 3/4" EMT conduit, as long as you are using THHN/THWN-2 conductors and following NEC requirements.
Here’s the breakdown:
- Each circuit: 20A, 120V → requires 12 AWG copper THHN (rated for 25A at 90°C, but limited to 20A by the 60°C termination rating per NEC 110.14(C)(1)(a)).
- You have:
- 4 circuits × 2 conductors (hot + neutral) = 8 current-carrying conductors
- 1 equipment grounding conductor (EGC) = 1 non-current-carrying conductor → Total: 9 conductors in the 3/4" EMT.
Conduit Fill Check (NEC Chapter 9, Table 1 & Table 4):
- 3/4" EMT has a 40% fill area of 0.213 in² (for more than 2 wires).
- Area of one 12 AWG THHN: 0.0133 in² (NEC Chapter 9, Table 5).
- 9 × 0.0133 = 0.1197 in² → well under 0.213 in². ✅ Conduit fill is acceptable.
Derating Check (NEC 310.15(C)(1)):
- More than 3 current-carrying conductors require derating.
- You have 8 current-carrying conductors (4 hots + 4 neutrals).
- Derating factor for 4–6 conductors: 80%
- Derating factor for 7–9 conductors: 70% (NEC Table 310.15(C)(1))
- 12 AWG THHN: 90°C ampacity = 25A
- Derated: 25A × 0.70 = 17.5A → This is below 20A!
Problem: The derated ampacity (17.5A) is less than the 20A breaker rating → violation of NEC 240.4(D) and 310.15.
Solution: You must either:
- Reduce the number of current-carrying conductors (e.g., use multi-wire branch circuits with shared neutrals — but only if properly handled per 210.4), or
- Upsize the wire to compensate for derating.
Recommended Fix: Use 10 AWG THHN for the hots and neutrals:
- 10 AWG THHN: 90°C ampacity = 35A
- Derated: 35A × 0.70 = **24.5A