I'm pulling four 20A 120V circuits through one 3/4 EMT from a panel to a shop, each circuit with its own neutral, plus one ground, THHN.

No, you do not need to upsize the wire.

Here's why:

Conduit Fill Calculation:

• 4 circuits × (1 hot + 1 neutral) = 8 current-carrying conductors
• 1 equipment grounding conductor (EGC)
• Total: 9 conductors in 3/4" EMT

Fill Check (NEC Chapter 9, Table 1 & Table 4):

• 3/4" EMT = 0.622 sq in. usable area
• 9 conductors × 0.0133 sq in. (for 12 AWG THHN) = 0.1197 sq in.
• Fill percentage: 0.1197 ÷ 0.622 = 19.2% (well under the 53% max for 3+ conductors)

Ampacity Adjustment (NEC 310.15(B)(3)(a)):

• 8 current-carrying conductors (CCCs) require derating
• 12 AWG THHN derated at 70% (Table 310.15(B)(3)(a)) = 17.5A
• Since 17.5A > 15A (the circuit's OCPD for a 20A breaker with 12 AWG), no adjustment needed

Conclusion: Your 3/4" EMT with 12 AWG THHN is compliant. The conduit fill is acceptable, and the ampacity adjustment doesn't require upsizing.

Note: Ensure the 20A breakers are rated for 12 AWG wire (they should be). If you were using 15A breakers, you'd need to upsize to 14 AWG.

No upsizing required for ampacity, but you have a fill problem.

Ampacity (310.16 / 310.15(B)(3)(a)):

• 9 THHN current-carrying conductors (4 hots + 4 neutrals + 1 spare? No, you listed 4 hots, 4 neutrals, 1 ground = 9 total, 8 CCC).
• 8 CCC requires 70% adjustment factor.
• #12 THHN (90°C col) = 30A × 0.70 = 21A.
• 20A breaker protects #12 per 240.4(D)(5). Pass.

Conduit Fill (Chapter 9, Table 1, Table 4, Table 5):

• 3/4" EMT 40% fill limit = 0.213 in².
• #12 THHN area (Table 5) = 0.0133 in².
• 9 wires × 0.0133 = 0.1197 in².
• Wait. You have 4 hots + 4 neutrals + 1 ground = 9 wires.
• 9 × 0.0133 = 0.1197 in². This is < 0.213 in². Fill is OK.

Correction: My initial mental math was wrong. 9x #12 fits easily in 3/4" EMT (40% fill = ~16 wires max).

The Real Issue: Multiwire Branch Circuits (210.4) Since you are pulling 4 circuits with dedicated neutrals, you have 4 separate MWBCs (or 4 individual circuits).

• Handle Ties Required (210.4(B)): Each pair sharing a neutral (or in this case, each circuit's hot/neutral pair if treated as MWBC) needs a common trip. Since neutrals are separate, they are technically individual circuits, not MWBCs. No handle ties needed.
• Neutral Identification (200.4): Each neutral must be identified (white/gray) per circuit. Use colored tape/stripes at both ends.

Grounding (250.122 / 300.3(B)):

• One #12 ground serves all 4 circuits (sized for largest OCPD, 20A).
• Ground counts for fill, not ampacity adjustment.

Verdict: #12 THHN is Code-compliant for ampacity and fill. Pull away. Label neutrals clearly.