Square D 20 Amp Breaker — Wire Size, Install & Specs
A Square D 20 amp breaker takes 12 AWG copper on a 120 V branch circuit; the company's QO and Homeline lines carry a 10 kA interrupting rating and torque to 20 in-lb. This page covers wire-size choices for every common breaker rating from 20 A through 150 A, the install step-by-step, AIC ratings, and Square D product variants. Reviewed by a licensed PE.
What an amp breaker does
A circuit breaker is an automatic switch that opens the circuit when current exceeds its rating, protecting conductors and downstream equipment from overheating and short-circuit damage. The "amp rating" stamped on the handle (15, 20, 30, 50, 70, 100…) is the continuous current the breaker will carry indefinitely without tripping.
Square D — owned by Schneider Electric since 1991 — is the dominant North American residential and light-commercial brand. Their QO ("Qwik-Open") product line, introduced in 1955, is recognized by its distinctive plug-on design and visible trip indicator window. Hugo Stotz patented the thermal–magnetic miniature circuit breaker in 1924, and the modern breaker still uses his bimetallic + electromagnetic combination.
Sizing formulas — breaker, conductor, voltage drop
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- I_breaker = Required breaker rating, A
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- I_load,cont = Continuous load (3 hr or more), A
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- I_load,nc = Non-continuous load, A
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- I_cond = Conductor ampacity from Table 310.16, A
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- I_breaker = Breaker rating, A
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- Wire must equal or exceed breaker — except small conductor rule (NEC 240.4(D))
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- V_d = Line-to-line voltage drop, V
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- L = One-way length, m
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- I = Load current, A
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- R_ac = Conductor AC resistance, Ω/km
Reference values — breaker amp rating to wire size
| Breaker (A) | Cu wire (NEC 310.16, 75 °C) | Al wire | Typical use |
|---|---|---|---|
| 3 | 14 AWG | n/a | Control panel, transformer secondary |
| 15 | 14 AWG | 12 AWG | Lighting, outlets in dwellings |
| 20 | 12 AWG | 10 AWG | Kitchen, bath, garage outlets |
| 30 | 10 AWG | 8 AWG | Dryer, A/C, water heater |
| 40 | 8 AWG | 6 AWG | Range, large A/C |
| 50 | 6 AWG | 4 AWG | EV charger, range, sub-panel |
| 55 | 6 AWG | 4 AWG | Industrial motor branch |
| 60 | 6 AWG | 4 AWG | Sub-panel feeder, hot tub |
| 70 | 4 AWG | 3 AWG | Sub-panel feeder |
| 80 | 4 AWG | 3 AWG | Industrial branch |
| 90 | 3 AWG | 2 AWG | Industrial branch |
| 100 | 3 AWG | 1 AWG | Sub-panel main |
| 125 | 1 AWG | 2/0 AWG | Service feeder |
| 150 | 1/0 AWG | 3/0 AWG | Square D QO panel main |
Standards covering molded-case and miniature breakers
| Document | Scope | Key clause |
|---|---|---|
| NFPA 70 (NEC) 240.4 | Conductor protection | Wire ampacity ≥ breaker rating |
| NEC 210.20 | Branch-circuit protection | 1.25 × continuous load factor |
| NEC 110.3(B) | Listed equipment | Use per the manufacturer's instructions |
| UL 489 | Molded-case circuit breakers | Square D QO/Homeline qualification |
| UL 943 | GFCI breakers | 4–6 mA trip threshold |
| UL 1699 | AFCI breakers | Series + parallel arc detection |
| IEC 60898 | European miniature breakers | B / C / D trip curves |
- Turn off the main Switch the main service disconnect OFF and lock-out / tag-out. Verify zero voltage at the panel bus with a non-contact tester before opening the deadfront.
- Pull the correct wire For a 20 A circuit, run 12 AWG copper THHN/NM-B (12-2 + ground for a single-pole 120 V circuit). Strip 5/8 in (16 mm) of insulation from each conductor.
- Snap the breaker on the bus Hook the back of the breaker on the panel mounting rail, swing it onto the hot stab. For a Square D QO breaker, you should hear a positive click. Confirm the breaker handle moves freely.
- Land the conductors Insert the bare hot under the breaker lug and torque to 20 in-lb (Square D QO spec). Insert the white neutral under the neutral bar at the same torque, and the bare ground under the ground bar.
- Energize and verify Re-install the deadfront, restore main power, switch the new breaker ON. Confirm 120 V hot-to-neutral at the receptacle and < 1 V hot-to-ground. Test the AFCI/GFCI function if installed.
Worked example — sizing the 240 V 30 A breaker for a tankless water heater
A 5.5 kW tankless heater drawing 5500 / 240 = 22.9 A continuous on a 240 V circuit. Apply NEC 210.20(A): 1.25 × 22.9 = 28.6 A → next standard size up is the 30 A breaker. Conductor: 10 AWG copper THHN (35 A ampacity at 75 °C) ≥ 30 A breaker, so wire is sized correctly. Run length 18 m, voltage drop: 2 × 18 × 22.9 × 0.00328 / 1000 of a kilometre = 2.7 V, which is 1.1 % — well under the 3 % branch-circuit limit.
Comparison — Square D QO vs. Homeline vs. PowerPact
| Aspect | QO | Homeline | PowerPact (B/H/J/L) |
|---|---|---|---|
| Market | Premium residential / light commercial | Builder-grade residential | Industrial / commercial MCCB |
| Mounting | Plug-on bus stab | Plug-on bus stab | DIN rail or bolt-on |
| Standard AIC @ 240 V | 10 kA | 10 kA | 25–65 kA (frame dependent) |
| High AIC option | QO-H 22 kA | HOM-H 22 kA | up to 200 kA (L-frame) |
| Trip indicator | Yes (visible window) | Position only | Yes + alarm contacts |
| Lug torque (20 A) | 20 in-lb | 15 in-lb | Per frame (35–80 in-lb) |
Square D breaker variants — 70 A, 150 A, and high-current sizes
Square D 70 amp breaker
The QO270 (two-pole 70 A) is a 240 V double-pole breaker often used for sub-panel feeders or larger HVAC outdoor units. It pairs with 4 AWG copper THHN at 75 °C terminations. AIC rating is 10 kA standard; QO270H lifts that to 22 kA.
Square D 150 amp breaker
Single 150 A panel mains use the QOM2150VH or PowerPact HJL150 frame. They accept 1/0 AWG copper or 3/0 AWG aluminum and torque to 250 in-lb. For whole-house generator transfer setups, this is a common interlock-kit size.
240 V 30 A and 240 V 50 A breakers
A 240 V 30 A breaker handles dryers (NEMA 14-30 outlet), small EV chargers, and tankless water heaters. A 240 V 50 A breaker is the standard for ranges (NEMA 14-50) and Level-2 EV chargers up to 11.5 kW. Both are double-pole units occupying two stab positions in the panel.
3 A breaker — control circuits
At the small end, 1–3 A miniature breakers (Square D Multi 9, IEC 60898 C-curve) protect 24 V control transformers, PLC power supplies, and instrument loops. These are DIN-rail mount, not panel plug-on units.
- How to wire 20 amp breaker?
- Run 12 AWG copper conductors (12-2 NM-B with ground for a 120 V circuit, 12-3 if it is a multi-wire branch circuit), strip 5/8 in of insulation, land the hot under the breaker lug torqued to 20 in-lb (Square D QO), the white neutral on the neutral bar, and the bare copper on the ground bar. Snap the breaker onto the bus stab, restore the main, and switch the new breaker on. The receptacle should read 120 V ± 5 % hot-to-neutral.
- What size wire for a 60 amp breaker?
- 6 AWG copper or 4 AWG aluminum is the NEC 310.16 75 °C choice for a 60 A breaker (65 A ampacity, derated to the next standard breaker size below). For a sub-panel feeder where the run length is significant, many designers up-size to 4 AWG copper to absorb voltage drop on a 200-foot run.
- What size wire for an 80 amp breaker?
- 4 AWG copper or 3 AWG aluminum (THHN/THWN-2) is the NEC 75 °C choice — 4 AWG copper is rated 85 A and 3 AWG aluminum is rated 100 A. Always confirm the panel terminal temperature rating; lugs rated 60 °C only force you to use 3 AWG copper instead.
- What size wire for a 90 amp breaker?
- 3 AWG copper (100 A at 75 °C) or 2 AWG aluminum (100 A at 75 °C). For runs longer than 75 ft on a 240 V circuit, up-size to 2 AWG copper to keep voltage drop under 3 %.
- What is the AIC rating of a Square D QO 20 A breaker?
- The standard Square D QO single-pole 20 A breaker has an interrupting rating of 10 kA (10 000 A) at 120/240 V AC. The QO-H (high-interrupting variant) reaches 22 kA at 120/240 V. The matching Schneider PowerPact MCCB jumps to 25–65 kA depending on frame size.
The continuous load on a branch circuit shall not exceed 80 percent of the rating of the overcurrent device, and the conductor ampacity shall be not less than the rating of the overcurrent device.
Sources
- NFPA 70 — National Electrical Code, Articles 210, 240, 408 (2023).
- UL 489 — Molded-Case Circuit Breakers, 14th edition.
- Square D QO / Homeline product catalogue (Schneider Electric, 2024).
- IEEE Std 141 — Red Book, Chapter 9.
- UL 943 — Standard for GFCI; UL 1699 — AFCI standard.
- IEC 60898-1 — European MCB performance standard.