AmpSizerWhat size Romex do you need?
Romex (NM-B) size
12 AWG copper conductors
that is 12/2 or 12/3 cable on a 20 A breaker · the ground is already in the jacket
2.6% drop at 40 ft
- Ampacity wants12 AWG
- Voltage drop wants12 AWG
- Current the conductor has to carry
- 20 A
- Usable ampacity after derating
- 20 A (table 20)
- Longest run before the drop passes 3%
- 45 ft
- In metric sizes
- 3.3 mm²
— had the ampacity and 240.4(D) still rejected it, at 0 A.
Romex is for dry, indoor locations only. Outdoors or underground it is UF, THWN in conduit, or SER.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
One circuit check a month
MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What size Romex do you need for a circuit?
The AmpSizer Romex result for a 20 amp circuit is 12/2 NM-B — 12 AWG copper, which is 20 amps in the 60 °C column. Over a 40 ft run at 120 V that drops 2.6%, so distance is not the constraint here.
The 60 °C column is the whole story with this cable. NM-B has 90 °C conductors inside it, and 334.80 makes you use the 60 °C ampacity anyway. That is why 12 AWG is 20 A in Romex and 25 A as THHN in conduit, with the same copper inside.
Two conductors plus a ground is written 12/2 and has a black, a white and a bare. 12/3 adds a red, for a three-way switch, a shared neutral or a 240 V circuit that also needs 120.
The short version of this whole page is a five-row table, and it is worth memorising because it covers almost every circuit in a house.
| Cable | Copper | Breaker | What it normally feeds |
|---|---|---|---|
| 14/2 | 14 AWG | 15 A | Lighting, bedroom and living room receptacles |
| 12/2 | 12 AWG | 20 A | Kitchen and bathroom receptacles, garage, laundry |
| 12/3 | 12 AWG | 20 A | Three-way switching, shared-neutral pairs |
| 10/3 | 10 AWG | 30 A | Electric dryer, small water heater |
| 8/3 | 8 AWG | 40 A | Electric range, cooktop |
| 6/3 | 6 AWG | 55 A | Large range, some sub feeds |
The breaker column is the 60 °C ampacity capped by 240.4(D), which for 14, 12 and 10 gauge is the same number either way. From 8 AWG up the cap disappears and the 60 °C figure governs on its own — which is why 8/3 is a 40 A cable and not a 50 A one, even though 8 AWG THHN in conduit carries 50.
When Romex is the wrong answer
Outside, or anywhere damp. NM-B is dry-location cable. Use UF, or individual conductors in conduit.
Exposed where it can be damaged. Garages, workshops and unfinished basements usually want it in conduit or behind protection, and some jurisdictions simply do not allow it exposed at all.
In commercial buildings. Many occupancy types prohibit NM entirely, and the substitute is MC cable or conduit. This is a local rule as much as a national one, and it is worth asking before you buy a reel of it.
Frequently asked questions
What size Romex for a 20 amp circuit?
12/2 for a normal receptacle or lighting circuit, 12/3 if it needs a second hot or a travelling conductor. Never 14 gauge on a 20 amp breaker: 14 AWG is capped at 15 A by 240.4(D), and a 14-gauge cable on a 20 A breaker is the single most common wiring fault found in older houses.
Can I use 14 gauge on a 20 amp breaker?
No. 240.4(D) caps 14 AWG copper at 15 A regardless of what the ampacity table says, so a 14-gauge cable needs a 15 A device. This comes up constantly when somebody replaces a tripping 15 A breaker with a 20: the breaker stops tripping and the cable is now unprotected.
Why is 12/2 Romex only 20 amps when 12 AWG THHN is 25?
Because 334.80 makes non-metallic sheathed cable use the 60 °C column, even though the conductors inside are rated 90 °C. The sheath is the reason: the assembly is not built to shed heat the way individual conductors in a raceway are. The 90 °C rating is only there for derating arithmetic.
Does Romex get derated in insulation?
Yes, and it is easy to miss. NM cable run through thermal insulation or bundled with other cables for more than 24 inches picks up the adjustment factors of 310.15(C)(1), so a stack of cables leaving a panel through one hole and buried in blown insulation is a real derating case.
Is 10/3 the same as 10/2 with a ground?
No. The number after the slash counts the insulated conductors, and the ground is not counted — 10/2 is black, white and bare, 10/3 is black, red, white and bare. So 10/3 has four conductors in it and costs noticeably more, which is why it is worth being sure you need the red one.
How we calculate this
2 × amps × R × one-way feet ÷ 1,000 on the Chapter 9 Table 8 resistance
figures. The device is the standard rating from 240.6(A), capped by 240.4(D) at 15 A for 14 AWG, 20 A
for 12 AWG and 30 A for 10 AWG. The equipment grounding conductor is already in the cable.Worked example
With Load current 20, One-way run 40, System voltage 120 V, Metal Copper, this page works out 12 AWG copper conductors. that is 12/2 or 12/3 cable on a 20 A breaker · the ground is already in the jacket
- Current the conductor has to carry20 A
- Usable ampacity after derating20 A (table 20 )
- Longest run before the drop passes 3%45 ft
- In metric sizes3.3 mm²
Sources:
The markings on the jacket
NM-B is the modern non-metallic sheathed cable, dry indoor locations only. UF-B looks similar and is rated for direct burial and wet locations. They are not interchangeable and the jacket says which is which.
Colour is a convention, not a code rule. White jackets are usually 14 gauge, yellow 12, orange 10 and black 8 or 6 — a manufacturers’ habit that has become useful shorthand. Read the printed size anyway; imported and older cable does not follow it.
Stapling and protection are where NM installations fail inspection, not sizing: within 12 inches of a box, every 4.5 feet, and a nail plate wherever it passes closer than 1.25 inches to the face of a stud.