30 amp wire size
A 30 amp circuit is normally wired with 10 AWG copper. In NM-B cable, 10 AWG copper is rated exactly 30 A in the 60 °C column, and NEC 240.4(D) sets 30 A as the most it may be protected at, so the two limits meet at the same breaker.
Minimum wire size for 30 A
| Conductor | 60 °C column | 75 °C column |
|---|---|---|
| Copper | 10 AWG | 10 AWG |
| Aluminum | Not listed | 8 AWG |
Usually chosen: 10 AWG copper, as 10/3 NM-B (orange sheath) for a dryer, 10/2 NM-B for a water heater, or 10 AWG THHN in conduit. In aluminum the minimum is 8 AWG.
Breaker rating taken as the required ampacity; 240.4(D) limits applied. The charts list no 60 °C aluminum, and 60 °C copper only to 2 AWG (95 A), the largest NM-B. Manufacturer chart values (2017 NEC basis). Verify against the NEC edition adopted locally.
This rating serves the classic 240 V appliances of a house: the electric clothes dryer and the electric water heater. The answer box gives the minimum for copper and aluminum; below it are the appliance rules that lead to 30 A, the cable choice for a dryer, and the drop on longer runs.
Where 30 amp circuits are used
Electric dryers are the most common 30 A load. The NEC load calculation counts a household dryer at 5,000 W or its nameplate rating, whichever is larger (220.54), and most residential dryers are built for a 30 A, 120/240 V circuit with a NEMA 14-30 receptacle.
A tank-type electric water heater is the other. NEC 422.13 treats a storage water heater of 120 gallons or less as a continuous load, so the branch circuit is rated at least 125% of the nameplate. A typical 4,500 W, 240 V element draws 18.75 A; 125% is about 23.4 A, which rounds up to a 25 A or, far more often, a 30 A breaker with 10 AWG copper.
Other 30 A circuits include central air conditioners and heat pumps whose nameplate lists a minimum circuit ampacity in the mid-20s, small electric furnaces and duct heaters, 240 V shop equipment such as table saws and compressors, and the 120 V NEMA TT-30 outlet used for travel trailers and small RVs. For HVAC equipment the nameplate's minimum circuit ampacity and maximum overcurrent protection govern instead of general rules.
Voltage drop at 30 A
Loaded to 30 A, 10 AWG copper stays within 3% to about 96 ft at 240 V and only about 48 ft at 120 V. Dryer and water heater runs inside a house are usually well under the 240 V limit; a TT-30 RV pedestal at the end of a long driveway is where 8 AWG or 6 AWG becomes necessary.
A dryer seldom draws its full rating for the whole cycle, so the table overstates the typical drop. A water heater, by contrast, really does run near its nameplate current for long periods, which makes the full-load figures a fair guide.
| One-way length (ft) | Drop at 120 V (%) | Drop at 240 V (%) | Copper for 3% at 120 V | Copper for 3% at 240 V |
|---|---|---|---|---|
| 25 | 1.55 | 0.77 | 10 AWG | 10 AWG |
| 50 | 3.10 | 1.55 | 8 AWG | 10 AWG |
| 75 | 4.65 | 2.32 | 8 AWG | 10 AWG |
| 100 | 6.20 | 3.10 | 6 AWG | 8 AWG |
| 150 | 9.30 | 4.65 | 4 AWG | 8 AWG |
| 200 | 12.39 | 6.20 | 3 AWG | 6 AWG |
| 250 | 15.49 | 7.75 | 2 AWG | 4 AWG |
| 300 | 18.59 | 9.30 | 2 AWG | 4 AWG |
Full rating on a single-phase circuit, copper at 75 °C, stranded.
Breaker pairing
10 AWG copper pairs with a breaker of 30 A or less. A dryer outlet is a two-pole 30 A breaker; a water heater with a 4,500 W element may sit on a 25 A or 30 A two-pole breaker depending on the 125% result and local practice.
There is no next-size-up option at this size. 10 AWG copper at 75 °C reads 35 A, but 240.4(D) caps it at 30 A, so a 35 A or 40 A breaker on 10 AWG is not permitted unless a specific rule, such as the motor and air-conditioning provisions of Articles 430 and 440, allows it.
NM-B cable or THHN in conduit
A modern dryer circuit is four-wire: two hot conductors, a neutral for the 120 V timer, motor and lights, and a separate equipment grounding conductor. That means 10/3 NM-B with ground. Since 1996 the NEC has required the four-wire arrangement for new dryer circuits (250.140); older three-wire circuits, with the neutral grounding the frame, may remain where they already exist.
A water heater has no 120 V load, so 10/2 NM-B with ground is enough, and the white conductor is re-marked as a hot conductor at both ends. In conduit, 10 AWG THHN is common for commercial and garage runs, and its 90 °C rating is useful where several circuits share the conduit.
Copper or aluminum
On the manufacturer charts 10 AWG aluminum reads 30 A at 75 °C, but 240.4(D) limits 10 AWG aluminum to 25 A, so a 30 A circuit in aluminum needs 8 AWG.
Aluminum conductors this small are unusual, and dryer receptacles and water heater terminals are often not rated for them. Where aluminum is used, the terminations must be marked AL/CU and the conductors made from the AA-8000 series alloys the NEC requires for aluminum building wire.
Common mistakes
Reusing a three-wire dryer circuit for a new install
A three-wire 10/2 circuit with no separate ground is allowed to remain only where it already exists. A new dryer circuit, or one being extended, is run as 10/3 with ground to a 14-30 receptacle.
Putting a 40 A breaker on 10 AWG for a heater
When a breaker trips on a water heater, the answer is to find the cause, not to raise the breaker. 10 AWG copper may never be protected above 30 A under 240.4(D).
Forgetting the water heater's 125%
A 5,500 W element draws about 22.9 A at 240 V; at 125% that is about 28.6 A, which still fits a 30 A circuit, but a larger tankless unit will not and needs its own calculation.
Mismatching the receptacle
A 30 A circuit requires a 30 A receptacle under 210.21(B)(3). Fitting a 50 A receptacle to a 30 A circuit, a common shortcut at RV hookups, invites a 50 A plug onto wire sized for 30 A.
Frequently asked questions
Is 10 gauge wire good for 30 amps?
Yes. 10 AWG copper is rated 30 A in the 60 °C column and NEC 240.4(D) permits it on a 30 A breaker, which makes it the standard size for 30 A circuits.
What size wire does a dryer need?
Most electric dryers need a 30 A, 120/240 V circuit wired with 10/3 copper cable with ground and a NEMA 14-30 receptacle. The dryer's installation instructions confirm the rating.
Can 12 gauge wire be used on a 30 amp breaker?
No. 12 AWG copper is limited to 20 A protection by 240.4(D). A 30 A breaker requires 10 AWG copper or larger, with the exceptions written for motor and air-conditioning circuits.
What wire size is needed for a 30 amp RV outlet?
A TT-30 RV outlet is 120 V, 30 A and is wired with 10 AWG copper. Because the full current flows at 120 V, long runs to a parking pad often need 8 AWG or 6 AWG to hold the voltage up.
Does a water heater need a 30 amp circuit?
Many do. A 4,500 W or 5,500 W, 240 V water heater is sized at 125% of its nameplate current, which leads to a 25 A or 30 A circuit, normally wired with 10 AWG copper.