40 Amp Wire Size

The wire size for a 40 amp breaker is 8 AWG copper on 75°C terminations, which is what almost every residential breaker and panel lug is listed for. Aluminum is also 8 AWG. That is the whole answer for a standard installation. The reasoning behind it is the part most charts leave out, and it is what tells you when the standard answer stops applying.

40 Amp Wire Size in Copper

8 AWG copper is rated 50 amps in the 75°C column of NEC Table 310.16. A 40 amp breaker sits ten amps under that, a margin of 25 percent, and that headroom is the reason 8 AWG is a comfortable rather than a marginal choice at this current. It is also why a 40 amp circuit in 8 AWG copper tolerates a moderate amount of derating — a warm attic, or a fourth current-carrying conductor in the raceway — before it stops being adequate.

The temperature column matters more than the conductor's insulation. NEC 110.14(C) sets the ampacity by the lowest-rated termination in the circuit, so 8 AWG THHN rated 90°C landing on a breaker listed 75°C is a 75°C circuit and reads 50 amps, not the 55 amps its insulation would allow. In the 60°C column the same conductor reads exactly 40 amps, which is the figure that governs NM-B and UF-B cable.

40 Amp Wire Size in Aluminum

8 AWG aluminum is rated 40 amps at 75°C. That meets a 40 amp load exactly, with nothing left over.

This is a genuine judgement point rather than a technicality. The conductor is legal — an ampacity equal to the load satisfies the code — but every allowance that reduces ampacity now takes the circuit below its breaker. Run that conductor through an attic above 30°C, or bundle it with a second circuit in the same conduit, and the corrected ampacity drops under 40 amps while the breaker stays at 40. The copper equivalent absorbs the same correction and still clears.

In practice, aluminum at 40 amps is uncommon for a different reason: the savings that make aluminum worth the extra handling appear on long, heavy runs, and a 40 amp branch circuit is neither. Where aluminum does appear at this size it is usually a feeder to a detached structure. If you use it, size up to 6 AWG aluminum — rated 50 amps at 75°C — rather than accepting zero margin.

Why 8 AWG and Not 10 AWG

10 AWG copper reaches exactly 40 amps in the 90°C column of Table 310.16, so the number looks like it works. It does not, for two separate reasons, and the second one is absolute.

The first is that the 90°C column is almost never available. It applies only where every termination in the circuit is listed 90°C, and residential breakers and lugs are listed 60/75°C. On the 75°C column that a real circuit reads, 10 AWG copper is 35 amps — five amps short.

That rule is worth stating precisely, because readers arrive at this page expecting it to be the obstacle and it is actually the answer. 240.4(D) does not limit 8 AWG — the small-conductor rule stops at 10 AWG, so nothing caps the conductor you are going to use. What 240.4(D) does at 40 amps is eliminate the size below it. The rule does not constrain the answer; it creates it.

There is one exception, and it is narrow. NEC 240.4(D) opens with the words "Unless specifically permitted in 240.4(E) or (G)", and Table 240.4(G) sends air-conditioning and refrigeration circuit conductors to Article 440. The small-conductor rule therefore never reaches them, which is why an HVAC nameplate can legitimately call for a 40 amp breaker on a 10 AWG conductor — that equipment is sized from its nameplate, not from Table 310.16. In every other case, 10 AWG on a 40 amp breaker is wrong.

Search this question and you will find 8 AWG, 6 AWG and occasionally 10 AWG given as the answer. Three of those disagreements have a specific cause, and knowing which one is in play tells you which page to believe.

Sites That Say 6 AWG

6 AWG is the answer to the 50 amp question, and the two amperages are adjacent enough that pages conflate them. 6 AWG copper is rated 65 amps at 75°C, which is a size and a half more conductor than a 40 amp circuit needs. It is never wrong to install — oversizing is always permitted — but it costs more, it is stiffer to terminate, and on a 40 amp breaker it buys margin you already had.

There is one case where 6 AWG genuinely is the 40 amp answer: a long run where voltage drop rather than ampacity sets the size. A page that recommends 6 AWG without saying which of those two reasons applies is not giving you enough to judge it.

Sites That Say 10 AWG

10 AWG appears because 10 AWG copper reads exactly 40 amps in the 90°C column of Table 310.16, and a page reading across that row without applying NEC 240.4(D)(7) will report it. That is a real number from a real table, quoted without the rule that caps it at 30 amps. It is the most dangerous disagreement of the three, and it is the reason this page states the cap alongside the ampacity rather than in a footnote.

Sites That Give One Number for Both Materials

8 AWG is correct for copper and technically correct for aluminum, so a page can print "8 AWG" for both and be defensible. What it hides is that copper has 10 amps of margin and aluminum has none, which is the difference between a circuit that survives a derating factor and one that does not. A single figure covering both materials is accurate and incomplete at the same time.

NEC Edition

Table 310.16 has changed between editions, and pages reproduced from older code books circulate without a date. The figures here follow NFPA 70-2023, and the 8 AWG and 10 AWG copper and aluminum rows were confirmed against multiple independent reproductions before publication. A chart with no edition on it cannot be checked against yours.

A 40 amp circuit in a house is a 240 volt circuit. Not because 40 amps requires 240 volts, but because everything that draws 40 amps in a dwelling is a heating or motor load built for it.

40 Amp at 240V

A 40 amp 240 volt circuit delivers 9.6 kW. The same conductor at 120 volts delivers 4.8 kW, and there is no common household appliance that wants 4.8 kW badly enough to draw 40 amps on a single hot leg when it could draw 20 on two.

The circuit is fed by a two-pole breaker occupying two slots in the panel, picking up both busbars so the two hot conductors are 240 volts apart. A 40 amp two-pole breaker is a standard rating under NEC 240.6(A), which is part of why 40 amps is such a common design target: it is a size you can actually buy, and it is the next standard rating above the 30 amp circuit that so many appliances outgrow.

220V, 230V or 240V?

220 volts, 230 volts and 240 volts all describe the same residential circuit, and the wire size is the same for all three. The nominal supply in the United States is 120/240 volt single-phase, and that is the figure the NEC and Table 310.16 work from. The older designations — 110/220 and 115/230 — describe the same service under earlier nominal voltage standards, and they survive in conversation, in appliance nameplates and in search queries.

So 220v 40 amp wire size, 240v 40 amp wire size and 40 amp 220 wire size all have the same answer: 8 AWG copper. Nothing about the conductor changes. Voltage determines how much power the circuit delivers and how much voltage drop matters over distance; current alone determines the conductor size, and the current is 40 amps in all three cases.

Three-Wire or Four-Wire?

Whether a 40 amp circuit needs three conductors or four is decided by whether the appliance uses 120 volts internally — which is the 8/3-versus-8/2 question in its practical form.

A 240 volt load that is purely 240 volts — a hardwired EV charger, an air conditioning condenser, a tankless water heater element — needs two hot conductors and an equipment grounding conductor. That is 8/2 cable: two insulated conductors plus a ground. There is no neutral because nothing in the appliance runs at 120 volts.

An appliance with 120 volt components inside it — a range with a clock, an oven light and a receptacle, or a spa pack with 120 volt controls — needs a neutral as well. That is 8/3 cable: two hots, a neutral and a ground. A sub-panel is always four-wire, because the circuits downstream of it will include 120 volt ones.

For new work the equipment grounding conductor is not optional. NEC 250.140 requires ranges, wall ovens, counter-mounted cooking units and clothes dryers on new branch circuits to be grounded through an equipment grounding conductor; the three-wire arrangement that bonded the appliance frame to the neutral is permitted only on existing branch circuits that have no EGC available.

When You'd See 120V at 40 Amps

Essentially never in a dwelling. A 40 amp 120 volt branch circuit is not prohibited — 40 is a standard overcurrent rating and nothing in the code bars it at 120 volts — but no common residential appliance is built to draw it, so the configuration does not occur in practice.

People search for it for two reasons, and both are worth answering. The first is that they do not yet know what voltage their circuit is, which the two-pole breaker in the panel settles in a glance. The second is that they are working on a 12 volt DC system — a vehicle, a boat, a solar installation — where 40 amps at low voltage is entirely ordinary. That is a different calculation, governed by voltage drop over the run rather than by Table 310.16, and it is decided by voltage drop over the run rather than by Table 310.16. Set the calculator above to DC at 12 volts: a 3 percent budget is 0.36 volts, and 40 amps over even a short run wants far more copper than the same load at 240.

40 amps sits at a real boundary in a house: above the 30 amp dryer and water heater circuits, below the 50 amp range and welder circuits. The loads that land on it are specific, and in two cases the code provision governing them is specific to this amperage.

Electric Ranges, Wall Ovens and Cooktops

What makes the section genuinely useful is the tension between that minimum and what installers actually run. Table 220.55, Column C, gives a single range of 8¾ to 12 kW a calculated demand of 8 kW, which is 33.3 amps at 240 volts. So a 40 amp circuit is legally sufficient for a range right up to 12 kW, and it is simultaneously the code's floor for anything above 8¾ kW. Yet the common trade practice for a full-size range is 50 amps, because the incremental cost of 6 AWG over 8 AWG on a short kitchen run is small and it removes any argument about a future larger appliance.

So a 40 amp range circuit is correct, and you will also see 50 amp circuits serving identical ranges. Where 40 amps is clearly the right answer is a separately circuited wall oven or cooktop, which draw less than a combined range, and a range under 12 kW sized to its calculated demand.

One provision specific to this circuit: 210.19(C) Exception No. 2 permits the neutral of a range circuit to be smaller than the ungrounded conductors, provided it carries at least 70 percent of the branch-circuit rating and is not smaller than 10 AWG. On a 40 amp circuit that threshold is 28 amps, which 10 AWG clears. The allowance matters when the range is fed with individual conductors or SE cable; standard 8/3 NM-B carries three full-size 8 AWG conductors and the question does not arise.

Sub-Panels

A 40 amp feeder is a light sub-panel: a shed, a small detached garage, a home office. It is not enough for a workshop with a table saw and a dust collector, where 60 to 100 amps is the honest starting point, and undersizing the feeder is the mistake people make here rather than undersizing the conductor.

The feeder is 8 AWG copper and it is always four-wire — two hots, a neutral and a separate equipment grounding conductor. Three rules govern the termination at the far end, and they are the ones most often got wrong:

• The neutral bar and the ground bar in the sub-panel must be isolated from each other, and the main bonding jumper — the green screw or strap bonding the neutral bar to the enclosure — must be removed (NEC 408.40, 250.24).
• The neutral and the equipment ground are bonded together at exactly one place in the system, the service disconnect, and a sub-panel is on the load side of it (NEC 250.32(B)).
• A sub-panel in a separate structure needs its own grounding electrode — a ground rod or rods — connected to its ground bar (NEC 250.32(A)), plus a disconnecting means at that structure (NEC 225.31).

EV Chargers

An EV charger is the load that fits a 40 amp circuit most exactly. A 32 amp Level 2 charger is a 40 amp circuit, and the arithmetic is precise. NEC 625.42 classifies electric vehicle charging as a continuous load, and NEC 625.41 then requires the branch-circuit overcurrent protection to be rated at not less than 125 percent of the equipment's maximum load. 32 × 1.25 = 40.

That is not a coincidence of rounding. The three common Level 2 ratings — 32, 40 and 48 amps — were chosen to land on the three standard circuit sizes of 40, 50 and 60 amps once the 125 percent factor is applied. A 32 amp charger on a 40 amp circuit in 8 AWG copper is the configuration the equipment was designed around.

The failure mode is the inverse, and it is common enough to be worth naming: a 40 amp charger does not go on a 40 amp circuit. Forty amps is its maximum load, so its circuit is 40 × 1.25 = 50 amps, in 6 AWG copper. Buying a charger by matching its amp rating to an existing breaker is how a continuous load ends up at 100 percent of its overcurrent device.

Most hardwired EVSE is 240 volts only, so the cable is 8/2 — no neutral.

Hot Tubs and Spas

40 amps suits a smaller 240 volt tub with a single pump and a modest heater. The industry standard for mid-size and large tubs is 50 amps, and some run 60, so this is one application where the nameplate decides and no general rule substitutes for it.

Two requirements apply regardless of the amperage, and both catch people out:

• GFCI protection. NEC 680.44 requires the outlet supplying a self-contained spa or hot tub to be GFCI protected, which in practice means a two-pole GFCI breaker or a GFCI spa panel. A listed self-contained unit with integral GFCI protection for all its internal parts is the exception.
• A maintenance disconnect. NEC 680.13 requires it to be readily accessible, within sight of the equipment, and not less than 5 feet horizontally from the inside wall of the tub — far enough that nobody reaches it from the water. The distance is measured from the water's edge along the shortest path someone would take to reach it, and a permanently installed barrier that forces a 5 foot reach path satisfies the rule in place of the raw distance.

Most 240 volt spa packs include 120 volt controls, so the run is usually four-wire: 8/3 plus ground, or individual 8 AWG conductors in conduit.

Air Conditioners and Heat Pumps

Air conditioning equipment is the one load where 40 amps does not mean 8 AWG, and it is the most misunderstood case in the cluster.

So a condenser with an MCA of 27 amps and an MOCP of 40 amps takes a 40 amp breaker on a 10 AWG conductor. That is legal because Table 240.4(G) routes air-conditioning circuit conductors to Article 440, and 240.4(D) applies only "unless specifically permitted in 240.4(E) or (G)" — so the small-conductor cap never reaches this circuit.

The pairing is worth stating plainly, because it is the same rule twice. NEC 240.4(D)(7) is what rules 10 AWG out everywhere else on this page — and for air-conditioning equipment it is the one provision the code lifts. The conductor that would be a violation on a range circuit is the conductor the manufacturer specified for this condenser.

That exception is anchored to the nameplate and goes no further. It is not a general permission to run 10 AWG on a 40 amp breaker: it applies to this equipment, on the strength of the MCA and MOCP printed on it, and to nothing else on this page. On a 40 amp range, cooktop, sub-panel, EV or spa circuit, 10 AWG remains a violation.

The rule to take away is that an HVAC circuit is sized from its nameplate, not from its breaker. The 125 percent continuous-load factor is already inside the MCA figure; applying it again oversizes the conductor for no benefit. Condensers are 240 volts only, so the run is 8/2 or 10/2 depending on the MCA — no neutral.

Electric Tankless Water Heaters

Included on code and market grounds rather than keyword grounds — there are no tankless keywords in the 40 amp file, but this is now among the most common reasons a homeowner adds a 40 amp circuit.

Whole-house electric tankless units do not run on one 40 amp circuit; they run on several. A 27 kW unit is typically three 40 amp two-pole circuits, each in its own 8 AWG copper run, and a 36 kW unit is four. The heater's internal elements are split across them deliberately, because a single circuit carrying 27 kW at 240 volts would be a 112 amp load.

The practical constraint is rarely the conductor. Three 40 amp two-pole breakers consume six panel slots and add 120 amps of connected load, which is why a tankless retrofit on a 100 amp service usually turns into a service upgrade first. Each circuit is 240 volts only, so each run is 8/2.

The conductor size is settled; how it is packaged is not. At 40 amps every common wiring method is available, which stops being true a size or two up.

NM-B (Romex)

NM-B works at 40 amps, and it works with no margin at all. NEC 334.80 holds non-metallic sheathed cable to the 60°C ampacity column whatever its conductors are individually rated, and 8 AWG copper in the 60°C column is exactly 40 amps. So 8/2 or 8/3 NM-B on a 40 amp breaker is compliant, and it is what most kitchen and garage circuits at this amperage are actually wired in.

It is worth being precise here rather than repeating the usual warning, because the 60°C rule is often quoted as though it disqualifies NM-B at this size. It does not. What it does is remove the headroom: the same 8 AWG copper reads 50 amps in conduit on 75°C terminations and 40 amps inside the cable jacket, and the ten amps you lose is the margin you would otherwise have for derating.

Where the rule does bite is one size up. At 50 amps, 8 AWG NM-B is no longer enough and the cable jumps to 6 AWG — which is why a 50 amp circuit in Romex costs noticeably more than a 40 amp one, and why the boundary between these two amperages is a real decision point rather than a formality. Insulation types and temperature ratings are covered on the wire sizes explained page.

THHN in Conduit

Individual THHN or THWN-2 conductors in EMT, PVC or flexible conduit is the method that gets the full 50 amps out of 8 AWG copper, because the conductors are 90°C rated and the circuit reads the 75°C column set by the terminations rather than the 60°C column set by a cable jacket.

That extra ten amps is not theoretical. It is what lets a 40 amp circuit in conduit survive a derating factor that the same circuit in NM-B would fail. For a run through a hot attic, a garage ceiling shared with other circuits, or any location where conductors will be bundled, conduit is the method that keeps the answer at 8 AWG.

A 40 amp 240 volt circuit in conduit is three conductors for a 240-volt-only load — two hots and a ground — or four where a neutral is needed. Conduit fill under NEC Chapter 9 is not a constraint at these quantities in any standard trade size.

Direct Burial and UF-B

A 40 amp circuit that crosses a yard — to a detached garage, a sub-panel or a hot tub — is either UF-B cable buried directly or individual conductors in buried conduit.

UF-B is rated for direct burial and, like NM-B, is held to the 60°C column, so 8 AWG UF-B is 40 amps: adequate, with the same absence of margin. Burial depth is set by NEC Table 300.5 and depends on the method and what is above it, which is a detail worth confirming against the table rather than assuming. Conduit is the more forgiving choice underground for a reason that has nothing to do with ampacity: a buried conduit can be repulled when the circuit needs to change, and a direct-buried cable cannot.

What Size Ground Wire for a 40 Amp Circuit?

10 AWG copper, or 8 AWG aluminum. The equipment grounding conductor is sized from NEC Table 250.122 against the rating of the overcurrent device, not from the ampacity table and not from the circuit conductors. A 40 amp breaker calls for 10 AWG copper, and that is true whether the circuit conductors are 8 AWG or something larger.

One rule catches people on long runs. NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally where the circuit conductors are upsized for voltage drop. Upsize the 8 AWG copper to 6 AWG for a long run and the 10 AWG ground has to grow with it — a step that is easy to miss when the reason for the upsize had nothing to do with grounding. Cable assemblies handle this themselves; individual conductors in conduit do not. The equipment grounding conductor sizes come from NEC Table 250.122, by overcurrent device rating rather than by conductor size: 14 AWG copper up to 20 amps, 12 AWG to 60, 10 AWG to 100, 8 AWG to 200.

Copper vs Aluminum for 40 Amps

At 40 amps the honest recommendation is copper, and the reason is margin rather than any general objection to aluminum. 8 AWG copper gives you 50 amps against a 40 amp breaker; 8 AWG aluminum gives you 40 against 40. On a branch circuit this short the material cost difference is a few dollars and the margin is worth more than the saving.

If aluminum is used anyway — usually on a feeder to an outbuilding, where the run is long enough for the economics to change — three requirements are not optional. Every lug, breaker and device the conductor lands in must be listed for aluminum and marked AL/CU or AL9CU. The stripped conductor wants an antioxidant compound. And the termination needs a torque wrench set to the value marked on the equipment, because an aluminum connection tightened by feel is the connection that runs hot. Go to 6 AWG aluminum rather than 8 if the circuit will see any derating at all.

Ampacity has no length column. A 40 amp circuit in 8 AWG copper is correct at any distance as far as Table 310.16 is concerned, and voltage drop is a separate limit that the ampacity table says nothing about.

The mechanism is straightforward: the conductor's resistance is fixed per foot, so the voltage lost across the run grows with length while the ampacity stays where it is. At 40 amps the current is high enough that the loss accumulates faster than it would on a lighting circuit, and the industry target is 3 percent on a branch circuit — 7.2 volts out of 240.

Two things work in this circuit's favor. It is a 240 volt circuit, so the same voltage lost is half the percentage it would be at 120 volts. And most 40 amp loads are close to the panel: a range, a cooktop, a condenser on the side of the house. The runs where drop becomes the deciding factor are the outliers — a sub-panel at the end of a long yard, an EV charger at the far end of a detached garage.

When to Upsize to 6 AWG

Upsize when the run is long, when the load is continuous, or when both apply at once. An EV charger is the case that combines them: it draws its full 32 amps for hours at a stretch, and it is frequently at the far end of the property.

Rather than publish a distance here, use the numbers for your actual run. The voltage drop calculator takes the conductor size, the load and the one-way distance and returns the percentage; voltage drop by gauge and run length scales linearly with distance, so a circuit at 1.5 percent over 50 feet is at 3 percent over 100. If the result exceeds 3 percent, the next step is 6 AWG copper — which is 65 amps at 75°C, so it also buys back all the derating headroom that the longer run may have cost. Remember to increase the equipment grounding conductor with it, under 250.122(B).

Matching an EV Charger's Rating to the Breaker

A 40 amp charger needs a 50 amp circuit, not a 40 amp one. The 125 percent continuous-load factor in NEC 625.41 is the whole point of the rule, and matching the charger's nameplate amps to an existing breaker quietly puts a continuous load at 100 percent of its overcurrent device. The charger that belongs on a 40 amp circuit is a 32 amp charger.

Treating a 40 Amp Circuit as a Dryer Circuit

A residential clothes dryer is a 30 amp circuit in 10 AWG copper. NEC 220.54 sets the load at 5,000 watts or the nameplate rating, whichever is larger, and a typical dryer draws 22 to 24 amps at 240 volts. Running 40 amps to a dryer outlet is not dangerous — the conductor is oversized, not undersized — but the receptacle configuration is different, and a 40 amp breaker will not protect a 30 amp dryer receptacle.

Sizing an HVAC Conductor from the Breaker

The nameplate MCA sizes the conductor; the breaker only has to be at or below the MOCP. Reading "40 amp breaker" off the unit and pulling 8 AWG is the common version of this, and it is merely wasteful. The damaging version is the reverse — assuming that because the nameplate permits 10 AWG at 40 amps for a condenser, the same conductor is acceptable on an ordinary 40 amp circuit. It is not, and 240.4(D)(7) is the reason.

Substituting Aluminum One-for-One

8 AWG aluminum is not 8 AWG copper. It is 40 amps against copper's 50, it needs AL/CU-listed terminations, and it needs torqued connections. Swapping the material without changing anything else leaves a circuit with no derating margin terminating in lugs that may not be listed for it.

Upsizing the Breaker Without the Conductor

A 40 amp breaker on a circuit wired for 30 is the most straightforward mistake on this list. The breaker protects the conductor, not the appliance, and a 40 amp device on 10 AWG copper defeats the only protection that conductor has. If a 30 amp circuit is tripping, the conductor is the thing that has to change.

The Nearest Size We Cover

If your load does not fit a 40 amp circuit, the nearest amperage covered here is wire size for 50 amps at 50 amps . It works through the conductor, the cable type and the grounding conductor for that amperage in full.

Reference Pages

Every conductor size and temperature column sits in one place on the wire size chart, including the maximum breaker each size may be protected at. For the AWG system, insulation types and derating as subjects rather than as a single lookup, start at wire size.