100 Amp Wire Size

A 100 amp conductor is 3 AWG copper or 1 AWG aluminum read straight from NEC Table 310.16, and a qualifying dwelling service is permitted one size smaller — 4 AWG copper or 2 AWG aluminum — under NEC 310.12. Everything on this page follows from which of those two you are installing.

The Three Cases the Code Treats Separately

Most pages answering this question give one number. The code gives three answers, and the difference between them is a full trade size.

• A service — the conductors from the utility's point of connection to the service disconnect of a dwelling.
• A feeder carrying the entire dwelling load — for example from a meter-main outside to the only panel inside.
• A sub-panel feeder that does not carry the entire load — a detached garage, a workshop, a shop panel, an addition.

A sub-panel feeding a garage fails that test twice: a garage is not a dwelling, and a panel carrying some but not all of the dwelling's load is outside the allowance regardless. That conductor is sized from Table 310.16 at the full 100 amps.

This is the same shape as the distinction on the 50 amp wire size page, where 310.12 fails for the opposite reason — a 50 amp service sits below the section's 100 ampere floor, so no allowance exists there at all.

100 Amp Wire Size in Aluminum

Aluminum is the normal conductor at this ampacity, not the alternative. Service entrance and feeder runs at 100 amps are where aluminum's cost and weight advantages are large enough to make it the default, and it is what most utilities and electricians actually install.

1 AWG aluminum is rated 100 amps in the 75°C column of NEC Table 310.16 — exactly the load, with nothing spare. That is the size for a sub-panel feeder or any run the 83 percent allowance does not reach.

2 AWG aluminum is rated 90 amps, which is short of 100 but clears the 83 amps a qualifying dwelling service needs. That is the service size, and the arithmetic behind it is below.

4 AWG aluminum is rated 65 amps and is not a 100 amp conductor in any case.

100 Amp Wire Size in Copper

3 AWG copper is rated 100 amps at 75°C — again exactly the load — and is the sub-panel feeder size. 4 AWG copper is rated 85 amps and is the service size under the 83 percent allowance.

Copper at 100 amps is a deliberate choice rather than a default: it costs substantially more per foot on a run this size, and its advantage is a smaller conductor for the same ampacity, which matters where conduit fill or termination space is tight.

Why the Service Size Is Smaller

The 83 percent allowance is not a rounding convention. It exists because a dwelling service never carries its full nameplate rating continuously — the diversity of household loads means the calculated demand is well below the service rating almost all of the time.

The arithmetic is checkable:

• 100 amps × 0.83 = 83 amperes of required ampacity
• 4 AWG copper is 85 amps at 75°C — clears 83
• 2 AWG aluminum is 90 amps at 75°C — clears 83
• 6 AWG copper is 65 amps and 4 AWG aluminum is 65 amps — neither clears it

So the allowance moves the answer exactly one trade size and no further, and it moves it only for the two cases NEC 310.12 names.

Yes for a service, no for a sub-panel feeder — and that single distinction is why the question is asked so often and answered so badly.

#4 Copper on a Service

4 AWG copper is rated 85 amps in the 75°C column, which is less than 100 and looks wrong on its face. It qualifies for a dwelling service because NEC 310.12 lowers the required ampacity to 83 amps, and 85 clears 83.

So a 100 amp dwelling service in 4 AWG copper is correct, common and code compliant. A reader who has been told "4 AWG is too small for 100 amps" has been given the Table 310.16 answer to a question the code answers elsewhere.

#4 Copper on a Sub-Panel Feeder

On a feeder that does not carry the entire dwelling load, 4 AWG copper at 85 amps is undersized by a full trade size. The conductor for that case is 3 AWG copper, and this is the single most consequential error on competitor pages: a chart that prints "100 amp = 4 AWG copper" without naming the case sends a reader to a detached garage with a conductor rated 85 amps behind a 100 amp breaker.

Is 3 AWG Enough for a 100 Amp Service?

Yes, in every case. 3 AWG copper is rated exactly 100 amps at 75°C, so it satisfies the sub-panel feeder requirement directly and exceeds what a service needs. It is the right answer when you do not know which case you have, and it is what to specify when a run might later be extended to serve loads beyond the dwelling.

The aluminum equivalents follow the same pattern: 2 AWG aluminum for a qualifying service, 1 AWG aluminum for anything else.

A 100 amp dwelling service takes 2 AWG aluminum or 4 AWG copper, and aluminum is what is normally installed.

What Counts as a Service

A service is the conductor run from the utility's point of connection to the service disconnecting means. It is not the same thing as the feeder to a panel inside the house, and it is not the same thing as a sub-panel feed — though the first two share a conductor size because NEC 310.12 covers both.

If the conductors run from a meter-main outside to the only panel inside, they are a feeder carrying the entire dwelling load, and the 83 percent allowance applies to them exactly as it does to the service.

Service Entrance Cable

SEU is the flat oval cable used for the service run itself. Its two insulated conductors are wrapped in a bare concentric neutral, which means it has no separate equipment grounding conductor — that is fine for a service, where the neutral and ground are bonded at the disconnect, and it is what makes SEU unsuitable for a feeder.

SER is the round four-conductor version with a separate equipment grounding conductor. It is the cable for a feeder or a sub-panel feed.

USE-2 is the underground type for a service lateral. Standard SER is not listed for direct burial.

Grounding a 100 Amp Service

A service is grounded through a grounding electrode conductor, sized from NEC Table 250.66 against the size of the largest ungrounded service conductor — not from the breaker, and not from the table a feeder uses.

For a 100 amp service the service conductors are 2 AWG or smaller, so Table 250.66 calls for an 8 AWG copper grounding electrode conductor. NEC 250.66(A) caps it separately: where that conductor's sole connection is to a rod, pipe or plate electrode, it need not be larger than 6 AWG copper or 4 AWG aluminum whatever the table says.

How Far a Service Can Run

Service conductors are usually short, and voltage drop rarely decides their size. Where it does is a long lateral — a house set well back from the road, or a meter on a pole at the property line. The conductor's resistance is fixed per foot, so the voltage lost grows with the run while the ampacity stays where it is, and at 100 amps the loss accumulates quickly.

Use the calculator above with your own one-way distance. If the result exceeds 3 percent, the fix is the next conductor size up. Voltage drop by gauge and run length follows one rule worth carrying: drop is proportional to distance, so a feeder at 2 percent over 100 feet is at 3 percent over 150.

A 100 amp sub-panel feeder takes 1 AWG aluminum or 3 AWG copper, read from NEC Table 310.16 at the full 100 amps with no allowance applied.

Why a Sub-Panel Does Not Get the Service Size

NEC 310.12's 83 percent allowance reaches conductors supplying the entire load of a dwelling. A sub-panel that serves a garage, a workshop, an addition or any subset of the house is not carrying that entire load, so the allowance does not apply and the conductor is sized at 100 amps.

The practical consequence is one trade size, and it is the difference between a conductor rated 85 amps and one rated 100 behind the same breaker.

Four-Wire, Always

A sub-panel feeder is four conductors: two ungrounded, one grounded neutral and one equipment grounding conductor. Three rules govern the far end:

• The neutral bar and the ground bar 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 at exactly one point 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).

The Equipment Grounding Conductor

A feeder is grounded through an equipment grounding conductor, sized from NEC Table 250.122 against the rating of the overcurrent device. For a 100 amp breaker that is 8 AWG copper or 6 AWG aluminum.

Table 250.122 and Table 250.66 both produce 8 AWG copper at 100 amps, which is exactly why so many sites treat them as one table. They are not. One is sized from the service conductor and one from the breaker, they diverge at other ampacities, and using the wrong one is a habit that produces a wrong answer the first time the numbers stop coinciding.

The Neutral Is a Third Question

The grounded conductor of a feeder is sized for the maximum unbalanced load under NEC 220.61, and never smaller than the equipment grounding conductor minimum. On a qualifying dwelling service or feeder, NEC 310.12(C) permits it smaller than the ungrounded conductors. A sub-panel serving only 240 volt equipment carries almost no unbalanced load, and a sub-panel serving lighting and receptacles carries a great deal — so this is a calculation rather than a lookup.

How Far to a Detached Garage

This is the run where distance genuinely decides the conductor. A feeder to a detached structure is commonly 50 to 150 feet, and at 100 amps the voltage lost over that distance is a real design constraint rather than a footnote.

Set your own one-way distance in the calculator above and read the percentage. The conductor that satisfies ampacity at 50 feet may not satisfy a 3 percent drop target at 150, and the answer is to go up a size rather than to accept the drop — a sub-panel that starts 5 percent low has no headroom left for the circuits inside it.

Remember that a longer run changes two other things. NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally where the circuit conductors are upsized for voltage drop. And the separate structure still needs its own disconnect and grounding electrode.

A 100 amp main panel is fed by either a service or a feeder, and which one it is decides the conductor — the panel itself does not.

A Main Panel Fed by a Service

Where the panel is the service disconnect, its conductors are service conductors and NEC 310.12 applies: 2 AWG aluminum or 4 AWG copper.

A Main Panel Fed by a Feeder

Where a meter-main outside feeds a panel inside, and that panel carries the whole house, the conductors are a feeder carrying the entire dwelling load — still NEC 310.12, still the same two sizes. Where the panel carries only part of the load, it is a sub-panel and takes 1 AWG aluminum or 3 AWG copper.

At 100 amps the branch-circuit wiring methods drop away and the service and feeder types replace them.

NM-B Is Effectively Out

NEC 334.80 holds non-metallic sheathed cable to the 60°C ampacity column. 3 AWG copper is 85 amps in that column, short of 100, so NM-B is not a practical 100 amp wiring method. This is the point at which the cable that serves most residential branch circuits stops being an option at all.

SER

Type SE Style R is the four-conductor feeder cable: two ungrounded conductors, a neutral and a separate equipment grounding conductor, under a sunlight-resistant jacket. It is what a sub-panel feed is normally run in.

The trap is thermal insulation. NEC 338.10(B)(4)(a) limits SER to the 60°C column where it passes through thermal insulation, which is exactly what a feeder run through an insulated wall or attic does. A conductor chosen from the 75°C column and then buried in insulation is no longer the conductor you sized.

SEU

Type SE Style U is the flat oval service cable, with a bare concentric neutral and no separate equipment grounding conductor. That is correct for a service and wrong for a feeder, because a four-wire feeder needs an EGC that SEU does not have.

USE-2 and Direct Burial

USE-2 is the underground service lateral type. Standard SER is not rated for direct burial, so a buried run is either USE-2 or individual conductors in buried conduit. Burial depth comes from NEC Table 300.5 and depends on the wiring method and what covers it.

Individual Conductors in Conduit

THHN or THWN-2 in EMT, PVC or flexible conduit is the most forgiving option for a long feeder: the conductors read the 75°C column set by the terminations rather than a cable jacket's 60°C column, and a buried conduit can be repulled when the load changes.

Aluminum Terminations

Every lug, breaker and device an aluminum 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. At 100 amps these are service and feeder terminations carrying the whole house, so a connection tightened by feel is a connection that runs hot under the largest load in the building.

A 100 amp service is the code minimum for a one-family dwelling, not a comfortable size, and for a growing number of houses it is the binding constraint rather than a safe default.

The Code Minimum

NEC 230.79(C) requires the service disconnecting means for a one-family dwelling to be rated not less than 100 amperes, 3-wire. NEC 230.79 also requires it to be not less than the calculated load under Article 220. So 100 amps is a floor, and the calculation decides whether it is also the answer.

Where 100 Amps Runs Out

The loads that exhaust a 100 amp service are the ones being added to houses now. A Level 2 electric vehicle charger draws 32 to 48 amperes continuously. Heat pump backup heat strips run 10 to 20 kW, which is 40 to 80 amperes. Either one on top of an existing full household load commonly pushes a 100 amp service past what an Article 220 calculation supports.

That does not make the upgrade automatic. A documented load calculation is what decides it, and load management equipment that prevents a charger and a heat pump from running at full draw simultaneously can keep a 100 amp service viable where a naive breaker-count would say it cannot.

When the Upgrade Is the Real Answer

Where the calculation does not support the load, the answer is a larger service rather than a larger conductor — the conductor is not the limit, the service rating is. A 200 amp service is the usual upgrade, and under the same NEC 310.12 allowance it lands on 2/0 copper or 4/0 aluminum — two sizes up from 100 amps, not twice the conductor.

Using the Service Size on a Sub-Panel Feeder

The most consequential error at this ampacity. 4 AWG copper is a correct 100 amp service conductor and an undersized sub-panel feeder, and a chart that prints one number without naming the case is how the second happens.

Using the Wrong Grounding Table

A service takes a grounding electrode conductor from NEC Table 250.66, sized against the service conductor. A feeder takes an equipment grounding conductor from NEC Table 250.122, sized against the breaker. They both give 8 AWG copper at 100 amps, which hides the error until the habit is carried to an ampacity where they differ.

Running SER Through Insulation Without the Derate

NEC 338.10(B)(4)(a) puts SER in the 60°C column where it passes through thermal insulation. A feeder sized from the 75°C column and then run through an insulated wall has been derated after the fact.

Using SEU for a Feeder

SEU has a bare concentric neutral and no separate equipment grounding conductor. A four-wire feeder needs an EGC, so SEU cannot serve one however convenient its profile is.

Assuming 100 Amps Is Adequate Because It Is the Minimum

NEC 230.79(C) sets 100 amperes as the floor for a one-family dwelling. It is not a statement that 100 amps suits the house, and the Article 220 calculation is what answers that.

Sizing a Long Feeder on Ampacity Alone

A conductor that satisfies 100 amps at the panel can be well below target at a detached garage 150 feet away. Ampacity has no length column, and the equipment grounding conductor has to grow with any upsize under NEC 250.122(B).

The Nearest Size We Cover

If your load does not fit a 100 amp circuit, the nearest amperage covered here is 50 amp wire size 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 the wire sizes hub.