Sizing a feeder circuit under the National Electrical Code comes down to four steps: calculate the load using Article 220, select a conductor from Table 310.16 whose ampacity meets that load, apply any correction factors for heat or conduit fill, and match the overcurrent device to the result. Each step has rules that can shift the answer significantly, and skipping any one of them is how feeders end up undersized, overheating, or failing inspection.
Start With the Load Calculation
You cannot pick a wire size until you know how much current the feeder will actually carry. NEC Article 220 lays out the method. Add up the total connected load in volt-amperes for all lighting, receptacles, and equipment on the branch circuits the sub-panel will serve.1Mine Safety and Health Administration. Article 220 – Branch Circuit and Feeder Calculations
The calculation then splits loads into two categories. A continuous load runs at maximum current for three hours or more; commercial lighting and HVAC are the usual examples. Everything else is non-continuous. The NEC requires both the feeder conductors and the overcurrent device to be rated for the full non-continuous load plus 125 percent of the continuous load.1Mine Safety and Health Administration. Article 220 – Branch Circuit and Feeder Calculations A sub-panel carrying 40 amps non-continuous and 80 amps continuous needs a feeder sized for at least 140 amps: 40 plus 80 times 1.25.
Article 220 also applies demand factors. These account for the fact that every connected load almost never runs simultaneously at full capacity. A 200-unit apartment building doesn’t need feeders sized as though every stove, dryer, and air conditioner is on at once. Demand factors shrink the calculated load to a realistic maximum, which in turn permits smaller, cheaper conductors. Skip them and you overspend on copper; miscalculate them in the other direction and you get tripped breakers.
The Residential 83 Percent Allowance
For single-phase 120/240-volt dwelling units, NEC 310.15(B)(7) lets you size feeder conductors rated 100 to 400 amps at 83 percent of the feeder rating rather than the full rating. A 200-amp residential feeder can use conductors rated for 166 amps and still satisfy the code. The allowance reflects how diverse residential loads actually behave, with heating, cooling, cooking, and other heavy draws rarely peaking together. It applies only to individual dwelling units. Commercial buildings and common-area feeders in apartment complexes get no such break.
Pick the Conductor From Table 310.16
Once you have the calculated load in amperes, Table 310.16 points you to a conductor size. The table lists allowable ampacity by wire material, insulation temperature rating, and conductor size. Most feeder installations use conductors with 75°C-rated insulation because that matches the terminal rating of standard panelboard equipment. The common 75°C ampacities for feeder-size conductors:
- 6 AWG copper: 65 amps (aluminum: 50 amps)
- 4 AWG copper: 85 amps (aluminum: 65 amps)
- 3 AWG copper: 100 amps (aluminum: 75 amps)
- 2 AWG copper: 115 amps (aluminum: 90 amps)
- 1/0 AWG copper: 150 amps (aluminum: 120 amps)
- 2/0 AWG copper: 175 amps (aluminum: 135 amps)
- 3/0 AWG copper: 200 amps (aluminum: 155 amps)
- 4/0 AWG copper: 230 amps (aluminum: 180 amps)
These values assume no more than three current-carrying conductors in the raceway and an ambient temperature of 30°C (86°F). Change either condition and the ampacity drops.
Copper or Aluminum
Aluminum conductors cost significantly less than copper for the same ampacity, which is why they are the default for larger feeders like a 100-amp or 200-amp sub-panel run. But aluminum carries installation requirements copper does not. Aluminum forms an oxide layer when exposed to air, and that oxide is an insulator. Left untreated, it creates a high-resistance connection point that generates heat under load, so an anti-oxidant compound rated for the application must be applied to every aluminum termination. Terminals and splicing devices must also be identified for use with aluminum. Copper and aluminum cannot share a terminal unless the device is specifically listed for mixed metals; the two metals expand at different rates and corrode when in direct contact.2National Electrical Manufacturers Association. Copper-Clad Aluminum Conductor Requirements in the National Electrical Code
Aluminum also has higher electrical resistance than copper, so it needs a larger conductor to carry the same current. A 100-amp feeder in copper calls for 3 AWG; the same feeder in aluminum needs 1 AWG or 1/0 AWG depending on the specific installation conditions. The larger wire means larger conduit, so check the total materials cost before assuming aluminum is cheaper on every job.
Apply Derating for Heat and Conduit Fill
Table 310.16 ampacities assume ideal conditions. Real installations rarely match them. Three adjustments commonly change the usable ampacity of a conductor.
Ambient Temperature
When the air around a conductor exceeds 30°C (86°F), the conductor cannot shed heat as effectively and its ampacity must be reduced. This matters for feeders routed through attics, boiler rooms, or outdoor locations in hot climates. A 75°C-rated conductor in a 40°C (104°F) environment retains only 88 percent of its table ampacity. At 50°C (122°F), that falls to 75 percent. A 2 AWG copper conductor rated for 115 amps under normal conditions is good for roughly 86 amps if it runs through a 50°C attic. Miss this correction and the conductor can operate above its safe temperature limit without ever tripping the breaker.
Conduit Fill
When more than three current-carrying conductors share a raceway, each conductor’s ampacity must be reduced because the bundled wires collectively produce more heat than they can dissipate. Four to six conductors in the same raceway drop each conductor to 80 percent of its table ampacity. Seven to nine conductors bring it down to 70 percent. Grounding conductors and neutral conductors carrying only unbalanced current don’t count toward the total. Spare conductors do count, since they could be energized later. When elevated temperature and conduit fill both apply, the two factors multiply, and the usable ampacity can shrink dramatically.
Terminal Temperature Limits
Even with 90°C-rated insulation, you usually can’t use the 90°C ampacity for sizing. Terminal connections on most equipment limit the allowable ampacity. For equipment rated 100 amps or less, conductor ampacity is generally based on the 60°C column of Table 310.16 unless the equipment is listed for higher-temperature conductors. For equipment rated above 100 amps, the 75°C column applies. The 90°C rating comes into play only as a starting point when you apply derating factors. You can begin at the higher 90°C ampacity, apply the corrections, and use the result so long as the derated ampacity doesn’t exceed the 75°C (or 60°C) column value for that conductor size.
Check Voltage Drop on Long Runs
The NEC recommends limiting feeder voltage drop to no more than 3 percent, with total voltage drop across the feeder and branch circuits combined not exceeding 5 percent. These figures appear as informational notes rather than mandatory requirements, but treating them as a ceiling is sensible. A feeder operating at 6 or 7 percent voltage drop makes motors run hotter and draw more current, dims lighting noticeably, and causes sensitive electronics to misbehave.
Voltage drop becomes a real factor on long runs. A 100-amp feeder running 150 feet in copper needs to be upsized beyond the minimum ampacity requirement to stay within 3 percent. The calculation uses the conductor’s resistance per foot, the length of the run (doubled for the round trip), and the operating current. Many electricians rely on online calculators or reference charts, but the principle is simple: longer runs and higher currents both increase voltage drop, and the only remedy is a larger conductor.
Match the Overcurrent Device
NEC 215.3 requires feeder overcurrent protection in accordance with Article 240. The breaker or fuse protecting the feeder must be rated for the full non-continuous load plus 125 percent of any continuous load, which mirrors the conductor sizing rule. The breaker rating and the conductor ampacity should align. A feeder with conductors sized for 125 amps gets a 125-amp breaker, not a 150-amp breaker that would let the conductors carry more current than they can safely handle.
Standard breaker sizes come in fixed increments: 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, and so on. NEC 240.4(B) allows the next standard size up when the calculated load falls between standard ratings, provided the conductors are not part of a multi-outlet branch circuit. A feeder with a 97-amp calculated load can use a 100-amp breaker with conductors rated for at least 100 amps.
Size the Grounding Conductor
Every feeder must include an equipment grounding conductor that provides a low-resistance path for fault current back to the source. When a short circuit energizes a metal enclosure or conduit, the EGC carries enough current to trip the breaker quickly rather than leaving the fault sitting on metal parts someone might touch. The EGC can be a separate wire, bare or green-insulated, or in some installations the metal raceway itself serves the purpose. The NEC sizes the EGC based on the rating of the upstream overcurrent device using Table 250.122, not on the conductor ampacity. In other words, a 200-amp breaker determines the EGC size whether the circuit conductors are sized at exactly 200 amps or upsized for voltage drop.
Keep Neutral and Ground Separate at the Sub-Panel
This is where feeder installations most often go wrong, and it affects what you need to run. At the main service panel, the neutral and the equipment grounding conductor are bonded together and connected to the grounding electrode system. At every sub-panel downstream, they must stay separate. The neutral bar in the sub-panel must be isolated from the metal enclosure, and the bonding screw or strap that would connect them has to come out. A separate ground bar, bonded directly to the enclosure, terminates the equipment grounding conductors.
When neutral and ground are incorrectly bonded in a sub-panel, return current from normal operation splits between the neutral wire and every grounded metal path in the building, including conduit, water piping, and structural steel. Stray currents on surfaces that should carry zero current create shock hazards and interfere with sensitive equipment. The fix is to leave the bonding screw out, isolate the neutral bar, and run a four-wire feeder (two hots, one neutral, one ground) to every sub-panel. That four-wire requirement affects your conduit sizing, so build it into the plan from the start rather than discovering it at rough-in.
Outdoor and Underground Runs Change Your Conductor Choice
Feeders routed outdoors or underground face moisture that indoor installations don’t. The conductor insulation must be rated for wet locations, which means a type with a “W” in the designation such as THWN-2 or XHHW-2. The NEC treats the interior of any above-grade outdoor raceway as a wet location, so even conductors inside conduit need wet-rated insulation when installed outdoors. The ampacity math is the same; the insulation type is what changes. Burial depth, PVC expansion fittings, and grounding at separate buildings are governed by their own code sections and are worth working through before the trench is open.
Putting the Steps Together
A workable feeder sizing sequence looks like this. Calculate the connected load, separate continuous from non-continuous, and apply demand factors. Add 125 percent of the continuous portion to the non-continuous portion to get the minimum required ampacity. For a residential dwelling-unit service or main feeder between 100 and 400 amps, apply the 83 percent allowance. Pick a conductor from Table 310.16 at the 75°C column (or 60°C for equipment at or below 100 amps) that meets or exceeds that number. If the raceway holds more than three current-carrying conductors or runs through a hot location, derate and verify the result still covers the load. On runs long enough to matter, check voltage drop and upsize if the number exceeds 3 percent. Choose the next standard breaker size that matches the conductor ampacity, size the equipment grounding conductor from Table 250.122, and keep neutral and ground separate at the sub-panel. The feeder that follows every step will carry its load without complaint for the life of the installation.