A feeder wire is any electrical conductor that carries power from a main service panel (your meter and main breaker) to a secondary distribution point, like a subpanel in a garage, workshop, or another part of a building. It sits in the middle of your electrical system: downstream from the utility connection but upstream from the individual circuits that power your outlets, lights, and appliances. If branch circuits are the small roads leading to each house on a street, feeder wires are the highway connecting neighborhoods to the city center.
Feeders vs. Branch Circuits
The National Electrical Code (NEC) draws a clear line between these two types of wiring. A branch circuit runs from the final breaker or fuse to the outlets and devices you actually plug things into. A feeder, covered under NEC Article 215, runs between the main panel and a downstream panel or large piece of equipment. The practical difference matters because feeders carry much higher loads, supplying power that then gets divided among many individual branch circuits. That means they require heavier wire gauges, larger conduit, and their own sizing calculations.
How Feeder Wire Is Sized
Feeder conductors must be large enough to handle the total load of everything downstream. The NEC requires that the wire’s capacity (called ampacity) equal at least 100% of the noncontinuous load plus 125% of any continuous load, meaning any load that runs for three hours or more. This 25% buffer on continuous loads prevents the wire from overheating during sustained use.
For common residential scenarios, here’s what the copper wire sizing looks like based on NEC Table 310.16:
- 60-amp subpanel: 8 AWG copper minimum (rated for 70 amps at standard temperature ratings)
- 100-amp subpanel: 4 AWG copper minimum (rated for 125 amps)
- 200-amp subpanel: 3/0 AWG copper minimum (rated for 310 amps at 75°C)
The NEC also recommends keeping voltage drop on feeders to 2% or less. Voltage drop is the small loss of electrical pressure that happens over distance. The longer the wire run, the more voltage you lose, so longer runs between panels often require stepping up to a larger wire gauge than the ampacity alone would demand.
Copper vs. Aluminum Feeder Wire
Most feeder wire is either copper or aluminum, and the choice comes down to a tradeoff between performance and cost. Copper conducts electricity about 70% better than aluminum, which means copper feeders lose less energy as heat and can use smaller wire for the same load. For high-demand applications where efficiency matters over the long term, copper pays for itself through lower energy losses.
Aluminum, however, weighs roughly one-third as much as copper and costs 40 to 60% less per kilogram. For large feeder runs, especially overhead lines or long underground pulls, aluminum is significantly cheaper to buy, ship, and install. The tradeoff is that aluminum feeders need a larger cross-sectional area to carry the same current, so the cable itself will be physically bigger. Modern aluminum feeder wire uses an AA-8000 series alloy designed to reduce the connection problems that gave older aluminum wiring a bad reputation.
Insulation Types for Feeder Wire
The letters stamped on feeder wire describe its insulation, and picking the right type depends on where the wire will be installed.
THHN is one of the most common choices for indoor feeder runs. It uses PVC insulation with a nylon jacket and handles temperatures up to 90°C in dry locations. It works well pulled through conduit inside a building but isn’t ideal for wet environments, where its temperature rating drops to 75°C.
XHHW uses cross-linked polyethylene insulation, which gives it better thermal stability and chemical resistance. It’s rated for 90°C in both wet and dry conditions, making it a stronger choice for outdoor, underground, or industrial feeder installations where moisture or chemical exposure is a concern.
SER (Service Entrance Rated) cable is a bundled cable commonly used for the short feeder run between a meter base and the main panel, or from a main panel to a nearby subpanel. It contains multiple conductors and a bare ground wire inside a single outer jacket.
What a Feeder Circuit Includes
A complete feeder circuit isn’t just the hot wires carrying current. It also includes a neutral conductor and an equipment grounding conductor. The hot conductors are sized to the full load, but the neutral only needs to handle the maximum unbalanced load, which is the difference between the two hot legs in a typical 240-volt system. In many residential feeders, the neutral can be smaller than the hot wires because the loads on each leg are roughly balanced.
The grounding conductor has its own sizing rules based on the size of the overcurrent protection device (the breaker) upstream. For example, a feeder protected by a 400-amp breaker requires a grounding conductor no smaller than 3 AWG copper, regardless of the actual load on the circuit. This ensures a safe path for fault current if something goes wrong.
Underground Feeder Installations
When feeder wire runs underground, such as from a house to a detached garage, burial depth requirements protect the cable from accidental damage. UF (underground feeder) cable buried directly in the ground must sit in a trench at least 24 inches deep. Running the wire through conduit reduces that requirement to 18 inches, since the conduit itself provides mechanical protection.
For direct-burial installations without conduit, laying treated lumber planks a couple of inches above the cable as you backfill the trench adds a layer of protection. If someone digs in that area years later, the plank warns them before a shovel can reach the wire. Underground feeder runs also benefit from XHHW-rated conductors, since the insulation handles long-term moisture exposure without degrading.