How Far Can a Subpanel Be From the Main Panel? Voltage Drop Decides

A subpanel can sit a few inches from the main panel or a few hundred feet away. What changes with distance is the wire.

Short answer: the NEC sets no maximum distance between a main panel and a subpanel. The practical limit is voltage drop on the feeder: the longer the run, the larger the conductors need to be to stay near the recommended 3%. A subpanel in a detached building also needs its own disconnect and grounding electrode system, at any distance.

Is there a maximum distance in the NEC?

No. A feeder protected by a breaker in the main panel has no length limit in the code. The distance numbers people run into belong to other rules:

• 10 ft and 25 ft are the feeder tap rules (NEC 240.21(B)(1) and (B)(2)). They cover conductors tapped off a feeder with no overcurrent device where they connect. A subpanel fed from its own breaker isn't a tap.
• "Nearest the point of entrance" applies to service conductors reaching the service disconnect (NEC 230.70(A)(1)) and to the disconnect at a separate building (NEC 225.32). It says nothing about how far a subpanel can be from the main.

Why voltage drop sets the practical limit

Every foot of wire has resistance, so some voltage is lost along the way. Lose too much and motors run hot and start poorly, lights dim, and heating equipment puts out less.

The NEC's guidance is an informational note to NEC 215.2(A): no more than about 3% drop on a feeder, and 5% for feeder plus branch circuit combined. Informational notes aren't enforceable, but they're the design standard electricians work to, and equipment installation instructions, which are enforceable under NEC 110.3(B), can set their own minimum voltage.

The standard approximation, the same one our voltage drop calculator uses for single-phase 120/240 V:

Voltage drop = 2 × K × one-way length (ft) × amps ÷ circular mils

K is 12.9 for copper and 21.2 for aluminum. Drop rises in straight proportion to length and current: double either and the drop doubles.

Worked example: a 60 A feeder at 150 and 300 feet

Take a 60 A feeder at 240 V carrying 48 A, which is 80% of its rating. The code minimum is 6 AWG copper or 4 AWG aluminum from the 75 °C column (NEC Table 310.16 with 110.14(C)). In NM cable, NEC 334.80 moves copper up to 4 AWG; see wire size for 60 amps.

For 6 AWG copper (26,240 circular mils) at 150 ft: 2 × 12.9 × 150 × 48 ÷ 26,240 = 7.08 V, and 7.08 ÷ 240 = 2.95%.

What the numbers show:

• At 150 ft, the ampacity minimum sits right on the line. 6 AWG copper just passes and 4 AWG aluminum just misses. Either leaves little of the 5% total for branch circuits, which is why going one size up is common.
• At 300 ft, the minimum is far off. 6 AWG copper loses 5.90%, the whole 5% budget gone before a single branch circuit, leaving 225.8 V at the subpanel. It takes 3 AWG copper or 1/0 aluminum to get back under 3%; 1 AWG aluminum lands at 3.04%.
• Lighter loads drop less. At 24 A, every figure halves. The design current comes from the load calculation, covered in how to size a subpanel.
• 120 V loads feel it more. A 120 V load returns on the neutral, and the same 7.08 V is 5.9% of 120 V. On a long feeder, spreading 120 V circuits evenly across both legs matters.

Run your own length and load through the voltage drop calculator.

To see whether an existing long feeder is sagging, no panel work is needed: plug a receptacle tester with a voltage readout, such as the Klein RT250, into a receptacle on the subpanel and read it with the big loads off, then again with a heavy plug-in load such as a space heater running on the same circuit. The dip covers the feeder and that branch circuit together, and a large one is worth showing your electrician; take the two readings a minute apart, since utility voltage also drifts.

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What upsizing for distance changes

• The ground wire grows too. When the hots are upsized for voltage drop, NEC 250.122(B) requires a wire-type equipment grounding conductor to grow in proportion to their circular-mil area, keeping the fault path in step with the larger conductors.
• The terminals have to accept it. Larger conductors must fit the listed range of the feeder breaker's and subpanel's lugs.
• The conduit gets bigger to hold the larger conductors.
• The breaker doesn't have to change. It's still sized for the load. The bigger conductors just carry that load with less drop.

What distance doesn't change

• A feeder sized for the load. Ampacity of at least the noncontinuous load plus 125% of the continuous load (NEC 215.2(A)(1)), with overcurrent protection sized the same way (NEC 215.3).
• Protection at the main-panel end. The feeder breaker sits where the conductors receive their supply (NEC 240.21) and protects them at their ampacity (NEC 240.4). It also protects the subpanel, which must be rated at least as high as that breaker (NEC 408.36).
• Four wires and an isolated neutral. Two hots, a neutral and an equipment grounding conductor, with neutral and ground kept apart at the subpanel (NEC 250.24(A)(5), 250.142(B)). See subpanel grounding and neutral separation.

When the subpanel is in another building

A detached garage, shop or barn adds requirements that have nothing to do with distance:

• A disconnecting means at the building (NEC 225.31), at a readily accessible location nearest where the feeder enters (NEC 225.32). A subpanel with a main breaker usually serves.
• Its own grounding electrode system, connected to the incoming equipment grounding conductor, with no neutral-ground bond at that panel (NEC 250.32).
• Burial depth for the trench, which depends on the wiring method.

All three are covered in feeding a detached building. For planning circuits inside the building, see garage or shop subpanel.

Why is this electrician work?

The feeder lands in the main panel, where the main lugs stay live even with the main breaker off. Sizing it takes a load calculation, the right ampacity column for the wiring method, a voltage-drop check and a proportional ground, followed by a permit and inspection. Give your electrician the run length measured along the route the conductors will take, not the straight-line distance.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 215.2(A): feeder minimum ampacity; its informational note gives the 3% / 5% voltage-drop recommendation
• NEC 215.3: feeder overcurrent protection
• NEC 240.4, 240.21: conductor protection, located where conductors receive their supply; 240.21(B) holds the 10 ft and 25 ft tap rules
• NEC Table 310.16, 110.14(C), 334.80: conductor ampacity, termination temperature, NM cable ampacity
• NEC 250.122(B): equipment grounding conductor increased with upsized conductors
• NEC 250.24(A)(5), 250.142(B): no neutral-ground connection downstream of the service disconnect
• NEC 225.31, 225.32: disconnecting means for a separate building and its location
• NEC 250.32: grounding electrode system at a separate building
• NEC 408.36: panelboard overcurrent protection

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Voltage drop calculator
• What is a subpanel?
• Main breaker vs main lug
• Check the service first: home electrical load calculator

More in our electrical panel guides.