🧑‍🔧 Wire a Subpanel

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Work out the current an individual load draws, then size the feeder that runs from the main panel out to a garage or workshop subpanel so the far end still gets full voltage.

Adding a subpanel for a garage, a workshop or a shed is really three sizing questions asked in order: how much current do the loads draw, what conductor is rated to carry it, and is the voltage still good at the far end of what is often a much longer run than a typical branch circuit. This guide starts small, with one load, and works out to the feeder.

As with the generator guide, this is meant to help you understand the numbers behind a subpanel install, not to replace an electrical permit or a licensed electrician where your jurisdiction requires one. Subpanel work touches your main service, and getting the grounding and bonding wrong there is genuinely dangerous.

What a subpanel adds, and what it does not

A subpanel is a second distribution point fed by a single feeder from the main panel. It moves the breakers close to the loads, so a workshop needs one heavy cable from the house instead of a dozen light ones. The feeder does not have to carry every breaker's rating added together. It has to carry the current the loads can actually draw at the same time, and the main panel has to have the capacity to give it.

That is why this project asks three questions in order. How much current do the loads draw, worked out load by load? What conductor and breaker are rated for that current, which is an ampacity question answered from code tables? And is the voltage still good at the far end of the run? The calculators here answer the first and the third. The second is a code lookup, and the answer is the larger of what ampacity and voltage drop each demand.

Four wires, and why the neutral stays separate

A modern subpanel feeder has four conductors: two hots, a neutral and a ground. In the subpanel the neutral and the ground go to separate bars. They are tied together only at the main service, because a second connection would let normal load current wander onto the ground wires and metal enclosures. A separate building also normally gets its own grounding electrode. Details vary by code edition and jurisdiction, so treat this as the outline your electrician or inspector fills in.

Steps

1. 1⚡Ohm's Law
   Start with one load you plan to feed from the subpanel. A resistive load such as a heater is just a resistor, so leave one field at 0 and Ohm's Law solves it from the other two. A nameplate usually gives watts and volts rather than ohms, so first work out the resistance (volts squared ÷ watts), enter it with the voltage, and read the current. Add up the currents of everything that can run at once to get the feeder's design current.
   🧮 Open the Ohm's Law calculator
2. 2🔋Voltage Drop
   The subpanel feeder is usually the longest run in the whole job. Enter the design current, the one-way length of the run, and the resistance of the conductor you are considering (ohms per 1,000 ft, from a wire table), to see the actual voltage drop over that distance. A run to a detached garage or shed can easily be long enough that a feeder sized only for ampacity shows up as dim lights and struggling motors at the far end. "📋 Copy Values" carries the current, length and system voltage straight into Cable Sizing next.
   🧮 Open the Voltage Drop calculator
3. 3🪢Cable Sizing
   Size the feeder conductor for voltage drop. "📥 Paste Values" fills in the same current, length and system voltage from Step 2, so just add the voltage drop you will allow. It returns the copper area that limit needs and the next standard size up, which is a number to bring to the supply house or your electrician. It is a voltage-drop answer only: the conductor must also have the ampacity for its breaker, and the larger of the two sizes wins.
   🧮 Open the Cable Sizing calculator

Worked example: a workshop 240 ft away on a 60 A feeder

A workshop 240 ft from the house is to be fed by a 60 A, 240 V subpanel feeder. The loads that could run together are a 4,500 W, 240 V heater, a 240 V compressor drawing 12 A, and two 120 V tool circuits of 12 A each, one on each leg. The question is what conductor the feeder needs.

What it adds up to

The chain turned four load currents into a design current of 42.75 A per leg, showed that the obvious 6 AWG conductor would drop 3.4 percent over 240 ft, and landed on 4 AWG. The cheap part of a long feeder is the copper: trenching and conduit cost far more than the difference between two conductor sizes, so on a buried run it usually pays to go up a size now rather than dig it up later.

What the chain left out is worth stating plainly. The main panel's capacity, the demand calculation your code requires, the breaker sizes, the conduit fill, the grounding electrode and every inspection are outside these calculators. They are the reason this is a job for a permit and, in many places, a licensed electrician.

Notes & limitations

• The commonly cited guidance keeps voltage drop under about 3% for a branch circuit or a feeder alone, and about 5% total from service to the furthest outlet. This guide checks the feeder in isolation, so leave some of that 5% budget for the branch circuits downstream of the subpanel.
• A subpanel also needs its own grounding and bonding scheme (typically a separate ground bus, with the neutral bonded to ground only at the main service, and a grounding electrode at a separate building). That is a code and safety question this guide only touches on, and worth confirming with your local code or an electrician before you wire anything.

Mistakes that make a subpanel look right and behave wrong

These are the errors that pass a quick look and show up months later.

• Sizing the feeder by ampacity alone. On a long run the voltage-drop check can demand a larger conductor than the breaker rating does, as the example shows.
• Adding every breaker's rating together. The feeder carries the demand, the current that can really be on at once, not the sum of the panel's labels.
• Forgetting that only the 240 V loads draw equal current in both legs. The 120 V loads must be balanced across the two legs, or one leg carries far more than the average and its conductor and breaker with it.
• Bonding the neutral to ground in the subpanel. It sends load current onto ground wires and metal enclosures, which is a shock hazard.
• Spending the whole voltage-drop budget on the feeder. About 3 percent for the feeder and 2 percent for the branch circuits behind it is the usual split of a 5 percent total.

When to bring in an electrician

Work inside the main panel involves live bus bars that stay energized even with the main breaker off, and anything on the utility side of the meter is never a do-it-yourself job. Most jurisdictions require a permit and an inspection for a subpanel, and many require a licensed electrician.

An electrician can also do the load calculation that decides whether your service can take another 60 A at all, which this project cannot. If the service is near its limit, that answer changes the whole plan before a single cable is bought.

Frequently asked questions