Overview: what a subpanel is and the key safety rules
A subpanel is a remote load center fed from your main/distribution panel and used to distribute circuits closer to the loads they serve. Feeders to a subpanel are normally a 4‑wire feeder: two hot conductors, one insulated neutral (grounded conductor), and one separate equipment grounding conductor; the neutral must be isolated (not bonded) to the subpanel enclosure — neutral‑to‑ground bonding remains at the service disconnect/main panel only[1][3][4].
Before any work on panels, always de‑energize the service and follow the panel manufacturer's instructions for torque and termination values; manufacturer manuals show the neutral bus isolated for subpanel use and often ship with a removable bonding screw/strap to allow separation of neutral and ground[4][5].
Step 1 — Make a load list and pick a sizing approach
Start with a written list of every device and circuit you plan to put on the subpanel (lighting, dedicated 240 V tools, HVAC, EV charger, etc.), mark which loads are continuous, and estimate wattage or amps for each item; you can then either perform the NEC Article 220 load calculation or hand this list to a licensed electrician for a permit‑grade sizing and review[1][6][10].
Remember continuous loads must be treated as 125% when sizing feeders (continuous loads should not exceed 80% of the feeder rating) — include that in your math or in the instructions you give your electrician[1][6].
The feeder must be protected by an overcurrent device (a breaker) in the main/distribution panel; the feeder breaker rating is used to determine the minimum equipment grounding conductor (EGC) size per NEC Table 250.122[1][4][9].
NEC Table 250.122 sets the minimum EGC size based on the rating of the overcurrent protective device protecting the feeder; always verify the correct table entry for the feeder breaker you choose before finalizing conductor sizes[9][1].
Typical field examples used by trades and online guides: 60 A feeders are commonly run with 6 AWG copper or 4 AWG aluminum; 100 A feeders commonly use 3 AWG copper or 1 AWG aluminum (some guides show #4 copper or #2 aluminum depending on assumptions). These are starting points only — actual conductor choice depends on which NEC ampacity column you must use (60°C/75°C/90°C), conductor type, termination temperature ratings, and voltage‑drop needs[6][11][12][13].
As a common example often cited in reference material: for a 100 A feeder many references list a minimum copper EGC of #8 AWG based on Table 250.122; verify that against the actual feeder breaker rating and the NEC table in force locally[9][11].
Step 3 — Conductor ampacity, terminals, and voltage drop
Conductor ampacity and sizing must follow NEC ampacity tables (for example Table 310.15(B)(16) or the applicable edition adopted by your jurisdiction), consider conductor insulation type and terminal temperature ratings, and apply derating when multiple current‑carrying conductors are in the same raceway[1][10].
Voltage drop and run length affect conductor sizing — use manufacturer or industry voltage‑drop calculators to confirm whether you need to upsize the feeder for long runs[10][12].
Step 4 — Neutral and grounding connections at the subpanel
Do not bond neutral and ground in a subpanel. Remove the neutral bonding screw or strap, or use a neutral bus that is insulated from the enclosure and install a separate equipment grounding bus that is bonded to the enclosure. Many panel manufacturers’ manuals show the panel ships with a removable bonding screw/strap for exactly this purpose[4][5][3].
Keep the neutral bus isolated from the enclosure and ensure the EGC is connected to a grounding bus that is bonded to the panel enclosure and back to the service ground via the feeder EGC[3][4].
Detached buildings: grounding electrode systems and mandatory bonds
If the subpanel is in a detached structure served by a feeder, a grounding electrode system (for example, ground rod(s)) is required at the detached building and must be connected in accordance with NEC 250.32 in addition to the 4‑wire feeder. The grounding electrode conductor bonds the local electrodes to the grounding bus at the subpanel and is a required connection, not optional[1][3].
Permits, inspections, and manufacturer instructions
Most jurisdictions require a permit and inspection to install a subpanel; the NEC is adopted locally and authorities having jurisdiction (AHJs) may amend requirements, so check with your local AHJ before you start[1][7].
Follow the load center manufacturer’s installation manual for neutral isolation details, torque values, and conductor termination instructions; manufacturers explicitly instruct de‑energizing service before panel work[4][5].
Common code traps and practical homeowner checklist
Using a 3‑wire feeder or bonding neutral and ground at the subpanel — this is not allowed for subpanels in the same building and violates the neutral isolation rule; use a 4‑wire feeder and keep neutral isolated[1][3].
Skipping a grounding electrode at a detached structure — a grounding electrode system is required at detached buildings served by a feeder[1][3].
Mismatching feeder breaker rating and conductor/EGC sizes — the feeder breaker determines minimum EGC per Table 250.122 and influences conductor ampacity requirements; select the breaker consistent with your conductor choices and load calculation[1][9][4].
Ignoring terminal temperature ratings and derating rules — pick conductor sizes using the correct NEC ampacity column and derate when multiple conductors share a raceway[1][10].
Not checking voltage drop for long runs — voltage drop can force upsizing the feeder even when ampacity would allow a smaller conductor[10][12].
Failing to obtain permits or consult the AHJ — always check local permit requirements and have the work inspected[1][7].
Not following the panel manufacturer's removable bonding‑screw instruction — many panels include a removable screw/strap for neutral/ground separation; follow the manual when converting the panel to subpanel service[4][5].
Practical final steps before energizing: have the work inspected as required, ensure all terminations are torqued to manufacturer specs, verify the neutral bus is isolated, verify the grounding bus is bonded to the enclosure, and when re‑energizing, check for any unexpected voltage readings or tripped devices — if unsure, have a licensed electrician verify the installation[4][5][1].
When to call a licensed electrician
Hire a licensed electrician if you do not have panel experience, when the load calculation is complex (service upgrades, large continuous loads, EV charging, or HVAC), when local code interpretations are unclear, or to obtain and pass the required inspection — electricians also confirm conductor choices against the actual termination temperature ratings and local NEC edition[1][6][7].
If your project includes adding a major 240 V appliance or EV charger, plan the subpanel feeder and breaker selection around that equipment’s nameplate load and consider the site guide on adding dedicated 240 V circuits for practical context.
Key references and tools
NEC (NFPA 70) for formal rules, load calculation in Article 220, grounding in Article 250, and conductor ampacity tables — consult the actual code edition adopted by your AHJ[1].
Manufacturer installation manuals (Schneider/Square D, Eaton) for neutral isolation instructions, torque values, and termination guidance[4][5].
NEC Table 250.122 reproduction for EGC minimum sizes and industry/online guides that show typical feeder conductor examples — always verify these examples against the NEC tables and your installation specifics[9][11][13].
Manufacturer/industry voltage‑drop calculators such as Southwire’s online tools to check whether the run length requires upsized conductors[10][12].
Bottom line
Installing a subpanel safely is a combination of proper planning (a written load list and continuous‑load accounting), following NEC rules for a 4‑wire feeder with an isolated neutral, sizing conductors and the EGC to the feeder breaker per NEC tables, addressing grounding electrodes for detached buildings, following manufacturer termination instructions, and obtaining required permits and inspections. When in doubt — or where load calculations or local code interpretations are complex — hire a licensed electrician to size and install the feeder and subpanel[1][9][4][3].