Subpanel Feeder & Sizing Calculator
Calculate subpanel feeder conductors (hot, neutral, ground), conduit size, voltage drop, and detached structure NEC 250.32 grounding rules.
Subpanel Feeder Specifications
NEC 215 • NEC 225- 4-Wire Feeder: Must run 2 Hots, 1 Insulated Neutral, and 1 Equipment Ground (no 3-wire grandfathering for new work).
- Unbonded Neutral (Floating): Discard green bonding screw. Neutral must NOT touch panel metal.
- Auxiliary Ground Rods: You must install a separate grounding electrode system (min 2 ground rods >=6ft apart) at the detached building bonded to the ground bar.
Main panel load calculations, trenching, conduit runs, and municipal permits.
About This Calculator & Field Methodology
Precision feeder sizing engine for residential and light-commercial electrical subpanels. Built to NFPA 70 / NEC Article 215 (Feeders), NEC Article 225 (Outside Branch Circuits & Feeders), and NEC 250.32 (Grounding for Separate Buildings). Computes hot conductor gauge, neutral reduction per NEC 220.61, equipment grounding conductors (EGC Table 250.122), conduit schedule, and detached garage grounding electrode requirements.
Standard formulas derived from international trade engineering standards (NEC Table 310.16, ACCA Manual J 8th Ed, ACI Concrete Mix Manual). Actual physical installations may require local authority having jurisdiction (AHJ) approval.
ToolHub Electrical Engineering Group
Commercial Electrical Inspector & Master Electrician · Master Electrician & IAEI Certified Electrical Inspector
Strict compliance with NEC, ACCA, and ACI code standards.
Frequently Asked Questions
Q:What wire size do I need for a 100-Amp subpanel?
For a 100-Amp subpanel with 75°C terminations per NEC Table 310.16, you need #3 AWG Copper conductors (or #2 AWG Copper for full continuous duty). If using Aluminum, you need #1 AWG Aluminum (or #2 AWG Aluminum Mobile Home Feeder / MHF where local jurisdictions permit residential derate).
Q:Why must the neutral and ground be separated in a subpanel?
Under NEC 250.24(A)(5) and 250.142, the neutral (grounded conductor) and equipment ground may ONLY be bonded together at the main service disconnect. In a subpanel, bonding neutral and ground creates parallel return paths across equipment metal frames, causing dangerous continuous objectionable current on conduit and metal boxes, resulting in severe shock and fire hazards.
Q:Do I need ground rods at a detached garage with a subpanel?
Yes. NEC Article 250.32(A) mandates that any detached structure fed by a multiwire branch circuit or feeder must have its own grounding electrode system. You must install a minimum of two 5/8-inch by 8-foot ground rods driven at least 6 feet apart, bonded to the subpanel ground bar with a continuous #6 AWG bare copper grounding electrode conductor.
Q:Can I run a 3-wire feeder to a detached garage?
No. The 2008 NEC eliminated the former 3-wire exception for new installations. All new feeders to detached buildings must be 4-wire (2 Hots, 1 Insulated Neutral, and 1 Equipment Grounding Conductor). Existing 3-wire installations installed under older code cycles may remain grandfathered only if no continuous metallic paths exist between buildings.
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