Step-by-Step Wire Gauge Selection Workflow

1. Start With the Load

Determine the expected amperage draw. Reference the equipment nameplate or use NEC load calculation methods (NFPA 70®).

2. Match to the Breaker

These wire sizes are consistent with NEC 240.4(D) and standard field practice for copper conductors:

Note: For continuous loads, NEC 210.20(A) requires the circuit to be rated at 125% of the load.

3. Check Insulation Ratings

THHN is typically rated for 90 °C in dry locations (Southwire THHN Spec Sheet). However, ampacity must be based on the lowest-rated component in the circuit, which often means using the 60 °C or 75 °C column in NEC Table 310.16.

4. Calculate Voltage Drop

Voltage drop should not exceed 3% for branch circuits and 5% overall including feeders, per NEC 210.19(A)(1). Use an online calculator such as the Schneider Electric Voltage Drop Calculator to determine if upsizing is necessary. Avoid using fixed distance rules—they can lead to undersized conductors or unnecessary upsizing.

5. Adjust for Bundling and Ambient Temperature

If more than three current-carrying conductors share a raceway or cable, NEC 310.15(C)(1) requires ampacity derating. Additional factors such as high ambient temperatures or enclosed conduit can also trigger derating. For full details, see EC&M on 310.15.

Installation and Commissioning Best Practices

Installation

• Use solid conductors for fixed runs where possible

• Use stranded wire in flexible or vibration-prone areas

• Avoid nicking the conductor during stripping

• Follow manufacturer torque specs (Eaton Torque Guide)

• Use antioxidant compound on aluminum terminations if specified

• When feasible, leave 20% headroom on ampacity to account for future loads or environmental changes

Commissioning

• Label both ends of all conductors

• Photograph junctions before they’re enclosed

• Save NEC tables and product cut sheets in your project folder

• Document torque specs for inspection

• Keep product packaging for reference

Troubleshooting Reference

Contractor FAQ

Can I run 14 AWG on a 20 A breaker?
No. NEC 240.4(D) only allows 14 AWG to be protected by a 15 A breaker.

How far can I run 12 AWG before voltage drop becomes a problem?
It depends on load, voltage, and environment. Use the Southwire Voltage Drop Calculator to verify.

Can aluminum be used in residential wiring?
Yes—for feeders or service entrances. You must use listed connectors per NEC 110.14(C), and apply antioxidant compound only if the lug requires it (source).

What’s the difference between THHN and XHHW?
THHN has a PVC jacket with nylon coating. XHHW has a cross-linked polyethylene jacket and offers better moisture and chemical resistance (Service Wire comparison).

Can I mix solid and stranded wires in a single box?
Yes, but only with UL-listed connectors rated for mixed conductors.

What’s the ampacity of 6 AWG copper at 90 °C?
75 amps per NEC Table 310.16, assuming terminals are also rated 90 °C and derating isn’t required.

Does NM-B need conduit?
Indoors: No.
Outdoors: Yes, unless replaced with UF-B (The Spruce).

Should I retorque lugs?
Yes. Do it once after installation, and then periodically based on the terminal’s specification.

Code and Compliance Quick Reference

Contractor Sourcing Checklist

• Breaker size and amp draw

• Conductor material (copper/aluminum)

• Wire gauge and insulation type

• Run environment (wet, dry, conduit)

• Voltage drop calculation

• Listed connectors and torque tools

• NEC tables and spec sheets

• Labeling tools and documentation plan

Summary

• Wire gauge decisions should be based on real-world conditions—not just breaker size or distance.

• Always calculate voltage drop; don’t rely on distance-based shortcuts.

• Derate when bundling or installing in high ambient temperatures.

• Aluminum is allowed in many applications, but only with compliant materials and installation methods.

• Good documentation—including labels, photos, and torque logs—makes inspections faster and callbacks less likely.

Need help?
Revco Lighting & Electrical Supply has supported Long Island’s trade professionals since 1978.

• Visit a branch

• Call (631) 283-3600

  Shop wire and cable