Selecting the correct wire size for a 400 amp service is critical for safety, code compliance, and long term performance. This guide covers conductor types, temperature ratings, voltage drop, and installation methods that impact ampacity.
Use this information as a starting point for preliminary planning and always confirm final specifications with local authority having jurisdiction and licensed electricians.
| Wire Type | Material | Minimum Size (AWG/kcmil) | Typical Use |
|---|---|---|---|
| THHN/THWN in conduit | Copper | 3/0 AWG | Branch circuits and feeders |
| XHHW in conduit | Aluminum | 250 kcmil | Cost effective feeders |
| Service entrance cables | Copper or Aluminum | 250 kcmil to 4/0 AWG | Lateral connections to panel |
| RHW in conduit | Copper | 250 kcmil | Heavy industrial feeders |
| Bus bars in panels | Copper | 3/0 AWG and up | Main lugs and grounding |
Understanding 400 Amp Service Requirements
A 400 amp service typically feeds large residential subpanels, multi family properties, small commercial buildings, or light industrial loads. National Electrical Code requirements focus on conductor ampacity, overcurrent protection, and grounding for this service level. Proper sizing prevents overheating, nuisance tripping, and fire risk.
For copper conductors in open air or conduit, 3/0 AWG often matches a 400 amp breaker at standard temperature ratings. Aluminum alternatives usually require 250 kcmil to reach similar performance while accounting for derating factors.
Conductor Ampacity and Temperature Rating
Temperature Ratings Impact Ampacity
Insulation temperature rating directly affects ampacity. THHN wire with a 90 degree Celsius rating allows higher breakers compared to 60 or 75 degree Celsius rated insulation. Select conductors with matching or higher temperature ratings to circuit breaker and environment.
Conduit Fill and Derating
Multiple conductors in conduit require derating factors that reduce allowable ampacity. Ambient temperature above 30 degrees Celsius, grouping with heat producing equipment, and embedded installations in thermal insulation also lower usable current capacity.
Voltage Drop and Practical Sizing
Long feeder runs for 400 amp services can experience significant voltage drop, affecting motor performance and sensitive electronics. For runs over 100 feet, upsizing to 4/0 AWG copper or 300 kcmil aluminum is common to keep voltage drop below 1 to 1.5 percent.
Calculate voltage drop using conductor resistance, reactance, and circuit length rather than relying solely on ampacity tables. This ensures stable voltage at the service panel under full load conditions.
Installation Methods and Protection
Conduit, Busway, and Cable Types
Use rigid metal conduit, EMT, or flexible conduit for 400 amp feeders to protect conductors and simplify future changes. Busway offers an alternative for complex layouts where adding circuits later is expected. Select appropriate service entrance cables where the connection point enters the building.
Main Breaker and Coordination
Size the main breaker or fused switch to match conductor ampacity, typically 400 amps for copper 3/0 AWG. Coordinate with feeder overcurrent devices, motor loads, and transformer inrush current to avoid nuisance shutdowns during startup surges.
Key Takeaways for 400 Amp Service Wiring
- Select copper or aluminum conductors based on ampacity, voltage drop, and cost for feeders and service lateral.
- Confirm ampacity after conduit fill and ambient derating to avoid overheating.
- Size overcurrent protection to match conductor rating at 125 percent for continuous loads.
- Plan for voltage drop on long runs and consider upsizing conductors.
- Coordinate main breaker, grounding, and local authority requirements before installation.
FAQ
Reader questions
Can I use THHN in conduit for a 400 amp service?
Yes, THHN copper conductors sized at 3/0 AWG are commonly permitted for 400 amp services when installed in conduit and properly derated for conduit fill and ambient temperature.
What is the minimum aluminum wire size for 400 amps?
Aluminum conductors typically require 250 kcmil for a 400 amp service, depending on insulation type, temperature rating, and local code allowances.
How does voltage drop affect 400 amp feeders?
Long conductors create resistance that reduces voltage at the load, so upsizing to 4/0 AWG copper or 300 kcmil aluminum is recommended for runs over 100 feet to limit voltage drop.
Is busway required for 400 amp service panels?
Busway is optional but helpful for future expansion and complex layouts; many installations use standard insulated conductors in conduit with a properly rated main breaker.