Feeder Wire Size Chart – How to Determine the Right Size
Choosing the correct feeder wire size is essential for safety, efficiency, and compliance with electrical codes. If the feeder wire is too small, it can overheat, cause voltage drop issues, or even create a fire hazard. If it is oversized, it leads to unnecessary material costs. This is why electricians, engineers, and even DIY enthusiasts refer to a feeder wire size chart before making decisions.
Table of Contents
Table of Contents
In electrical installations, feeder wires carry electricity from the main service panel to subpanels or large appliances. The wire size depends on the amperage, voltage, conductor material, and distance of the run. The National Electrical Code (NEC) provides guidelines for choosing the right feeder wire size, and there are also calculation methods to ensure accuracy.
Understanding feeder wire sizing is not just about looking at a chart — it’s about knowing the factors behind those numbers. In this guide, you’ll learn how to use a feeder wire size chart, how to calculate wire size, and the recommended sizes for common feeder ratings such as 100A, 200A, 300A, and 400A. You’ll also understand technical rules like the 83% rule for continuous loads, and get answers to common questions like whether 3/0 or 4/0 wire is larger.
Key Takeaways
- Feeder wire size depends on amperage, voltage, distance, and conductor type (copper or aluminum).
- Voltage drop must be kept under 3% for feeders according to NEC recommendations.
- The 83% rule applies when calculating conductor size for continuous loads.
- Using a feeder wire size chart ensures compliance and avoids guesswork.
- Copper wire has higher ampacity than aluminum for the same gauge.
Use our online tool AWG to mm² Calculator
Feeder Wire Size Chart – Understanding the Basics
A feeder wire size chart shows the minimum conductor sizes for specific amperages based on standard conditions. The ampacity values are generally taken from NEC Table 310.16 (or equivalent tables in other regions).
The chart considers the conductor type, insulation temperature rating, and installation method. Here is a commonly used feeder wire size chart for copper and aluminum conductors (75°C insulation rating, standard installation conditions, voltage drop not yet considered):
| Feeder Amp Rating | Copper Wire Size | Aluminum Wire Size |
|---|---|---|
| 20A | 12 AWG | 10 AWG |
| 60A | 6 AWG | 4 AWG |
| 100A | 3 AWG | 1 AWG |
| 125A | 1 AWG | 2/0 AWG |
| 150A | 1/0 AWG | 3/0 AWG |
| 200A | 2/0 AWG | 4/0 AWG |
| 300A | 350 kcmil | 500 kcmil |
| 400A | 600 kcmil | 1,000 kcmil |
Note: These sizes are based on typical NEC guidelines for 75°C-rated conductors and may vary with installation method, ambient temperature, and distance. For long runs, a larger size may be required to reduce voltage drop.
What Size Wire for a 200 Amp Feeder?
For most standard installations, a 200 amp feeder requires:
- Copper: 2/0 AWG
- Aluminum: 4/0 AWG
If the distance from the main panel to the subpanel is long (e.g., over 100 feet), upsizing the wire is recommended to minimize voltage drop. For example, you may use 250 kcmil copper instead of 2/0 AWG for a long run.
Use our online tool Conduit Fill Calculator (NEC Standard)
What Size Feeder Wire for 100 Amp Service?
A 100 amp feeder usually requires:
- Copper: 3 AWG
- Aluminum: 1 AWG
This is suitable for typical home subpanels or detached buildings. If running a 100A feeder to a workshop 150 feet away, upsizing to 1 AWG copper or 2/0 AWG aluminum might be needed.
What Size Wire for 400 Amp Feeder?
A 400 amp feeder is common in commercial or large residential setups. You will need:
- Copper: 600 kcmil
- Aluminum: 1,000 kcmil
Due to the high current, voltage drop and conduit size become critical. At this amperage, parallel conductors are often used to meet NEC requirements.
How Do I Calculate Wire Size?
While charts make it easier, you can calculate wire size using these steps:
- Determine the load in amps – based on NEC load calculation methods.
- Apply the 83% rule – for continuous loads (running for 3 hours or more), multiply the load by 1.25 to size the wire.
- Check ampacity tables – find a wire size that can handle the adjusted load.
- Account for voltage drop – use voltage drop calculators to check if upsizing is needed for long distances.
For example, if you have a 150A continuous load:
150 × 1.25 = 187.5A → choose a conductor that handles at least 187.5A.
Use our online tool House Wiring Cable Size Calculator
What is the 83% Rule for Feeder Wires?
The 83% rule (sometimes stated differently) comes from NEC Article 215.2. It allows feeder conductors to be sized at 83% of the total load rating in some situations, particularly where demand factors are applied.
This is not to be confused with the 125% rule for continuous loads. The rule is often applied in service entrance calculations where diversity is considered.
Sub Panel Wire Size Calculator
A sub panel wire size calculator helps determine the right conductor size based on amperage, voltage, phase, conductor material, and distance. It factors in:
- Load amperage
- Voltage (120/240V or 3-phase)
- Distance to sub panel
- Conductor type (copper/aluminum)
- Allowable voltage drop
For example, a 100A subpanel at 100 feet using copper would typically require upsizing from 3 AWG to 1 AWG copper to maintain voltage drop under 3%.
What Size Wire for 300 Amps?
A 300 amp feeder needs:
- Copper: 350 kcmil
- Aluminum: 500 kcmil
As with higher amperages, check if parallel conductors are more economical.
Which is Bigger, 3/0 or 4/0 Wire?
4/0 wire is bigger than 3/0 wire. In AWG (American Wire Gauge), as the number before “/0” increases, the conductor gets larger. 4/0 is commonly used for 200A aluminum feeders and large service entrances.
Use our online tool Amps to Wire Size Calculator
What Size Feeder Wire Should a 20 Amp Circuit Have?
A 20 amp feeder or branch circuit typically requires:
- Copper: 12 AWG
- Aluminum: 10 AWG
For feeders at 20 amps, this is rare since feeders usually handle higher loads, but this size is common in branch circuits for receptacles and lights.
Advanced Voltage Drop Considerations for Feeder Wires
Voltage drop is the reduction in voltage between the power source and the load due to resistance in the wire. NEC recommends no more than a 3% drop for feeders. For example:
- A 200A feeder at 240V over 150 feet may drop over 3% if using minimum chart sizes. Upsizing from 2/0 to 3/0 copper can solve the issue.
- Voltage drop is more critical for long runs and heavy loads like motors.
Final Tips for Choosing Feeder Wire Size
- Always check the feeder wire size chart first for a baseline.
- Adjust for continuous loads using the 125% rule.
- For long runs, increase wire size to reduce voltage drop.
- Consider copper for higher efficiency and aluminum for cost savings.
- When in doubt, consult a licensed electrician and verify NEC compliance.
By combining a feeder wire size chart with proper calculations, you ensure a safe, code-compliant, and cost-effective electrical installation. For projects like subpanels, large appliances, or commercial feeders, precision in sizing will protect both the equipment and the wiring system for years to come.
Use our online tool Wire Size Calculator by Load and Length
FAQs
What size wire for 500 amp service?
Typical selections: 600 kcmil copper or 750 kcmil aluminum are commonly used, but exact size depends on insulation temp-rating and installation conditions.
Always verify against NEC Table 310.15(B)(16), terminal ratings, and voltage-drop requirements.
What size wire for 320 amp service?
Common practical choices: 350 kcmil copper or 600 kcmil aluminum (subject to insulation temp rating).
Confirm with NEC ampacity tables, ambient/temp correction factors and local code.
What size feeder wire should a 20 amp circuit have?
Standard: 12 AWG copper THHN/TW or equivalent for a 20 A branch circuit.
Use 10 AWG if long run (to limit voltage drop) or if aluminum is required.
What size line for method feeder?
Feeder conductor size is chosen from the calculated load current, NEC ampacity tables, and method of installation — there is no single “method” size.
Compute load current, apply demand/adjustment factors, then select conductor with adequate ampacity and voltage-drop performance.
How to tell which wire is the feed?
The feed is the conductor(s) delivering power from the source (meter/breaker) to the panel — it’s usually the largest conductors and labeled LINE or L1/L2/L3 and connected to the service/meter side.
Use continuity/voltage checks (with power off/on safely) and trace from the supply device to confirm.
Which is correct to size the conductors for a motor feeder?
Size feeders using the motor full-load current (FLA) from the nameplate and NEC Article 430 rules — feeder conductors are typically sized ≥125% of motor FLA for continuous duty and per 430.22/430.24 protections.
Also account for starting currents, short-circuit protection, and voltage-drop limits.
What size wire to run 20 amps 100 feet?
To limit voltage drop to ~3%, use 10 AWG copper for a 20 A load at 100 ft; 12 AWG may work for short runs but can have excessive drop at 100 ft.
If using aluminum or very sensitive equipment, upsize further and check calculated voltage drop.
Is thicker wire always better?
Thicker decreases resistance, heat and voltage drop, but increases cost, conduit fill, and may exceed terminal ratings or require larger connectors.
Choose size to meet ampacity, code, voltage-drop, mechanical and economic constraints — not simply “bigger is better.”
What size wire for a 30 amp circuit?
Standard: 10 AWG copper for a 30 A branch circuit (or 8 AWG if long run to control voltage drop).
Select conductor type and insulation per application and code.
What is the formula for feeder size?
Calculate load current: single-phase I= P/(V x PF) , three-phase I= P/( 1.732 x V x PF) Then select a conductor whose NEC ampacity ≥ calculated current (apply 125% for continuous loads), include correction/multiplier factors and verify voltage-drop.
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