The wire size for a 600 amp service refers to the specific conductor gauge and the number of parallel runs required to safely distribute 600 amps of current without exceeding the thermal limits of the insulation or the termination ratings of the equipment. Because a single conductor capable of carrying 600A is physically impractical to bend and terminate, the direct answer relies on parallel runs: for a standard copper installation using the 75°C column, you need two parallel sets of 350 kcmil or three parallel sets of 4/0 AWG. If using aluminum, you need two parallel sets of 500 kcmil or three parallel sets of 250 kcmil.

Choosing the correct configuration changes everything about your physical installation. It dictates your conduit trade size, the physical dimensions of your pull boxes, the bending radius required at sweeps, and the specific lug terminations on your main breaker. Think of parallel wires like adding extra lanes to a highway; instead of building one impossibly wide road (a single massive cable), you build multiple standard lanes to handle the traffic volume safely.

SAFETY WARNING: A 600A service entrance involves lethal utility-side voltage and massive fault currents. Always de-energize the upstream utility feed, lock/tag out the disconnect, and verify the bus is dead with a Category IV rated multimeter before touching any conductors. Local codes frequently require a licensed electrical contractor and utility coordination for service entrance work.

The Core Math and Conductor Sizing Table

To size conductors for 600 amps, we must look at the National Electrical Code (NEC) Table 310.16. The most critical rule for services over 100A is NEC 110.14(C), which mandates that we use the 75°C ampacity column for termination limits, even if your wire insulation is rated for 90°C (like THHN).

Below is the data-dense reference table for the most common 600A configurations. This assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a single raceway.

Material Configuration (Sets) Wire Size per Phase Ampacity per Conductor (75°C) Total Ampacity Min Conduit Size (Trade Size)
Copper 2 Parallel Sets 350 kcmil 310A 620A 3-inch
Copper 3 Parallel Sets 4/0 AWG 230A 690A 3-inch
Aluminum 2 Parallel Sets 500 kcmil 310A 620A 3.5-inch
Aluminum 3 Parallel Sets 250 kcmil 205A 615A 3-inch
Copper 4 Parallel Sets 1/0 AWG 150A 600A 3-inch

Worked Numeric Example: Voltage Drop Calculation

Let’s calculate the voltage drop for a 200-foot underground feeder run using two parallel sets of 350 kcmil Copper on a 208Y/120V 3-phase system carrying a full 600A load.

The formula for 3-phase voltage drop is: VD = (√3 × K × I × L) / (CM × Sets)

  • √3 = 1.732
  • K (Copper) = 12.9 ohms-cmil/ft
  • I (Current) = 600A
  • L (Length) = 200 ft
  • CM (Circular Mils for 350 kcmil) = 350,000
  • Sets = 2

VD = (1.732 × 12.9 × 600 × 200) / (350,000 × 2)
VD = 2,681,136 / 700,000 = 3.83 Volts

On a 208V system, a 3.83V drop is 1.84%. This is well under the NEC recommended 3% maximum for feeders, proving that two sets of 350 kcmil copper is electrically sound for this distance.

Where You Meet 600A Services in Practice

You rarely see a 600A service on a standard single-family home. You will typically encounter this size in specific commercial and multi-family scenarios:

  • Multi-Family Dwellings: A 4-plex or 6-plex apartment building where the main service feeds a meter bank, and the calculated load (using NEC Article 220 optional or standard calculations) lands between 450A and 580A.
  • Commercial Strip Malls: A small retail plaza with 3-4 tenant spaces, each requiring a 150A to 200A panelboard.
  • EV Charging Hubs: A commercial parking lot hosting six 50kW DC fast chargers. Each charger pulls roughly 70A to 80A at 480V, pushing the total service demand past 500A.
  • Heavy Workshops: Facilities running multiple large CNC machines, industrial air compressors, and heavy MIG/TIG welding bays simultaneously.
Bench & Jobsite Insight: When pulling two sets of 350 kcmil, the bending radius is brutal. Standard 90-degree PVC sweeps will kink the wire or make pulling impossible. Use 36-inch radius sweeps (long sweeps) and apply high-quality wire pulling lubricant (like Polywater J) generously. A 3-inch conduit is the minimum, but stepping up to a 3.5-inch or 4-inch conduit for the main sweeps will save your crew hours of physical strain.

Common Confusions and NEC Code Traps

When sizing for 600 amps, even experienced journeymen can trip over specific NEC articles. Here is what people commonly confuse or get wrong:

1. Using the 90°C Column for Ampacity

THHN wire is rated for 90°C, and 350 kcmil Cu at 90°C is rated for 350A. Some builders mistakenly think two sets (700A total) is enough, or they try to use smaller wire based on the 90°C column. This is a code violation. Unless the main breaker lugs are explicitly rated and marked for 90°C (which is exceedingly rare for 600A equipment), NEC 110.14(C) forces you to use the 75°C column. Always size based on 75°C.

2. Neutral Conductor Sizing

Can you reduce the neutral wire size on a 600A service? NEC 220.61 allows reduction of the neutral for certain unbalanced loads, but there is a massive catch. If your 600A service feeds a commercial space with heavy non-linear loads (LED lighting, VFDs, server racks, EV chargers), harmonic currents will stack on the neutral. In these cases, the neutral must be sized at 100% of the phase conductors. Furthermore, many local AHJs (Authorities Having Jurisdiction) simply mandate a full-size neutral for any service over 400A to eliminate future harmonic overheating risks.

3. Parallel Conductor Rules (NEC 310.10(H))

You cannot just throw two random cables in a trench. NEC 310.10(H) strictly requires that parallel conductors be:

  • The exact same length (cut from the same continuous spool if possible, or measured precisely).
  • The same conductor material (all copper or all aluminum, never mixed).
  • The same cross-sectional area.
  • The same insulation type.
  • Terminated in the same manner.
If one set is 10 feet longer than the other, the resistance changes, the current divides unevenly, and the shorter set will overheat and melt the insulation while the breaker remains closed.

Step-by-Step Verification and Termination

Once the conductors are pulled into the 600A main breaker panel (such as a Square D QDP or Eaton 600A main frame), the termination process requires precision. Loose connections at 600A will arc, oxidize, and eventually cause a catastrophic phase-to-ground fault.

  1. Strip and Prep: Use a heavy-duty cable stripper. Do not nick the copper strands. If using aluminum, immediately apply an anti-oxidant compound (like Noalox) to the stripped conductor to prevent galvanic corrosion.
  2. Seat the Conductor: Ensure all strands are fully seated inside the mechanical lug barrel. No stray strands should be visible outside the lug.
  3. Torque to Spec: Do not guess the tightness. Use a calibrated torque wrench. For a standard 600A mechanical lug, the manufacturer spec (check the label inside the panel door) is typically between 350 in-lbs and 500 in-lbs. Use a calibrated break-over torque wrench to ensure you hit the exact value without over-tightening and stripping the lug threads.
  4. Verify Grounding: The Grounding Electrode Conductor (GEC) for a 600A service does not scale infinitely. Per NEC 250.66, the maximum required size for a copper GEC is 3/0 AWG, regardless of how large your phase conductors are. Ensure this is properly bonded to the grounding bus and the utility ground.
  5. Megger Test: Before energizing, use a megohmmeter (megger) at 1000V DC to test phase-to-phase and phase-to-ground insulation integrity. You should read >100 Megohms. Anything less indicates damaged insulation from the pull.

By strictly adhering to the 75°C ampacity column, respecting the physical realities of parallel pulls, and torqueing to exact manufacturer specifications, a 600A service will operate safely and efficiently for decades.