The 250 kcmil copper resistance in ohms per 1000 ft is the inherent opposition to direct current flow measured across a 1000-foot length of 250,000 circular mil copper conductor, which sits at exactly 0.054 ohms at 75°C. In a real circuit or installation, this baseline resistance dictates your voltage drop and I²R heat generation, directly determining whether your heavy feeder will maintain safe terminal temperatures and deliver adequate voltage to the load panel. Most commonly, apprentices and DIYers confuse the linear scaling of kcmil with the inverse scaling of AWG, or they mistakenly use DC resistance values when calculating AC voltage drop for long runs where skin effect matters.

The Direct Answer: According to NEC Chapter 9, Table 8, the DC resistance for uncoated 250 kcmil copper wire is 0.054 ohms per 1000 ft at 75°C (167°F). At a standard ambient 20°C (68°F), the resistance drops to approximately 0.043 ohms per 1000 ft.

The Data: 250 kcmil and Adjacent Feeder Resistance Specs

When pulling heavy feeders, you rarely just look at one wire size. You compare adjacent sizes to see if stepping up to 300 kcmil or 350 kcmil yields a meaningful reduction in voltage drop that justifies the higher material cost and the physical difficulty of bending thicker wire in conduit. The table below pulls directly from standard NEC Chapter 9, Table 8 parameters for uncoated copper conductors at the 75°C temperature column—the standard termination rating for circuits over 100A per NEC 110.14(C)(2).

Wire Size (kcmil) Cross-Sectional Area DC Resistance @ 75°C (Ω/kft) DC Resistance @ 20°C (Ω/kft) Max Ampacity (75°C Column)
250 kcmil 250,000 cmil 0.054 0.0431 255A
300 kcmil 300,000 cmil 0.045 0.0359 285A
350 kcmil 350,000 cmil 0.038 0.0304 310A
400 kcmil 400,000 cmil 0.033 0.0264 335A

Note: Coated (tinned) copper wire has a slightly higher resistance—roughly 0.057 Ω/kft for 250 kcmil at 75°C—due to the tin alloy's lower conductivity, though this rarely changes the final wire size selection in standard commercial or residential work.

Worked Example: Calculating Voltage Drop on a 250A Feeder

Let’s move from theory to the jobsite. Imagine you are feeding a 200A continuous load (like a large commercial HVAC unit or a subpanel serving heavy machinery) located 200 feet away from the main switchgear. You are using 240V single-phase power.

Step 1: Sizing the Conductor (NEC Compliance)
Because the load is continuous (operating for 3 hours or more), NEC Article 210.19(A)(1) requires us to multiply the load by 125%.
200A × 1.25 = 250A minimum required ampacity.
Looking at the 75°C column in NEC Table 310.16, 250 kcmil copper is rated for 255A. It passes the ampacity test. We will protect it with a standard 250A breaker.

Step 2: Calculating Actual Voltage Drop
Voltage drop is calculated using the actual operating current (200A), not the derated continuous sizing current. Think of voltage drop like water pressure loss in a long garden hose; the longer the hose and the higher the flow rate, the more pressure you lose at the nozzle before it reaches the end.

  • Current (I): 200A
  • One-way Distance (D): 200 ft (0.2 kft)
  • Resistance (R): 0.054 Ω/kft (from Table 8 at 75°C)

First, find the resistance of the one-way run:
R_run = 0.054 Ω/kft × 0.2 kft = 0.0108 ohms

Next, calculate the total voltage drop for a single-phase circuit (which requires multiplying by 2 to account for both the ungrounded 'hot' conductor and the grounded 'neutral' or second hot leg):
V_drop = 2 × I × R_run
V_drop = 2 × 200A × 0.0108 ohms = 4.32 Volts

Finally, find the percentage drop on a 240V system:
% Drop = (4.32V / 240V) × 100 = 1.8%

Result: A 1.8% voltage drop. This is well under the NEC's recommended maximum of 3% for branch circuits and feeders (NEC 210.19 Informational Note), meaning 250 kcmil copper is both legally compliant and electrically efficient for this run.

Where You Meet This in Practice

You don't pull 250 kcmil wire for standard bedroom receptacles. You meet this specific resistance profile in high-current infrastructure where a fraction of an ohm translates to massive heat and wasted energy.

Mains Voltage Safety: Working with 250 kcmil feeders means dealing with 200A to 300A+ service capacities. Always de-energize the main service disconnect, lock out and tag out (LOTO) the upstream utility feed if possible, and verify dead with a CAT IV rated multimeter before touching any lugs. Local codes frequently require a licensed electrical contractor for service entrance work.
  • Residential Service Entrances: While 4/0 AWG is common for 200A residential services, custom luxury homes or properties with multiple EV chargers (like dual Tesla Wall Connectors) and all-electric heating often upgrade to 300A or 320A services. 250 kcmil is the baseline starting point for these heavy residential feeders.
  • Commercial Subpanels: In retail or light industrial spaces, 250 kcmil is frequently pulled through 2-inch or 2.5-inch PVC or EMT conduit to feed 225A or 250A distribution panels powering rooftop HVAC units and lighting arrays.
  • Large Solar Inverter Tie-ins: Commercial solar arrays utilizing 250kW+ string inverters often require parallel sets of 250 kcmil or 300 kcmil copper to handle the massive DC-to-AC output current without exceeding the 3% voltage drop limit at the point of common coupling (PCC).
  • Industrial Motor Feeds: Large 150HP to 200HP 480V 3-phase motors draw significant starting current. While the running ampacity might fit in 250 kcmil, the voltage drop during across-the-line starting can cause contactors to chatter or drop out if the wire resistance is too high over long distances.

For deeper verification on your specific runs, always cross-reference your calculations with authoritative tools like Southwire's official calculator tools or Cerrowire's sizing and voltage drop resources, which account for specific insulation types and conduit materials.

Common Confusions: kcmil vs. AWG and DC vs. AC Impedance

When ordering wire or reading schematics, two major points of confusion lead to costly mistakes on the bench and in the field.

1. The Scaling Direction: kcmil vs. AWG
In the American Wire Gauge (AWG) system, a larger number means a smaller wire (e.g., 10 AWG is smaller than 6 AWG). This inverse logarithmic scale stops at 1 AWG. Once you cross into the 'ought' sizes (1/0, 2/0, 3/0, 4/0), the physical cross-section grows, but the naming convention gets clunky. Enter kcmil (thousands of circular mils), formerly known as MCM. kcmil scales linearly with physical area. A 250 kcmil wire has exactly 250,000 circular mils of copper cross-section. A 500 kcmil wire has exactly twice the copper and roughly half the resistance. Remembering this linear relationship makes mental math for parallel conductor runs much easier.

2. DC Resistance (Table 8) vs. AC Impedance (Table 9)
The 0.054 ohms/kft figure is the DC resistance. For 60Hz AC power on wire sizes up to about 350 kcmil, the DC resistance and AC resistance are nearly identical. However, as you push into 400 kcmil, 500 kcmil, and larger, the skin effect begins to force AC current to the outer perimeter of the conductor. This effectively reduces the usable cross-sectional area for AC current, raising the AC resistance above the DC baseline. If you are designing a run with 500 kcmil or parallel sets of 250 kcmil, you must consult NEC Chapter 9, Table 9 for AC impedance, factoring in whether your conduit is magnetic (steel) or non-magnetic (PVC/aluminum), as magnetic conduit introduces reactance that further alters the total impedance.

For complete code compliance and standard definitions, always refer to the latest edition of the National Fire Protection Association (NFPA) NEC portal. Remember that while NEC guidelines provide the baseline for safety and efficiency, your local Authority Having Jurisdiction (AHJ) has the final say on specific installation methods and derating requirements.