6/4 SO Cord Ampacity and Amp Rating | 45 A Or 55 A?

About ten amps of headroom separate a portable cord that runs cool from one that runs warm, and the only thing deciding which you get is how many conductors inside the jacket are actually carrying current. Published 6/4 SO cord ampacity and amp rating figures land at 45 amps for exactly that reason: the number assumes the usual three-phase load, three conductors live and a fourth doing ground duty.

The name spells out the build — four conductors, each one 6 AWG. Suppliers sell it as 6/4 SO or 6/4 SOOW with a 600-volt jacket, and the spec sheet prints a single figure, but the same copper is good for more in a different arrangement. That gap is where most sizing mistakes begin.

How Many Amps Does 6/4 SO Cord Carry?

Nearly every 6/4 SO and 6/4 SOOW listing quotes 45 A because three-phase plus ground is the most common way this cable gets used.

Flexible cord ampacity comes from NEC Table 400.5(A)(1), which sorts cord by conductor size and by how many conductors are loaded. The four in “6/4” does not mean four conductors are always carrying current. One of them is a ground.

• Three current-carrying conductors: 45 A — three hots plus a ground, the standard three-phase setup.
• Two current-carrying conductors: 55 A — two hots plus a neutral, with the ground still inside the jacket.

The two uses look identical from the outside, which is why the mix-up happens. A 6/4 cord feeding a three-phase motor sits in the 45 A column. The same cord wired as two hots, a neutral, and a ground sits in the 55 A column. Same copper, ten amps apart.

Here is how the scenarios break down:

6/4 SO Cord Ampacity: The Details That Move The Number

Ampacity is a table value, not a fixed property of the copper, so anything that changes conditions around the cable changes the answer. Three factors move it most: how many conductors carry current, how hot the surrounding air is, and what the cord terminates into.

Table 400.5(A)(1) is built on a 30°C (86°F) ambient temperature. Run a 6/4 SO cord across a sun-baked roof deck or through a hot mechanical room and the allowable ampacity drops under NEC temperature derating rules. The jacket print matters here too — the insulation temperature rating describes what the cord tolerates, not just how much current it passes.

Terminations are a separate question the ampacity table never answers. A 45 A or 55 A figure describes the copper only, and says nothing about whether the plugs, lugs, and receptacles on the ends are rated for that load. Verify the device ratings before trusting the cord alone.

Comparing flexible cord to fixed wiring is the other frequent slip. Ampacity for NM-B or THHN comes from different NEC tables, and pulling a number from those tables for a portable cord is a code violation waiting to be written up.

What The Jacket Tells You Before You Buy

The print on a 6/4 SO jacket carries the conductor count, voltage rating, insulation temperature, and any listing mark, which makes it the fastest sanity check before money changes hands.

Pricing tracks the listing. When a job gets inspected, the listed jacket is usually the cheaper decision. Our tested 6/4 SO cord picks compare both versions side by side if you would rather skip the guesswork.

Sizing it takes four checks:

1. Count the conductors that will carry current — three means 45 A, two means 55 A.
2. Measure the ambient temperature where the cord will run and derate above 86°F.
3. Confirm the plugs, lugs, and receptacles are rated for the load.
4. Match the jacket’s voltage and temperature rating to the job, and confirm the listing mark if the work gets inspected.

FAQs

Is 6/4 SO cord the same as 6/4 SOOW?

Both carry four 6 AWG conductors. The SOOW jacket adds oil and water resistance along with an extra-hard service rating, and it is the version most suppliers stock for outdoor and industrial work. Where a job involves moisture, oil, or direct sun, that heavier jacket is the one to order.

Can A 6/4 SO Cord Feed A 50-Amp Circuit?

With two current-carrying conductors, the listed ampacity is 55 A, which clears 50 A on the conductor side. Terminations, plugs, and the load type still have to suit a 50 A circuit, and the ampacity table addresses none of them. Where code requires fixed wiring, a cord is not a substitute.

Why Do Suppliers List 6/4 SO Cord At 45 Amps Instead Of 55?

Three-phase plus ground is the most common use, and that arrangement loads three conductors. The 45 A column is the honest figure for that job. The 55 A figure applies only when the cord is wired with two hots and a neutral, leaving fewer conductors carrying current.