3/0 THHN Wire Guide: Ampacity, Specs, Sizing, Installation, and Buying Tips

Type "3 0 thhn" into a search box and you probably already know the shape of the job: a feeder, a service lateral, a panel upgrade, or a piece of equipment that needs a serious conductor. What you want now is confirmation. Amperage at the right temperature column, strand count, conduit size, and whether copper or aluminum is the smarter buy.

We have spent decades building cable for projects that range from metro lines to cement plants, and the questions buyers ask about 3/0 THHN are remarkably consistent. So here is the practical version, numbers first, sales pitch left outside.

What "3 0 THHN" Actually Means

Three-oh, 3/0, and 000 AWG all point to the same conductor size. At roughly 167,800 circular mils, or about 85 mm², 3/0 sits near the top of the everyday building-wire range: one size above 2/0 and one size below 4/0. It shows up when a 200 A feeder, a long service run, or a heavy equipment drop needs more metal than 2/0 can comfortably deliver.

THHN describes the insulation system rather than a separate product family. The letters stand for Thermoplastic, High Heat-resistant, Nylon-coated. In practice that means a PVC insulation covered by a thin nylon jacket, a 600 V rating, a 90°C dry-location temperature, and usually a dual THHN/THWN-2 marking that also allows 75°C wet locations. Standard constructions are gasoline and oil resistant, and most are sunlight resistant as well.

So the full description of what you are buying is: a 3/0 AWG conductor, copper or aluminum, insulated and jacketed to a 600 V building-wire standard, ready to be pulled through conduit or raceway.

3/0 THHN Ampacity: Reading the Numbers Correctly

Ampacity is where most ordering mistakes begin, because 3/0 THHN has three different numbers depending on which temperature column and which conductor material you are looking at.

Two habits keep you out of trouble. First, the conductor can run at 90°C, but the lugs, breakers, and equipment it lands on usually cannot, so the usable ampacity is normally taken from the 60°C or 75°C column unless the termination is marked otherwise. Second, the 90°C number is your starting point for correction factors: high ambient temperatures, more than three current-carrying conductors in a raceway, or both, all reduce the figure before you ever reach the panel.

Always check the values against the code edition your inspector enforces. Tables get renumbered and revised, and the authority having jurisdiction has the final word.

Construction Details That Matter in the Field

At this size, the conductor is almost always stranded, and the strand pattern affects how the cable behaves on a pull. A 3/0 Class B compressed copper conductor is commonly built from 19 strands, which gives a good balance of flexibility and diameter. Overall diameter for a typical 3/0 copper THHN lands around 0.6 to 0.63 in, roughly 16 mm, so plan your bending radius and pulling setup accordingly.

The PVC insulation does the dielectric work and the nylon jacket protects it during installation, which is why THHN resists abrasion so well in conduit. Because the jacket is thin, a nick during pulling can expose insulation, so inspection after the pull matters as much as the pull itself. Every reel we produce goes through the same conductor, insulation, and concentricity checks you can read about on our quality controls page.

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For jobs that call for a PVC-insulated building wire in this size class, that product family is the direct equivalent of the construction described above.

Where 3/0 THHN Fits Best

The most common applications we ship this size for:

• Service entrance and meter-to-panel feeders in residential and light commercial work.
• Panel-to-panel and subpanel feeders where a 200 A frame is involved.
• Motor feeders and equipment drops inside conduit, especially where voltage drop is already tight.
• Industrial cabinets, machine connections, and distribution runs inside plant buildings.
• Long runs where 2/0 would drop too much voltage and 4/0 would be overkill or hard to terminate.

Just as important is where it does not belong. Standard THHN is not a direct-burial cable, it is not a tray cable unless specifically listed as one, and it should not be used in wet locations unless the product carries the THWN-2 or equivalent wet-location rating. For continuous flexing, portable cordage or a flexible cable is the correct choice.

Sizing, Voltage Drop, and Conduit Checks

A repeatable sequence saves hours of rework:

1. Start from the calculated load, not from the breaker size.
2. Apply 125 percent to continuous loads before selecting the conductor.
3. Match the ampacity column to the lowest temperature rating in the circuit.
4. Apply ambient correction and conductor-count adjustment.
5. Check voltage drop, keeping branch circuits near 3 percent and the total near 5 percent.
6. Verify conduit fill and pulling tension before the reel leaves the yard.

Voltage drop is where 3/0 usually earns its cost. A 200 A load on a 150 ft run of 3/0 copper at 240 V loses only around 4 to 5 V, under 2 percent, which is why this size appears so often on longer service runs where 2/0 would have strained the limit.

Conduit sizing deserves the same attention. Three 3/0 conductors need a noticeably larger raceway than a typical branch circuit, and the fill tables, not a rule of thumb, should decide the number. Bending radius, pull length, and lubricant choice all matter more at this diameter than they do at 12 AWG.

THHN Compared With Other 3/0 Options

Before you commit, it helps to see the alternatives side by side:

• THHN/THWN-2: PVC insulation with a nylon jacket, 600 V, 90°C dry and 75°C wet. The default for conduit work.
• XHHW-2: cross-linked polyethylene insulation, rated 90°C in both dry and wet locations, often slightly smaller in diameter and a good choice for damp areas.
• USE-2 and RHW-2 types: built for direct burial and wet service, useful when the run leaves the building.
• Aluminum 3/0: lighter and often less expensive, but roughly one ampacity column lower, and it needs aluminum-rated lugs and proper surface preparation.

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If the route passes through wet or humid areas, the cross-linked polyethylene option above is usually the more comfortable long-term answer.

Buying 3/0 THHN Without Guesswork

A short checklist before you approve a purchase order:

• Confirm the jacket printing shows the dual THHN/THWN-2 rating if wet locations are involved.
• Decide copper or aluminum early, because lugs, plates, and torque values change with the answer.
• Ask for compressed or compact stranded conductor if conduit fill is tight.
• Order the reel or cut length you actually need; 3/0 leftovers are expensive to store.
• Request the test report and standard reference, typically UL 83 and the relevant ASTM conductor specification.
• Store reels indoors, off the ground, and keep the factory lagging on until the pull starts.

If you would like to know who is behind the cable before you specify it, our company background covers the factory, the certificates, and the way we work with project teams.

Quick Answers to Common Questions

Is 3/0 THHN the same as 3/0 THWN?

Not automatically. THWN is the wet-location version, and many modern cables carry both markings on the same jacket. Check the print legend rather than assuming.

How many amps can 3/0 THHN carry?

Copper 3/0 THHN is typically 200 A at 75°C and 225 A at 90°C; aluminum is typically 155 A and 175 A. Terminations and derating usually decide the real number.

Can 3/0 THHN go underground?

Only in a raceway and only when the cable carries a wet-location rating. Direct burial calls for a cable designed for it, such as a USE type.

Getting the Right Reel the First Time

Size, temperature column, material, and installation environment decide almost everything about a 3/0 THHN order. Get those four right and the rest is logistics: reel length, delivery date, and whether the conductor arrives with the jacket printing your inspector expects to see.

If a run is borderline between two sizes, run the voltage drop calculation before you round up. It is cheaper to spend ten minutes with a calculator than to pull a conductor that will not pass inspection.