Wire Size and Amperage Charts: Ampacity, Gauge and Load Guide

Wire Sizes and Amperage Ratings: What Actually Determines the Number

A wire's amperage rating, called ampacity, is the maximum current it can carry continuously without exceeding its insulation's safe temperature limit. It isn't a single fixed number stamped on the wire — ampacity depends on four things working together: conductor size (larger gauge, more heat dissipation), conductor material (copper carries more current than aluminum at the same gauge), insulation type (higher-temperature insulation like THHN handles more current before it degrades), and installation conditions (bundled conductors in a hot attic dissipate heat far worse than a single wire in free air).

This is why two people can look at the same AWG wire size and get different "correct" answers — one is reading the 60°C column, the other the 90°C column, and the termination equipment (the breaker or outlet it connects to) is usually what limits which column actually applies. For most residential circuits protected by 100A or smaller breakers, the 60°C column governs unless every terminal in the circuit is rated higher.

Copper Wire Ampacity Chart by Gauge

The table below covers the AWG sizes most commonly asked about, from small control-circuit gauges up through heavy feeder conductors, based on NEC Table 310.16 at 30°C ambient with no more than three current-carrying conductors in a raceway.

*20 AWG is a low-voltage/signal gauge and is not assigned a standard NEC branch-circuit ampacity; its safe current limit is typically set by the specific application's manufacturer specs. Values shown for 14 AWG through 2/0 AWG are copper conductor ampacities per NEC Table 310.16.

Two important limits sit on top of this table. First, NEC 240.4(D) caps standard overcurrent protection regardless of ampacity column: 14 AWG is capped at a 15A breaker, 12 AWG at 20A, and 10 AWG at 30A, even though the 75°C column technically allows slightly more current. Second, a 2/0 copper conductor's real-world ampacity — commonly asked as "how many amps is 2/0 copper good for" — sits at 175A on the 60°C column and 195A on the 75°C column, which is why 2/0 shows up so often in subpanel feeder and service entrance sizing.

Converting Amps to Watts

Watts, amps, and volts are related by a simple formula: watts = amps × volts. This is why "how many watts is 6 amps" doesn't have one single answer — it depends entirely on the voltage of the circuit. On a standard 120V household circuit, 6 amps equals 720 watts. On a 240V circuit, the same 6 amps equals 1,440 watts, twice the power, because voltage and current both contribute to total power delivered.

• 6A at 120V = 720W (small appliances, lamps, chargers)
• 6A at 240V = 1,440W (240V equipment running at partial load)
• To reverse the calculation, divide watts by volts to get amps — a 1,500W space heater on a 120V circuit draws 12.5A, close to the practical limit of a 15A circuit once continuous-load derating is applied

What Size Wire Feeds a 100 Amp Subpanel

For a 100 amp wire size in copper, 4 AWG copper conductors are the standard choice when using 75°C-rated equipment, delivering 85A at 60°C or 95A at 75°C — right at the edge of a 100A rating, which is why many electricians step up to 3 AWG or use 2 AWG for extra margin on longer runs. A more common and conservative approach for a 100 amp subpanel is 2 AWG copper, which comfortably exceeds 100A on the 75°C column and provides headroom for voltage drop over distance.

Distance matters here more than most people expect. A subpanel feeder run over roughly 75–100 feet often needs to be sized up beyond the minimum ampacity requirement specifically to control voltage drop, since NEC recommends keeping voltage drop under 3% for feeders and 5% total to the farthest outlet. A voltage drop calculator using the actual run length is the reliable way to confirm sizing rather than relying on ampacity tables alone for long runs.

50 Amp Circuits: Breaker Sizing and Common Loads

The wire gauge for a 50 amp breaker is 6 AWG copper, which delivers 55A at the 60°C column — enough margin above the 50A breaker rating to satisfy code. This is the same size used for what size wire for 50 amp service loads like electric ranges, ovens, and Level 2 EV chargers running near their rated capacity. Some installers use 8 AWG copper on the 75°C column, which lands at exactly 50A, but this leaves zero margin and is generally avoided in favor of the more forgiving 6 AWG option.

A quick reference for common 50A loads: electric ranges and ovens, Level 2 EV chargers configured for 40A continuous draw (requiring the 125% continuous-load rule, hence the 50A breaker), MIG/TIG welders, and standard 240V hot tub circuits.

6/3 or 6/4 Wire for a Hot Tub: Which One Applies

This question comes down entirely to whether the spa has 120V components — lights, controls, ozonators — in addition to its 240V heater and pump. If it does, a 4-wire feeder is required: two hot conductors, a neutral, and an equipment ground, commonly run as 6/4 (or 6/3 plus a separately pulled neutral conductor of the same gauge). If the spa is strictly 240V with no 120V loads, a 3-wire configuration — two hots and a ground — is permitted, and 6/3 is standard. The safest way to confirm which applies is to check the spa's own wiring diagram on the nameplate rather than assuming; using a 3-wire feeder on a spa that actually needs a neutral will leave 120V components non-functional or improperly grounded.

Regardless of which configuration applies, NEC Article 680 governs the full installation: a 240V, 50A GFCI breaker is standard, the maintenance disconnect must be readily accessible and located at least 5 feet horizontally from the tub's inside wall, and equipotential bonding of all metal components around the spa is required. Copper conductors with THHN or similar wet-rated insulation are strongly preferred over aluminum, since aluminum's greater thermal expansion tends to loosen connections in the vibration-prone environment around spa equipment.

What Kind of Wire to Run in Conduit

THHN is the most widely used general-purpose conduit wire for dry, indoor conduit runs — it's rated 90°C dry, cost-effective, and its thin PVC/nylon jacket makes it easier to pull through conduit with multiple bends. Most THHN sold today is dual-rated THHN/THWN-2, meaning the same wire can also be used in wet or outdoor conduit runs at a 75°C rating, which covers the majority of both indoor and outdoor residential and light commercial applications without needing a separate wire type.

For conditions where long-term moisture exposure, direct burial, or chemical/UV exposure is a factor — underground feeders, coastal installations, wastewater or industrial settings — XHHW-2 is the stronger choice. Its cross-linked polyethylene insulation holds a full 90°C rating in both wet and dry locations, resists abrasion and chemical exposure better than PVC, and remains flexible in cold temperatures, though it costs more and is bulkier to pull than THHN. Romex (NM-B cable) should not be substituted for either in conduit applications requiring wet-location or commercial-grade wiring — it's rated for dry, indoor residential use only.

Wire sizing and circuit installation are governed by the National Electrical Code and local amendments, and errors in sizing or installation carry real fire and shock risk. The figures above are general reference points; always verify wire size, breaker rating, and installation method against current local code, and have circuits above basic low-voltage work installed or inspected by a licensed electrician.