🛁What size wire does a hot tub need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Hot tub circuit

Nameplate current

One-way run

System voltage

Metal

Terminal rating

Ambient

°F

Current-carrying conductors in the raceway

8 AWG copper

on a 50 A device · 10 AWG ground · 3/4" conduit at 25.8% fill

1.6% drop at 50 ft

Ampacity wants8 AWG
Voltage drop wants10 AWG

Current the conductor has to carry: 50 A
Usable ampacity after derating: 50 A (table 50)
Longest run before the drop passes 3%: 93 ft
In metric sizes: 8.4 mm²

Use the current on the spa pack label. Four conductors is the usual pull, because most packs run a 120 V control board.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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What size wire does a hot tub need?

The AmpSizer hot tub result for a 50 amp spa 50 feet from the panel is 8 AWG copper on a 50 amp GFCI breaker, with a 10 AWG ground, in three-quarter inch PVC. Voltage drop over that run is 1.6%.

The wire is the easy part. NEC Article 680 adds three requirements that have nothing to do with ampacity: a GFCI device protecting the circuit, a disconnect within sight of the spa and at least five feet from the water, and bonding of the metal parts around it.

Most spas want four conductors rather than three, because the pack runs 120 volt controls off a neutral even though the heater and pumps are 240 volt. The label says which.

The spa sizes on the market, with the circuit each one needs at a normal distance.

Nameplate	Breaker	Copper	Ground	Conduit, 4 wires	Drop at 100 ft
20 A — plug-in 120 V spa	20 A	12 AWG	12 AWG	1/2 in	6.6% at 120 V
30 A	30 A	10 AWG	10 AWG	3/4 in	3.1%
40 A	40 A	8 AWG	10 AWG	3/4 in	2.6%
50 A	50 A	8 AWG	10 AWG	3/4 in	3.2%
60 A	60 A	6 AWG	10 AWG	1 in	2.5%

Copper at 75 °C, 240 V except the first row. The 120 V plug-in spa is the one with a real voltage-drop problem, and it is also the one nobody calculates.

Why the plug-in spa is the awkward one

A 120 V plug-in spa looks like the easy install and behaves like the difficult one. It draws close to 15 amps continuously through whatever extension cord it ends up on, at the voltage where percentage drop is worst.

Ten percent down on a 120 V spa means the heater delivers about 19% less heat, so the tub takes a day to warm instead of an afternoon, and the owner concludes the heater is faulty. The cure is a dedicated receptacle close to the spa, not a heavier cord.

What an inspector will look for

GFCI protection on the circuit, tested
A disconnect within sight, at least 5 ft from the water
An 8 AWG solid copper bond tying the metal parts together
Wet-location wiring methods — THWN-2 in PVC, or UF where permitted
No receptacle closer than 6 ft to the inside wall of the spa, and GFCI protection on the ones within 20

Frequently asked questions

What size wire for a 50 amp hot tub?

8 AWG copper for the conductors and a 10 AWG copper ground, on a 50 A GFCI breaker. In wet-location conduit that is usually 8 AWG THWN-2 in three-quarter inch PVC, four conductors. At 100 ft, voltage drop reaches 3.2% and 6 AWG becomes the better choice.

How far does the disconnect have to be from the hot tub?

At least 5 feet from the inside wall of the spa, and within sight of it, per 680.12 — readily accessible so somebody can shut the spa off without walking past it. Some jurisdictions publish a maximum distance as well. In practice a spa panel on a post about 6 feet away satisfies everyone.

Does a hot tub need a GFCI breaker or a spa panel?

It needs GFCI protection, and a spa panel is the usual way to provide it while also being the required disconnect — one enclosure doing two jobs. A GFCI breaker in the main panel plus a separate disconnect near the tub is equally valid and sometimes cheaper.

Can I use Romex to a hot tub?

No. NM cable is dry-location only, and the run to a spa is outdoors, underground or both. Use individual THWN-2 conductors in PVC conduit, or UF cable where it is permitted. Burial depth for PVC is generally 18 inches, less under a slab.

What has to be bonded around a spa?

680.26 covers it: metal parts of the spa structure, metal piping, metal fittings within reach, pump motors, and any metal within 5 feet of the inside wall — tied together with an 8 AWG solid copper bonding conductor. Bonding is not grounding; its job is to make sure everything nearby is at the same potential rather than to clear a fault.

How we calculate this

The conductor is sized from the nameplate current of the spa pack, which is already the total of its heater, pumps and controls. Ampacity is NEC Table 310.16 at the terminal temperature selected, corrected by Tables 310.15(B)(1) and 310.15(C)(1); voltage drop is 2 × amps × R × one-way feet ÷ 1,000 on the Chapter 9 Table 8 resistance figures, and the larger requirement governs. The device is the next standard rating in 240.6(A), and Article 680 requires it to provide ground-fault circuit-interrupter protection. The equipment grounding conductor is Table 250.122; conduit is Chapter 9, Table 4 at 40% fill for four conductors. The disconnect requirements are 680.12, and equipotential bonding is 680.26 — neither is a calculation, and both are conditions of the install.

Worked example

With Nameplate current 50, One-way run 50, System voltage 240 V, Metal Copper, Terminal rating 75 °C — the usual breaker, Ambient 86, Current-carrying conductors in the raceway 3, this page works out 8 AWG copper. on a 50 A device · 10 AWG ground · 3/4" conduit at 25.8% fill

Current the conductor has to carry50 A
Usable ampacity after derating50 A (table 50 )
Longest run before the drop passes 3%93 ft
In metric sizes8.4 mm²

Sources:

The parts of a spa install that are not wire

Bonding is separate from grounding and is inspected separately. An 8 AWG solid copper conductor ties the metal parts together. It is the requirement most often missed on a self-installed spa.

The disconnect is for the person in the water, which is why it has to be visible from there and far enough away that nobody can reach it while wet.

Burial depth and sleeving are local. PVC at 18 inches is the general rule, and there are shallower allowances under slabs and for GFCI-protected circuits. Ask before you dig, not after.