Wire Gauge for Grow Lights: What Size You Need

For most LED grow lights on 120 V power, a 14 AWG cord handles up to 15 amps over short runs, and a 12 AWG cord handles longer runs and heavier loads. The right size depends on three numbers: total amps, cord length and the breaker on the circuit. This guide shows how to work out each one, with examples for a single light, a full tent and a multi-light room.

TL;DR

• Convert watts to amps first: amps = watts ÷ volts. A 600 W light on 120 V draws about 5 amps.
• Lower AWG number means thicker wire. 12 AWG carries more current and loses less voltage than 14 AWG or 16 AWG.
• Keep cords short. A 14 AWG cord is rated for 15 amps only up to about 50 feet in common cord charts.
• Keep continuous grow loads at or below 80% of the breaker rating.
• Fixed wiring inside walls is a job for a licensed electrician, not a grower.

Table of Contents

• How wire gauge works
• Step 1: Find the amps your lights draw
• Step 2: Match the cord gauge to amps and length
• Step 3: Check the circuit, not just the cord
• Step 4: Choose the right cord type
• Low-voltage control cables are a separate case
• Common mistakes
• Our view on wire gauge
• Choosing lights that fit your circuit
• Quick reference by setup
• FAQ
• Sources

How wire gauge works

In the US, wire is sized with the American Wire Gauge (AWG) system. The number runs backward. A smaller number means a thicker conductor.

• 10 AWG: heavy duty, long runs and large loads
• 12 AWG: standard for 20 amp circuits and heavy-duty cords
• 14 AWG: standard for 15 amp circuits and general-purpose cords
• 16 AWG: light duty, lamps and small loads over short runs
• 18 AWG: very light duty, usually the lead on small fixtures

Thicker wire has less resistance. Less resistance means less heat and less voltage lost along the run. Grow lights run for 12 to 18 hours a day, so heat in a cord matters more than it does for a tool used for ten minutes.

Most search results for this topic cover landscape lighting or household fixtures. Those use different rules, often low-voltage systems. A plug-in LED grow light runs on 120 V or 240 V mains power, so the numbers below apply to line-voltage wiring and cords.

Step 1: Find the amps your lights draw

The wire does not care about lumens or PPFD. It cares about current. Use the actual wall draw printed on the fixture label or spec sheet, not the equivalent wattage on the box.

The formula is simple:

Amps = watts ÷ volts

Worked examples at 120 V:

Add the fan, the carbon filter fan, the controller and the dehumidifier on the same circuit. In a small tent the light is often only half the load. Our guide to calculating amps and watts for a grow room circuit walks through the full tally.

If you are still choosing fixtures, the LED grow light wattage guide explains how many watts per square foot a space needs. That number sets your amps.

Step 2: Match the cord gauge to amps and length

A cord has two limits. One is how much current the copper and jacket can carry. The other is how much voltage the cord loses over its length. Cord charts apply whichever limit is stricter.

One published extension cord chart lists cord ratings of 13 A for 16 AWG, 15 A for 14 AWG and 20 A for 12 AWG. It also notes that a standard household plug caps any cord at 15 amps, whatever the wire size. It applies a 5% voltage-drop budget at 120 V. Its figures by length:

Read the table like this:

1. Find your total amps from Step 1.
2. Find your cord length in the left column.
3. Choose the first gauge whose value is equal to or higher than your amps.
4. If you are between two sizes, choose the thicker wire.

Examples

• One 480 W light, 4 A, 10 ft from the outlet. Any listed 16 AWG or 14 AWG cord covers it. A 14 AWG cord leaves more margin.
• One 720 W light, 6 A, 50 ft run. Use 14 AWG at minimum. A 16 AWG cord at this length is at its limit.
• Two lights plus fans, 12 A, 100 ft run. 12 AWG is the only size in the table that holds 12 A at 100 ft. Better still, move the power source closer.

The cheapest fix for a voltage problem is a shorter cord. If you need more than 50 feet to reach an outlet, ask whether a new outlet near the grow space is a better answer.

Step 3: Check the circuit, not just the cord

A thick cord does not make a circuit safer. The breaker and the wiring in the wall set the ceiling.

Common pairings in US homes:

• 15 amp breaker with 14 AWG wiring
• 20 amp breaker with 12 AWG wiring

The NEC treats a load that runs three hours or more as continuous. Grow lights qualify. A continuous load should not exceed 80% of the breaker rating, as explained for homeowners here. That gives you these limits at 120 V:

So a 15 amp circuit can feed two 480 W lights and a few small fans. It cannot feed four 720 W fixtures. Those need a 240 V circuit, several 120 V circuits, or fewer lights.

Check what else shares the circuit. Bedroom outlets are often daisy-chained with lighting and other rooms. A hair dryer in the next room can trip a breaker you thought was lightly loaded.

When 240 V helps

Many commercial-grade LED fixtures accept 100 to 277 V input. At 240 V the same wattage draws half the amps. That means thinner wire, smaller voltage drop and more lights per breaker. Check the fixture label for its input range, and have an electrician install any new 240 V circuit.

Step 4: Choose the right cord type

Gauge is one part of a safe cord. The U.S. Consumer Product Safety Commission advises checking that a cord is listed by a recognized testing laboratory such as UL, Intertek (ETL) or CSA. The same CPSC notice warns that overheating can happen at the plug, at the socket or along the whole cord.

Use this checklist:

• Buy cords with a UL, ETL or CSA mark.
• Choose a three-prong grounded cord. Do not defeat the ground pin.
• Use cords rated for the environment. Humidity in a tent is high, and water and plugs do not mix. Keep plugs and power strips off the floor, away from drips and runoff.
• Run each cord fully uncoiled. A coiled cord under load holds heat.
• Do not chain extension cords or plug a power strip into a cord.
• Replace cords with cracked jackets, warm plugs or discolored prongs.

Plug-in timers, controllers and power strips have their own amp ratings. Make sure the rating at least matches the load, since a light-duty strip on a 12 A load becomes the weak link.

Low-voltage control cables are a separate case

Many LED fixtures use a dimming or control line that is separate from the power cord. These carry a signal, not power, so the gauge rules above do not apply to them. What matters is the connector type and the cable length the system supports.

For systems that need them, we stock:

• Daisy chain cables for RJ12 to 0-10V dimming
• Driver and dimming extension cables
• Controller connector cables for environment controllers
• Driver extension cables for PROGROW fixtures

Check each product page for compatibility with your fixture before ordering. Never use a power cord where a control cable is specified, or the reverse.

Common mistakes

• Sizing by the cord’s look. A thin cord with a heavy plug is still a thin cord. Read the gauge printed on the jacket.
• Using a 16 AWG cord for a full tent. It is a light-duty size. It fits a single small light over a short run.
• Counting only the lights. Fans, heaters and dehumidifiers add amps.
• Ignoring the 80% rule. A breaker rated for 15 A should not carry 15 A for 12 hours.
• Running a cord through a wall or under a rug. Cords are for temporary, visible use. Permanent wiring needs an installed outlet.
• Replacing a breaker with a larger one. The wire in the wall sets the limit. Only an electrician should change a breaker size.

Our view on wire gauge

We sell lights, kits and accessories, and we see the same pattern in support questions. Most cord problems come from long runs and loads that grew over time. A grower starts with one light, adds a second, then a third, and keeps the same 16 AWG cord.

Our advice is short:

• Plan the circuit before you plan the canopy.
• Pick fixtures with efficient wall draw. Fewer watts for the same output means fewer amps and thinner margins to manage.
• Keep power cords short and thick, and keep control cables separate.
• Add a dedicated circuit when a room passes about 12 amps on a 15 amp breaker.
• Bring in a licensed electrician for any new outlet, 240 V line or panel change.

Efficiency is also how our supplemental under-canopy lighting is used. It adds light where the canopy shades the lower plant. It is meant to raise yield per watt rather than to raise the total load. Add its amps to your tally like any other fixture.

— Scott

Choosing lights that fit your circuit

If your wiring is the limit, the fixture is the easiest thing to change. We carry top lighting and under canopy grow lights in a range of wattages. For single-light tents, look at the ThinkGrow LED Model-I or the Model-I Plus. For larger spaces, the ION 720W LED grow light is one option. Check the spec sheet on each page for the exact wall draw before you size cords.

Space is limited? See our small LED grow lights. They draw less current, so they work with ordinary cords and outlets. You can browse the whole range, including tents, kits and environmental controls, on the LED Grow Lights Depot homepage.

Quick reference by setup

These are planning figures based on the chart above, not wiring instructions. Your fixtures, local code and existing wiring decide the final answer.

FAQ

What gauge wire do I need for a grow light?

For a plug-in LED fixture on 120 V, a 14 AWG cord covers most single lights up to 15 amps over runs of about 50 feet. Use 12 AWG for longer runs or heavier loads. Always compute amps from the fixture’s actual wall draw first.

Can I use a 16 gauge extension cord for grow lights?

Only for light loads over short runs. The chart above rates 16 AWG at 13 A for 25 feet, and it falls to 6 A at 100 feet. If the cord feels warm, replace it with a thicker one.

How many grow lights can I run on one 15 amp circuit?

At 120 V, the 80% limit for continuous loads is 12 amps, or 1,440 watts. Subtract your fans and other equipment, then divide what is left by the wattage of one light. Two 480 W lights with small fans fit. Four 720 W lights do not.

Do I need an electrician to set up grow lights?

Not for plugging a light into an existing outlet with a correctly rated cord. You do need a licensed electrician to add outlets, install a 240 V circuit or change breakers. Ask for one before the room’s load goes above what the existing circuit can carry.

Sources

• Extension Cord Gauge Chart: AWG, Amps and Length
• The NEC 80% Rule Explained
• CPSC: Household Extension Cords Can Cause Fires

Recommended

• Grow Room Circuit Load: How to Calculate Amps and Watts