What Size Breaker for a Water Heater?

Quick Answer: Most residential electric water heaters require a 30-amp, double-pole breaker and 10-gauge wire. This applies to standard 4,500-watt and 5,500-watt tank water heaters operating on 240 volts, which covers the vast majority of homes.

However, the exact breaker size depends on your water heater's wattage, and getting it wrong can cause serious problems. An undersized breaker trips constantly. An oversized breaker won't trip when it should, creating a fire hazard.

"We see breaker sizing issues more often than you'd think, especially in older homes where someone upgraded to a larger water heater without upgrading the circuit," says a lead service technician at Dalmatian Plumbing. "The math is straightforward once you know what to look for."

This guide walks you through exactly how to determine the right breaker size for your water heater.

Water Heater Breaker Size Chart

Use this quick reference chart to find the correct breaker size based on your water heater's wattage:

Note: The 4,500W and 5,500W rows cover most residential water heaters. If yours falls in this range, you need a 30-amp breaker with 10-gauge wire.

Breaker Size by Tank Size

Many homeowners know their tank size but not their wattage. Here's a general guide:

Important: Always verify the actual wattage on your water heater's label rather than relying solely on tank size. Manufacturers vary.

How to Calculate Breaker Size (Step by Step)

If your water heater isn't in the chart above, here's how to calculate the correct breaker size yourself:

Step 1: Find Your Water Heater's Specifications

Locate the manufacturer label on your water heater (usually on the side). You need two numbers:

• Wattage (e.g., 4,500 watts)
• Voltage (typically 240V for standard heaters, 120V for small units)

Step 2: Calculate the Amperage

Divide wattage by voltage:

Amps = Watts ÷ Volts

Example: 4,500W ÷ 240V = 18.75 amps

Step 3: Apply the NEC 125% Rule

The National Electrical Code (NEC) requires breakers for continuous loads (anything running 3+ hours) to be rated at 125% of the calculated amperage. Water heaters qualify as continuous loads.

Required capacity = Amps × 1.25

Example: 18.75A × 1.25 = 23.44 amps

Step 4: Round Up to Standard Breaker Size

Standard breaker sizes are: 15, 20, 25, 30, 35, 40, 50, 60 amps.

23.44 amps rounds up to either a 25-amp or 30-amp breaker. Most electricians use 30 amps for standard water heaters because:

• It provides an additional safety margin
• 30-amp breakers are more common and readily available
• NEC 422.11(E)(3) allows explicitly 30-amp protection for water heaters up to 5,500W

Step 5: Match the Wire Gauge

The wire must be rated for the breaker size:

For a 30-amp breaker, you need 10-gauge wire. Using undersized wire is a fire hazard.

Why Water Heaters Need Double-Pole Breakers

Standard residential water heaters operate on 240 volts, which requires a double-pole breaker. Here's why:

Your home's electrical panel has two "hot" bus bars, each carrying 120 volts. A double-pole breaker connects to both bus bars simultaneously, combining them to provide 240 volts.

A single-pole breaker (like those used for lights and outlets) only connects to one bus bar and provides 120 volts, not enough for a standard water heater.

Visual identification: Double-pole breakers are twice as wide as single-pole breakers and have their switches tied together so both trip simultaneously.

Exception: Small point-of-use water heaters (under sinks, for example) may operate on 120V and use a single-pole breaker.

Two Heating Elements, One Wattage, Why?

If you look at your water heater's label, you might see "Upper Element: 4,500W," "Lower Element: 4,500W," and "Total Connected Watts: 4,500W." This confuses many homeowners.

Here's the explanation: The two elements never run simultaneously. A thermostat controls them sequentially; the upper element heats first, then shuts off while the lower element takes over. This is called "non-simultaneous operation."

The result: Your water heater draws only 4,500 watts at any given moment, not 9,000 watts. Your breaker only needs to handle the single-element load.

This design is intentional. If both elements ran simultaneously, residential water heaters would require much larger (and more expensive) electrical circuits.

Tankless Water Heater Breaker Requirements

Tankless water heaters are a different story. Because they heat water on demand rather than maintaining a tank, they draw significantly more power.

Key difference: Whole-house tankless units often require multiple breakers because a single breaker can't handle the load. A 27,000W tankless heater, for example, might need three separate 40-amp circuits.

This significantly impacts installation costs. If you're considering a tankless upgrade, have an electrician evaluate whether your panel can support the additional load.

Wire Types for Water Heater Installation

The type of wire matters as much as the gauge. Here are the standard options:

NM-B Cable (Romex)

• Best for: Indoor, dry locations
• Description: Two insulated conductors plus ground, wrapped in plastic sheathing
• Standard size: 10/2 NM-B (two 10-gauge conductors plus ground)

THHN/THWN Wire in Conduit

• Best for: Outdoor installations, wet/damp locations, exposed runs
• Description: Individual wires run through metal or PVC conduit
• Note: Required where the wire may be exposed to moisture or physical damage

MC Cable (Metal-Clad)

• Best for: Areas requiring physical protection, exposed runs
• Description: Insulated conductors wrapped in flexible metal armor
• Advantage: More durable than Romex, easier than running conduit

Your local electrical code may specify which type is required for your installation.

Dedicated Circuit Requirement

Water heaters should have a dedicated circuit, meaning no other appliances share that breaker. This isn't just a recommendation; it's an NEC requirement for continuous-load appliances.

Why it matters:

• Water heaters draw near-maximum current for extended periods
• Sharing a circuit risks overloading
• A dedicated circuit ensures the breaker accurately protects the water heater

If your water heater is on the same circuit as other outlets or appliances, it's not up to code and should be corrected.

Signs You Need to Upgrade Your Circuit

Consider upgrading your water heater circuit if:

The breaker trips frequently: Occasional trips during storms are normal. Regular trips indicate the circuit is undersized or there's a problem with the heater.

You're installing a higher-wattage replacement: If your old 4,500W heater is being replaced with a 5,500W or 6,000W model, verify the existing circuit can handle it.

You're upgrading to a tankless water heater: Tankless water heaters almost always require electrical upgrades. Budget for this when planning the switch.

The wire feels warm: Warm wires near the water heater or breaker panel indicate the circuit is working too hard. This is a fire hazard; call an electrician immediately.

You have old wiring: Homes with original wiring from before the 1980s may have undersized circuits that don't meet current code for modern water heaters.

Common Breaker Sizing Mistakes

Mistake 1: Forgetting the 125% Rule

Calculating amps without applying the 125% continuous load factor results in an undersized breaker that trips frequently.

Mistake 2: Mismatching Breaker and Wire

A 30-amp breaker with 12-gauge wire is dangerous. The breaker won't trip before the wire overheats. Always match wire gauge to breaker size.

Mistake 3: Using a Single-Pole Breaker for 240V

A 240V water heater on a single-pole breaker won't work correctly and creates safety hazards.

Mistake 4: Oversizing "For Safety"

A 50-amp breaker on a 4,500W water heater isn't safer; it's dangerous. The oversized breaker won't trip during a fault, allowing wires to overheat.

Installation Safety Checklist

If you're working on the water heater electrical:

1. Turn off power at the main panel — Verify with a voltage tester before touching any wires
2. Follow manufacturer specifications — The label tells you exactly what's required
3. Match breaker size to wire gauge — Never exceed wire capacity
4. Use a double-pole breaker for 240V — Required for proper operation
5. Secure all connections — Loose connections cause arcing and fires
6. Label the circuit — Mark the breaker clearly in your panel
7. Get a permit if required — Many jurisdictions require permits for water heater replacement

When in doubt, hire a licensed electrician. Electrical work done incorrectly creates fire hazards and may void your homeowner's insurance.

Frequently Asked Questions

What size breaker do I need for a 4500-watt water heater?

A 4,500-watt water heater at 240V draws 18.75 amps. Applying the NEC 125% rule (18.75 × 1.25 = 23.44 amps), you need a 30-amp double-pole breaker with 10-gauge wire.

Can I use a 20-amp breaker for a water heater?

Only for small point-of-use water heaters with a capacity of less than 3,000 watts. Standard 40-50 gallon tank heaters require a 30-amp breaker. Using a 20-amp breaker on a standard water heater will cause it to trip constantly.

Do I need a double-pole breaker for my water heater?

Yes, if your water heater operates on 240 volts (which most residential tank heaters do). A double-pole breaker connects to both hot bus bars to provide 240V. Small 120V point-of-use heaters can use single-pole breakers.

What gauge wire for a water heater?

For standard residential water heaters on a 30-amp circuit: 10-gauge wire. For 40-amp circuits: 8-gauge. For 20-amp circuits: 12-gauge. The wire gauge must match the breaker size.

Why does my water heater keep tripping the breaker?

Common causes include: an undersized breaker, a faulty heating element, a bad thermostat, a water leak causing an electrical short, or loose wire connections. If the breaker is correctly sized, have the water heater inspected.

Can I use a 40-amp breaker for a 4500-watt water heater?

No, this is oversized and dangerous. A 40-amp breaker won't trip during certain fault conditions, allowing wires to overheat before protection kicks in. Stick to a 30-amp breaker for 4,500W heaters.

When to Call a Professional

While understanding breaker sizing helps you make informed decisions, electrical work should typically be left to licensed professionals. Call an electrician if:

• You need to install a new circuit for a water heater
• Your current wiring appears damaged or undersized
• You're upgrading to a tankless water heater
• The breaker trips repeatedly after proper sizing
• You're not comfortable working with electrical panels

At Dalmatian Plumbing, we coordinate with licensed electricians on water heater installations to ensure both the plumbing and electrical work meet code requirements.

Schedule water heater service or call.