Generator Interlock vs Transfer Switch: Which Is Better?

When connecting a portable generator to your home, you have two main options: a generator interlock kit or a transfer switch. Both prevent dangerous backfeeding, but they work differently and have distinct advantages.

This guide compares both options in detail to help you choose the right solution for your situation.

What’s the Difference?

Generator Interlock

A generator interlock is a metal plate that attaches to your electrical panel cover. It creates a mechanical barrier that prevents your main breaker and generator breaker from being on at the same time.

How it works:

1. Turn OFF the main breaker
2. The interlock slides, allowing the generator breaker to turn ON
3. Start your generator
4. Turn on individual breakers as needed (don’t exceed generator capacity)

Transfer Switch

A transfer switch is a separate electrical device (usually next to your panel) with its own breakers for specific circuits. It switches those circuits between utility and generator power.

How it works:

1. Start your generator
2. Connect it to the power inlet box
3. Flip the transfer switch from “LINE” to “GEN”
4. Pre-selected circuits now run on generator power

Side-by-Side Comparison

Feature	Generator Interlock	Transfer Switch
Cost (equipment)	$50 - $200	$250 - $500
Installation cost	$150 - $400	$400 - $800
Installation time	1-2 hours	3-5 hours
Circuits available	All panel circuits	Only pre-selected (6-12)
User skill required	Moderate	Low
Load management	Manual (you manage)	Pre-defined
Automatic transfer	Never	Some models
Code compliance	Yes (when listed)	Yes
Panel modification	Minimal	More extensive

Generator Interlock: Detailed Look

How an Interlock Works

The interlock kit is a simple mechanical device:

1. Physical barrier - A metal plate that slides or pivots
2. Prevents simultaneous operation - Main and generator breakers can’t both be ON
3. Break-before-make - You must turn one OFF before the other can turn ON

This prevents backfeeding because your home is completely disconnected from the utility before generator power is applied.

Best Generator Interlock Kits

Interlock Requirements

For a proper interlock installation, you need:

1. Compatible panel - Check manufacturer listings
2. Open breaker space - Adjacent to main breaker
3. Generator breaker - Sized for your generator (usually 30A or 50A)
4. Power inlet box - Outdoor receptacle for generator
5. Generator cord - Connects generator to inlet box

Using an Interlock Safely

Before an outage (preparation):

1. Know your generator’s wattage capacity
2. List which circuits you’ll need to power
3. Calculate the total load of those circuits
4. Practice the switching procedure

During an outage:

1. Turn OFF the main breaker
2. Turn OFF all individual circuit breakers
3. Slide the interlock to the generator position
4. Start your generator outdoors
5. Let it warm up for 1-2 minutes
6. Turn ON the generator breaker
7. Turn ON individual circuits ONE AT A TIME
8. Monitor generator—don’t exceed capacity

When utility power returns:

1. Turn OFF all individual circuit breakers
2. Turn OFF the generator breaker
3. Slide interlock to utility position
4. Turn ON the main breaker
5. Turn ON individual circuits
6. Shut down generator

Transfer Switch: Detailed Look

How a Transfer Switch Works

A transfer switch is a completely separate switching device:

1. Pre-wired circuits - 6-12 circuits connected during installation
2. Two-position switches - Each circuit flips between LINE (utility) and GEN (generator)
3. Mechanical interlock - Circuits can’t connect to both sources simultaneously

Best Transfer Switches

Transfer Switch Circuit Selection

During installation, you choose which circuits connect to the transfer switch. Common choices:

Essential circuits (usually include):

• Refrigerator
• Freezer
• Sump pump
• Well pump
• Furnace/HVAC
• Security system

Nice-to-have circuits:

• Kitchen outlets
• Lighting (one or two rooms)
• Garage door opener
• Internet/router
• TV/entertainment

Cost Comparison

Equipment Costs

Component	Interlock	Transfer Switch
Switching device	$50 - $150	$250 - $500
Generator breaker	$20 - $50	Included
Inlet box	$50 - $100	Often included
Generator cord	$80 - $150	$80 - $150
Total Equipment	$200 - $450	$330 - $650

Installation Costs

Service	Interlock	Transfer Switch
Electrician labor	$150 - $300	$300 - $600
Permit	$50 - $150	$50 - $150
Total Installation	$200 - $450	$350 - $750

Total Project Cost

Scenario	Interlock	Transfer Switch
Budget	$400	$680
Mid-range	$650	$950
Premium	$900	$1,400

Potential savings with interlock: $280 - $500

Safety Comparison

Both options are safe when properly installed and used correctly. The key differences are:

Interlock Safety Considerations

Requires user discipline:

• Must turn off main breaker first
• Must manage loads manually
• Must follow proper sequence

Failure modes:

• User error (forgetting steps)
• Overloading generator
• Starting loads too quickly

Transfer Switch Safety Considerations

More foolproof operation:

• Physical switches prevent errors
• Pre-selected circuits can’t overload
• Simpler operation

Failure modes:

• Mechanical switch failure (rare)
• Improper initial circuit selection

Code Compliance

Both are NEC-compliant:

Requirement	Interlock	Transfer Switch
Backfeed prevention	Yes	Yes
UL listing available	Yes	Yes
Permits required	Yes	Yes
Professional install recommended	Yes	Usually required

Which Should You Choose?

Choose a Generator Interlock If:

• Budget is primary concern - Save $300-$500
• You’re electrically knowledgeable - Comfortable managing loads
• You want flexibility - Different circuits each outage
• Quick installation needed - Can be done in 1-2 hours
• You might upgrade generators - Easy to change later
• You have limited essential circuits - Don’t need more than 10-12

Choose a Transfer Switch If:

• Simplicity matters most - Just flip switches
• Multiple family members will use it - Easy operation
• You’re less electrically confident - Pre-selected circuits are safer
• You want consistent essentials - Same circuits every time
• You may add automation later - Some switches can upgrade
• Your local code prefers it - Check with building department

Hybrid Options

Some products combine features of both:

Load Center Interlocks

These install like interlocks but include a small subpanel for critical circuits. You get interlock flexibility plus dedicated essential circuit protection.

Upgradeable Transfer Switches

Products like the Generac Home Link can start as manual transfer switches and later upgrade to automatic transfer with a standby generator, giving you a future growth path.

Installation Considerations

Panel Compatibility

Interlocks: Must match your specific panel brand and model. Common compatibility:

• Square D Homeline/QO
• GE
• Siemens/Murray
• Cutler-Hammer/Eaton
• Challenger

Transfer switches: Compatible with most any panel since they connect to existing breakers.

Space Requirements

Interlocks: Need one open breaker space adjacent to main breaker.

Transfer switches: Need wall space next to main panel (usually 18” x 12” minimum).

Inlet Box Location

Both require an outdoor inlet box:

• Within 25-50 feet of generator location
• Protected from direct rain/snow
• Accessible but not easily damaged
• Away from windows (exhaust fumes)

Frequently Asked Questions

Frequently Asked Questions

Is a generator interlock as safe as a transfer switch?

Both are safe when properly installed and used correctly. Interlocks are code-compliant and UL-listed. The main safety difference is that interlocks require more user attention—you must manually turn off the main breaker before energizing the generator breaker.

Why would I choose an interlock over a transfer switch?

Interlocks cost less ($50-$200 vs $250-$500), are faster to install (1-2 hours vs 3-5 hours), and let you power any circuit in your panel rather than just pre-selected circuits. They're ideal for budget-conscious homeowners with electrical knowledge.

Can I use my whole panel with a generator interlock?

Yes, that's one of the main advantages. An interlock lets you use any breaker in your panel, as long as you don't exceed your generator's capacity. You'll need to manage loads manually by turning off breakers you're not using.

Are generator interlocks legal everywhere?

Generator interlocks are legal and code-compliant in most US jurisdictions when properly installed. However, some areas may have specific requirements or prefer transfer switches. Always check with your local building department and get required permits.

Making Your Decision

Consider these final questions:

1. Budget - Can you spend $300-$500 more for a transfer switch?
2. Skill level - Are you comfortable managing electrical loads?
3. Users - Will family members with varying experience operate it?
4. Flexibility - Do you need different circuits in different situations?
5. Local requirements - What does your building department require?