How to Size a Transfer Switch for Your Generator

Choosing the right size transfer switch is crucial for a safe and effective backup power system. Too small, and you’ll trip breakers or damage equipment. Too large, and you’ll waste money on capacity you can’t use.

This guide walks you through the exact process to size your transfer switch correctly, whether you’re using a portable generator or installing a whole-house standby system.

Understanding Transfer Switch Sizing

Transfer switch sizing involves two key factors:

1. Amperage rating - How much current the switch can handle
2. Number of circuits - How many circuits you can connect

These aren’t the same thing. A 30-amp switch might have 10 circuit positions, but you can only use about 7,500 watts total at any time—regardless of how many circuits are connected.

Step 1: Know Your Generator’s Output

Your generator determines the maximum power available. Here’s how to find the relevant specifications:

Generator Wattage to Amps Conversion

Generator Size	Running Watts	Amps at 240V	Amps at 120V
3,000W	3,000	12.5A	25A
5,000W	5,000	20.8A	41.6A
7,500W	7,500	31.25A	62.5A
10,000W	10,000	41.6A	83.3A
12,000W	12,000	50A	100A

Formula: Amps = Watts ÷ Volts

Common Generator and Switch Pairings

Generator Type	Typical Output	Recommended Switch
Small portable (2,000-4,000W)	15-20A	30A manual
Medium portable (5,000-7,500W)	20-30A	30A manual
Large portable (8,000-12,000W)	30-50A	50A manual
Small standby (10-16kW)	40-65A	100A automatic
Large standby (17-22kW)	70-90A	150-200A automatic
Whole-house standby (22kW+)	90A+	200A automatic

Step 2: Calculate Your Load Requirements

Before selecting a transfer switch, identify which circuits you need to power and their total load.

Essential Circuit Loads

Appliance/Circuit	Running Watts	Starting Watts
Refrigerator	150-400	1,200-2,000
Freezer	100-300	800-1,500
Sump pump (1/3 HP)	800	1,300
Sump pump (1/2 HP)	1,050	2,150
Well pump (1/2 HP)	1,000	2,100
Furnace blower	300-800	500-1,500
Gas water heater	0 (gas)	0
Electric water heater	4,500	4,500
Microwave	1,000-1,500	1,000-1,500
Lights (per room)	60-200	60-200
TV/Electronics	100-400	100-400
Garage door opener	550	1,100
Security system	50-100	50-100

Load Calculation Example

Let’s size a transfer switch for these essential circuits:

Circuit	Running Watts	Starting Watts
Refrigerator	200	1,200
Freezer	150	1,000
Sump pump	800	1,300
Furnace	500	1,000
Kitchen lights	150	150
Living room lights	150	150
Well pump	1,000	2,100
Total	2,950	6,900

This setup needs a generator that can handle at least 3,000 running watts plus surge capacity. A 5,000-7,500 watt generator with a 30-amp transfer switch would be appropriate.

Step 3: Choose the Right Amperage

Based on your generator and load calculations, select the appropriate transfer switch amperage:

30-Amp Transfer Switches

Best for: Portable generators up to 7,500 watts

Pros	Cons
Most affordable option	Limited total capacity
Wide product selection	Can’t run large 240V loads
Easy installation	May need to manage loads manually
Perfect for essential circuits	Not for whole-house

Recommended for:

• 6-10 essential circuits
• Homes that only need backup for basics
• Portable generator setups
• Budget-conscious installations

50-Amp Transfer Switches

Best for: Large portable generators (8,000-12,000W) or small standby generators

Pros	Cons
More capacity headroom	Higher cost
Supports larger loads	Fewer product options
Good for 240V circuits	More complex installation
Future-proof for upgrades	May be overkill

Recommended for:

• Running 240V appliances (dryers, ranges on generator)
• 8-12 circuit requirements
• Plans to upgrade to a larger generator later
• Homes with higher essential loads

100-200 Amp Transfer Switches

Best for: Standby generators and whole-house backup

Pros	Cons
Whole-house coverage	Significantly higher cost
Works with large standby generators	Requires professional install
Service-entrance rated options	Complex installation
No load management needed	Overkill for portable generators

Recommended for:

• Automatic standby generator systems
• Homes wanting full backup coverage
• Properties with high electrical demands
• Areas with frequent long outages

Step 4: Determine Circuit Count

Beyond amperage, consider how many circuits you need to connect:

How Many Circuits Do You Need?

Home Size	Essential Circuits	Comfort Circuits	Total Recommended
Apartment/Condo	4-6	2-4	6-8
Small home (1,000-1,500 sq ft)	6-8	2-4	8-10
Medium home (1,500-2,500 sq ft)	8-10	4-6	10-12
Large home (2,500+ sq ft)	10-12	6-8	16+

Essential circuits (must have):

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

Comfort circuits (nice to have):

• Kitchen outlets
• Lighting circuits
• TV/entertainment
• Garage door
• Electric water heater

Step 5: Special Considerations

240V Circuits

Some appliances require 240V power:

Appliance	Typical Load	Transfer Switch Requirement
Central AC	3,000-5,000W	50A+ switch, standby generator
Electric dryer	3,000-5,000W	50A switch with 240V circuit
Electric range	8,000-12,000W	Typically not backed up
Electric water heater	4,500W	50A switch with dedicated circuit
Well pump (240V)	1,500-3,000W	30A may work; 50A preferred

Generator Outlet Matching

Ensure your generator’s outlet matches your transfer switch:

Generator Outlet	Amps	Voltage	Compatible Switch
L14-30 (4-prong)	30A	120/240V	30A manual
L14-50 (4-prong)	50A	120/240V	50A manual
14-50 (RV style)	50A	120/240V	50A with adapter
Hardwired	Varies	120/240V	Matched to generator

Sizing Quick Reference

Use this quick reference to match your situation:

Your Situation	Generator Size	Recommended Switch
Power a few essentials during outages	3,000-5,000W	30A / 6-circuit manual
Keep home comfortable during outages	5,000-7,500W	30A / 10-circuit manual
Run most of the house	7,500-12,000W	50A / 10-12 circuit
Automatic whole-house backup	10-16kW standby	100A automatic
Full home coverage, large home	20kW+ standby	200A service-rated automatic

Common Sizing Mistakes

Mistake 1: Oversizing the Switch

The problem: Buying a 50A or 100A switch for a 5,000W generator.

Why it’s wrong: You’re paying extra for capacity you can’t use. Your generator limits total power, not the switch.

The fix: Match the switch to your generator’s output, with modest room for growth.

Mistake 2: Undersizing for Growth

The problem: Buying a 6-circuit switch when you’ll want 10 circuits.

Why it’s wrong: Adding circuits later requires expensive modifications or replacement.

The fix: Buy for your realistic future needs, not just today’s minimum.

Mistake 3: Ignoring Starting Watts

The problem: Sizing only for running watts.

Why it’s wrong: Motors need 2-3x their running watts to start. If multiple motors start simultaneously, you’ll overload the system.

The fix: Calculate starting watts and ensure your generator can handle the surge.

Mistake 4: Forgetting 240V Requirements

The problem: Buying a 30A switch when you need to run your well pump or AC.

Why it’s wrong: Many 240V appliances require more than 30A switches can effectively provide.

The fix: List all 240V circuits you need backed up before sizing.

Frequently Asked Questions

Frequently Asked Questions

What size transfer switch do I need for a 7500 watt generator?

A 7500 watt generator at 240V produces about 31 amps, so a 30-amp transfer switch is the correct match. Going higher (like 50A) won't improve performance since your generator limits the power available.

Can I use a 50 amp transfer switch with a 30 amp generator?

Yes, you can use a larger transfer switch than your generator output. However, this doesn't give you more power—you're still limited by your generator's output. It does allow for future generator upgrades.

How many circuits can I run on a 30 amp transfer switch?

A 30-amp transfer switch typically supports 6-10 circuits. The number of circuits you can actively use simultaneously depends on the total load—typically 6,000-7,500 watts maximum.

Do I need a 100 amp or 200 amp transfer switch?

This depends on your electrical service and whether you want whole-house backup. If your main panel is 200 amps and you want full coverage, you need a 200-amp service-rated transfer switch. For essential circuits only, 100 amps or less is sufficient.

Use Our Free Calculator

Don’t want to do the math yourself? Our transfer switch size calculator does the work for you:

Summary

Sizing a transfer switch correctly comes down to three key steps:

1. Know your generator’s output - This sets your maximum power capacity
2. Calculate your load - Add up essential circuits plus a 20% buffer
3. Match the amperage - Choose a switch rated for your generator’s output

For most homeowners with portable generators in the 5,000-7,500 watt range, a 30-amp, 10-circuit manual transfer switch is the ideal choice. It provides enough capacity for essential circuits without overspending on features you won’t use.

If you’re investing in a standby generator for true whole-house backup, work with your generator dealer to ensure the automatic transfer switch is properly matched to both the generator and your home’s electrical service.