Cord-and-plug connection of water heaters was once acceptable, but has not been allowed for at least the past 25 years by the National Electrical Code [NEC 422.16(A)]. The code does not approve a flexible cord connection unless "the fastening means and mechanical connections are specifically designed to permit ready removal for maintenance or repair and the appliance is intended or identified for flexible cord connection.” The pipe connections to a water heater make it not readily removable. A rated flexible hose-type connector might overcome that, but confirming that the water heater is rated for a flexible cord would be a second battle. 

    A kitchen disposal, dishwasher, and clothes dryer are examples of appliances that are specifically designed for ease of removal and qualify for a cord-and-plug connection. Also see our articles Does an electric water heater require a disconnect? and Does a tankless water heater require a pressure relief valve? and Does a water heater need a shut-off valve?

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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

• Why is my water heater making strange (rumbling, gurgling, knocking or banging) noises? 

• What can I do to make my water heater last longer?

• How can I determine the age of a water heater if the serial number is missing or decoding it is impossible? 

• How does a hydronic heating system work?

• What is the difference between a regular water heater and a direct vent water heater? 

• What is the difference between a regular water heater and a power vent water heater?

• What is backdrafting at a gas water heater? 

• How do I determine if a water heater is gas or electric?

• What does it mean when a gas appliance (water heater, furnace, or range) has been "red tagged"?  

• What's the valve with the flip-up handle on the water heater for?

• Why is an older water heater an insurance problem?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.