Why is it unsafe to bond neutral and ground wiring at subpanels?

Monday, June 18, 2018

To understand why it is problem, we have to start with the basic principle that a 120-volt circuit begins and ends at the transformer on a pole, or the ground, outside the home. The neutral wiring completes the loop of the circuit back to transformer from one of the two 120-volt hot wires that serve the panel. Ground wiring is intended as an alternate route when a “ground fault” occurs, which is essentially when the electric current strays from its intended path and may be dangerous. The ground wiring allows a fault to complete a circuit with an unimpeded surge of current that trips a breaker, stopping a potentially dangerous event.

    When the ground wires are connected (bonded) to the neutral at the main service panel, the current flows readily through the neutral from there to the transformer. But when the neutral and ground wires are connected further back at a subpanel, they both carry current back to the main panel. This means that you effectively have two neutral wires running in parallel. The ground wire should not carry current continuously, and there is the potential to electrify the metal components connected to it that the ground wiring is intended to protect from becoming electrically charged.

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Here’s links to a collection of our other blog posts about ELECTRIC PANELS:

What causes copper wires to turn green or black in an electric panel?  

What is the maximum number of circuit breakers allowed in an electric panel?

When should a corroded or damaged electric panel cabinet or disconnect box be replaced? 

What is a tandem circuit breaker? 

When did arc fault circuit interrupter (AFCI) breakers first become required?

Can an electric panel be located in a closet? 

Can an electric panel be located in a bathroom? 

Can you add circuit breakers by different manufacturers to an electric panel if they fit?

My circuit breaker won't reset. What's wrong?  

What is a split bus electric panel?

How do I identify a combination AFCI (CAFCI) circuit breaker? 

What does a circuit breaker with a yellow or white test button indicate? 

What is the maximum gap allowed between the front of a recessed electric panel box and the wall surface surrounding it? 

What are the requirements for NM-cables entering an electric panel box?

Why is a fuse box/panel an insurance problem for homebuyers? 

Why is bundled wiring in an electric panel a defect?

What is the difference between GFCI and AFCI circuit breakers? 

Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

What happens when you press the "TEST" button on a circuit breaker in an electric panel?

What is a Dual Function Circuit Interrupter (DFCI)? 

What is the difference between a Combination Arc Fault Circuit Interrupter (CAFCI) and an Arc Fault Circuit Interrupter (AFCI) circuit breaker?  

What is the difference between "grounded" and "grounding" electrical conductors? 

What does it mean when a wire is "overstripped" at a circuit breaker?

Why is an old fuse panel dangerous?  

Who is the manufacturer of those "bad" electric panels?

Why is the circuit breaker stuck in the middle? 

What is a double tap at a circuit breaker?

What is the right electric wire size for a circuit breaker in an electric panel?

What is the life expectancy of a circuit breaker? 

My circuit breaker won't reset. What's wrong? 

Why do some breakers in my electric panel have a "TEST" button on them?

What is the right size electric panel for a house? 

• What do I need to know about buying a whole house surge protector? 

What is the maximum allowed height of a circuit breaker (OCPD) above the floor?

• What is the maximum height you can mount an electric panel above the floor? 

• What is the code required clearance in front of an electric panel?

What is the main bonding jumper and where do it find it in an electric panel? 

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

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