Bonding, Grounding and the NEC 

Bonding, Grounding and the NEC Presented by The National Association of Certified Home Inspectors www.NACHI.org

The New Code … • The 1999 and 2002 editions of the NEC* have now clearly defined the separate and vitally important purposes of grounding and bonding in making safe electrical installations *The National Electrical Code (NEC) is a registered trademark of the National Fire Protection Association, www.nfpa.org.

The New Code … • Section 250-4 establishes new performance requirements which clarify what grounding and bonding are required to accomplish

The New Code … Section 250-2 introduces and defines the new terms: • Ground Fault • Ground Fault Current Path • Effective Ground Fault Current Path

The New Code … • These new definitions are in addition to the two important definitions in Article 100 which apply to Section 250-2

The New Code … These definitions are: • Grounded - Connected to earth • Bonded - The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed

The New Code … • The importance of grounding electrical equipment seems to be well understood • The purpose and intent of bonding to create a low impedance ground fault return path seems to be less understood

The New Code … • The importance of bonding is best described in the following graphics which review how bonding performs in a typical circuit

A Typical Circuit … L O A D • 100’ of Overhead Distribution Line • 25’ of Service Drop • 25’ of Service Entrance Conductor • 100’ of Branch Circuit Conductors

A Typical Circuit … L O A D Current flows…...

Path of Current Flow - Normal Operation L O A D From the transformer to our service …

Path of Current Flow - Normal Operation L O A D Through the overcurrent device to our load …

Path of Current Flow - Normal Operation L O A D Through the load returning to the service …

Path of Current Flow - Normal Operation L O A D And back to the transformer.

Path of Current Flow - Normal Operation L O A D What determines the amount of current that will flow in this circuit?

Path of Current Flow - Normal Operation L O A D The total resistance or impedance in the circuit will determine the amount of current that will flow in the circuit.

Things You Can Count On … • Ohm’s Law Works • We can change the code, or • Hire a different contractor, or • Use romex instead of EMT, but • E = I x R still works!

Overload and Short Circuit Conditions L O A D How is our circuit protected against overload and short circuit?

Overload and Short Circuit Conditions 15A Circuit Breaker L O A D The overcurrent device protects this circuit from both overload and short circuit.

Ground Fault Condition • So let’s talk about a ground fault condition … • Which certainly sounds like the one condition where grounding would be important and decide for ourselves whether … • Grounding provides protection for equipment or personnel under a ground fault condition

Ground Fault Condition L O A D What happens if the hot conductor comes into contact with our metal box?

Ground Fault Condition L O A D And our friend comes along and touches it? Is he in jeopardy?

Ground Fault Condition L O A D No … not at all … and why not?

Ground Fault Condition L O A D Because the transformer we’re looking at is notgrounded so there is no path through the earth for current to return to the transformer.

Ground Fault Condition L O A D Yes, that was a “trick” question … sorry about that … but the intent was to make a point.

Things You Can Count On … • No circuit – no current • Current does not flow unless there is a continuous path from one side of the source of supply to the other • Current cannot travel through the earth to return to a transformer unless the transformer is grounded

Ground Fault Condition L O A D So our friend in this situation is perfectly safe … however …

Ground Fault Condition L O A D What do we know about utility company transformers?

Ground Fault Condition L O A D They’re grounded … and, with this transformer grounded, our friend is in serious jeopardy.

Ground Fault Condition L O A D Because utility transformers are grounded, we need to do something to our equipment to keep our friend from being electrocuted.

Ground Fault Condition L O A D Can we protect our friend by grounding our metal equipment? Let’s take a look.

Ground Fault Condition L O A D Grounding our equipment provides a second path for fault current.

Ground Fault Condition L O A D The first path is through our friend to earth and back to the transformer.

Ground Fault Condition L O A D The new second path is through our metal equipment to earth and back to the transformer.

Fault Current Path • We need to open a 15A Circuit Breaker as quickly as possible. This will require a fault current of 60A to 75A (four to five times the rating of the breaker) • We can use Ohm’s Law to find out how much current will flow on our new path

Ground Fault Condition L O A D The voltage is 120V. We need to know the resistance in this circuit to calculate current.

Fault Current Path • Assuming a minimum of 5 ohms resistance through each grounding electrode, we know there is at least 10 ohms resistance in the fault path that we created by grounding our equipment

Fault Current Path • Therefore, using Ohm’s Law: • E = I x R • Transposed to: I = E / R • Where: I (current) = E (voltage) / R (resistance) • And so, I = 120 / 10 = 12A

Fault Current Path Only 12 Amps … • Will 12 Amps trip our 15A circuit breaker? Absolutely not!

With Equipment Grounded L O A D So the overcurrent device does not open and we have fried our friend!

Conclusion … • Grounding does not protect equipment or personnel from a ground fault!

The Bonding Connection L O A D The vital connection left out of our discussion until now is the bonding of metal equipment to the service neutral.

The Bonding Connection • Every piece of conductive metal which is a part of our system or likely to become energized … • Must be connected together by an electrically continuous metal-to-metal contact or by an equipment grounding conductor

The Bonding Connection • These connections create an electrically continuous, low resistance path from every part of our system back to the service equipment • At the service, these connections terminate on the neutral bus creating an: • “Effective Ground Fault Current Path”

The Bonding Connection L O A D These bonding connections let us use the neutral as a return path for fault current.

The Bonding Connection L O A D Bonding provides a third path for fault current to return to the source of supply.

Fault Current Path • We need to open a 15A circuit breaker as quickly as possible. This will require a fault current of 60A to 75A (four to five times the rating of the breaker) • We can use Ohm’s Law to find out how much current will flow on our new path

Fault Current Path The resistance in this path includes • 100’ - #2 AL OH Distribution .032 • 25’ - #4 AL Service Drop .013 • 25’ - #2 CU Service Entrance .005 • 100’ - #14 CU Branch Circuit .307 Resistance to the point of fault .357 ohms

The Bonding Connection .357 ohms L O A D .3 ohms The resistance from the point of fault through our metal equipment back to the neutral is assumed to be the same as the branch circuit wiring and 100’ of #14 cu has a resistance of .3 ohms.

The Bonding Connection .357 ohms .57 ohms L O A D .3 ohms The total resistance in this path created by bonding is .714 ohms.

Bonding, Grounding and the NEC 