Why does the extension cord ground have to be the same AWG as the conductors?

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Electric-Light

Senior Member
In 120v application, I doubt there's any difference in magnetic trip characteristic between 15 or 20A breaker. The difference in thermal trip provides appropriate protection of conductors from overload.

For 100' extension cord, a 15A motor FLA motor load needing a high starting torque will benefit from a 10AWG conductor extension cord, but a 16AWG will do for a 60W work light on a 100' cord.

Even with the 16/3 100' if the hot shorts to ground, there's enough current to activate the magnetic trip.

Since the ground conductor isn't used to carry current, it really isn't subject to overload. It just provides a bleeder ground or a path to short out so if the hot shorts to case, it will trip the breaker.

Why can't we have something like 10/2, 16-ground extension cords?
 

iwire

Moderator
Staff member
Location
Massachusetts
If I am using a cord I do not want the EGC to be the most likely conductor to break first due to use / misuse / flexing etc.

We are already dealing with that problem with the grounding pins on plugs.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110406-2036 EDT

Electric_Light:

Consider an example:

You are holding an electric drill. Aluminum shell. The shell is connected to the EGC. You are standing in a pool of muddy water in your bare feet. A dead short develops between the hot wire and the drill shell. To what voltage are you subjected?

Suppose the source is 120 V. Then that voltage is about 120 * R(EGC) / { R(HOT) + R(EGC) }. If the EGC equals the Hot wire size, then the result is about 60 V. If the EGC is twice the resistance of the hot, then the voltage is about 120 * 2 / 3 = 80 V.

Also the smaller you make the EGC relative to the Hot the greater is the power dissipation in the EGC relative to the Hot.

.
 

broadgage

Senior Member
Location
London, England
In many cases a reduced ground would be acceptable, but in other cases it would be less safe.
I doubt that the marginal saving in copper would be justified by the extra expense of special manufacture.
In years gone by it was common practice to use reduced size ground wires in fixed wiring, but most codes and regulations in most countries now require a full sized ground/earth on branch circuits. And even when not a legal requirement it is often regarded as good practice.
 
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