Undersized Ground in NM Cable

[Conduit Phil]

On older NM cable (late 1960s?) it was allowed to have an undersized ground wire. #16 on #14 NM... No longer allowed yet why are we allowed to have undersized ground (#10) on NM cables all the way up to #6? My understanding is that we don't need a full size ground because it isn't expected to carry current for any significant amount of time. Why doesn't the same hold true for smaller circuits?

 

[480sparky]

In a short-circuit situation, the grounding conductor only needs to be large enough to permit enough current to flow to allow the OCPD to operate. In smaller circuits with 14, 12, and 10, it takes the same size as the ungrounded conductors to achieve this.

In larger amp circuits, using 8 and 6, a #10 will still provide enough current flow.

 

[glene77is]

Why doesn't the same hold true for smaller circuits?

Conduit,

You are asking "WHY?"
I am on your side.

I've always wondered Why a smaller guage wire
should vaporize before tripping its OCPD.
It does not make obvious sense.

 

[480sparky]

The amperage rating of the breaker is not really relevant when a circuit is shorted to ground.

A 20 amp breaker will not trip once it reaches 20.00000001 amps. Nor will a 100 amp breaker open as soon as the current reaches 100.00000001 amps.

In a short circuit, the full load current available to the circuit will flow, and it only takes one or two cycles for it to reach that point. Depending on the transformer, resistance in the circuit, and other factors, it may be 5,000 amps and up.

The ground wire only needs to be capable of carrying that 5k+ amps until the OCPD opens. With a fuse, that's only 3-5 cycles. But a circuit breaker usually needs 5-15 cycles to reacte. So #18 and 16 won't do it. So 15a circuits need 14 ground, 20a needs 12, etc. Once you get to 40- and 50- amp circuits, the current flow will be higher due to the lower resistance of the larger ungrounded circuits. But in those cases, a #8 will still carry enough current long enough to operate the OCPD.

Get into larger circuits, like 100 amps and up, the ratio between the ungrounded and grounded is maintained.

 

[don_resqcapt19]

Using the formula in this document I worked out the short circuit withstand current for #16, #18 and #20 wire. The formula is based on a conductor starting temperature of 75?C and a rise to 150?C. The following numbers are based on a one cycle clearing time. #14, 1685 amps, #16, 1060 amps, #18, 665 amps and #20, 419 amps. 150?C is the maximum short time temperature limit for an insulated conductor without causing damage to the insulation.

The same document also looks at the current required to melt a copper conductor. The following currents will melt the respective conductor if they are applied for one cycle.
#14, 4353 amps, #16, 2737 amps, #18, 1722 amps and #20, 1082 amps.

 

[don_resqcapt19]

...
The ground wire only needs to be capable of carrying that 5k+ amps until the OCPD opens. With a fuse, that's only 3-5 cycles. But a circuit breaker usually needs 5-15 cycles to reacte. So #18 and 16 won't do it. So 15a circuits need 14 ground, 20a needs 12, etc....

If that statement is true, we have have a problem. The one cycle melting current for #14 is 4,353 amps.