Reduced-size grounding/bonding wire in vintage NM cable

[drcampbell]

I have a burning question about vintage NM cable with reduced-size grounding/bonding wires.

We recently encountered cable with 12AWG white & black wires and 16AWG bare, protected by 20-amp Pushmatic breakers. If my calculation is correct -- and this is what I'd like the larger community to confirm or deny -- it would be possible for there to be a 500^oC temperature rise in the grounding/bonding wire while the breaker carries 200% for two minutes.

Here's what I started with:
16 AWG copper wire:
- 0.0132 ohms per meter
- 0.00129 meter diameter
- 8920 kg per m³ density
- 385 kJ per kg·Kelvin thermal capacity
- 1083 ^oC melting point
20-amp breaker worst-case let-through current:
- 40 amps (200%) for two minutes (is this the worst-case I²t?)

And here's what I calculated:
- 1.31x10^-6 m² cross-section
- 0.0117 kg per meter
- 21.1 watts per meter
- 2530 Joules per meter (2 minutes)
- and an adiabatic temperature rise of 563 ^oC during a worst-case hot-to-ground bonded fault.

First of all, are my calculations correct & relevant?
Second of all, what, if anything, ought to be done about it? Shrug it off as capable of withstanding a worst-case fault without melting, knowing that the cable insulation will be damaged? Replace the 20-amp breakers with 15? Add 12AWG grounding/bonding wires?

 

[jumper]

Reduced EGCs for small branch circuits was legal until the '68 or '71 code.

If you do not modify the circuit, you do not have to do anything.

If you wish to upgrade, a proper sized EGC will be needed.

 

[winnie]

When you ask 'what ought to be done about it', are you asking what is mandated by code, or has the decision already been made to 'fix' this and you are asking what the best method is to fix it?

1) Your calculation is a reasonable approximation.

It is very unlikely that you will have a fault that is just the worst maximum that the breaker will carry, and the resistance will change substantially as the wire heats up, so any fault current is likely to change.

2) As what _should_ be done, code changed to prohibit these reduced size conductors. Clearly the design is not 'safe enough' for current code, but was considered 'safe enough' to grandfather. That is probably all that should be mandated.

3) If a design choice is made to 'fix' this issue, I would either replace the NM with new, or replace the breakers with something that offers ground fault detection. I would not run extra bonding wires; the effort (labor cost) to run such wires is similar to the effort to run new cables, and the materials cost is not much less.

-Jon

 

[drcampbell]

... the resistance will change substantially as the wire heats up ...
-Jon

Hmmm ... Right you are; I neglected to factor that into my calculations.
(at first glance, I think the resistance will increase with increasing temperature and make things worse)

 

[don_resqcapt19]

I don't think it is a big deal...as far as I know NM in Canada still has a reduced size EGC.