mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
I came across this NFPA ROCM PDF from 2012:
https://www.nfpa.org/Assets/files/AboutTheCodes/70/NEC-P02_ROCagenda_11_12.pdf
What strikes me is how aggressively manufacturers and UL are trying to convince CMP-2 that 500 amps of short circuit current is common at the service disconnecting means and that modern breakers trip magnetically at over 300 amps.
Yet, with a bit of research both are proven unlikely.
For example even with a 10kva pole pig 1,777 amps is observed (page 4):
Time current curves of common breakers show a 6-7x magnetic pickup (105 amps):
Even UL admits their own test results are likely off...
If the CMPs were fed realistic values a simple table listing a maximum run length for each wire size would suffice while still providing complete arc fualt protection coverage.
https://www.nfpa.org/Assets/files/AboutTheCodes/70/NEC-P02_ROCagenda_11_12.pdf
What strikes me is how aggressively manufacturers and UL are trying to convince CMP-2 that 500 amps of short circuit current is common at the service disconnecting means and that modern breakers trip magnetically at over 300 amps.
Yet, with a bit of research both are proven unlikely.
For example even with a 10kva pole pig 1,777 amps is observed (page 4):
Time current curves of common breakers show a 6-7x magnetic pickup (105 amps):
Even UL admits their own test results are likely off...
If the CMPs were fed realistic values a simple table listing a maximum run length for each wire size would suffice while still providing complete arc fualt protection coverage.
