L6-20P -> L6-30R
Lamar Owen
lowen at pari.edu
Wed Mar 19 15:22:30 UTC 2014
On 03/19/2014 09:51 AM, William Herrin wrote:
> Nobody is talking about putting an L6-20R on a 30 amp circuit. OP was
> talking about putting an L6-30P on a 20 amp appliance: a PDU that has
> its own 20 amp breaker. Big difference.
If the PDU isn't listed for 30A then it's the essentially the same
thing, safety-wise. Unless there is overcurrent protection at the source
of the feed to the conductors of the flexible cord (240.21) that meets
the ampacity of the conductors of said flexible cord, unless one of the
exceptions of 240.5 apply, then it's a potentially unsafe condition (NEC
doesn't directly apply to supply cords of appliances themselves; that's
what the 'listing' is for from UL or similar; see
http://ecmweb.com/code-basics/nec-rules-overcurrent-protection-equipment-and-conductors
for more info, and see UL's FAQ entry for modifications to listed
equipment at
www.ul.com/global/eng/pages/offerings/perspectives/regulator/faq/).
Just replacing an L6-20P with an L6-30P on a 20A-listed PDU would be
unsafe and (IMO) unwise, since the breaker in the input of the PDU does
not protect the flexible cord's conductors from internal overcurrent
faults. A 20A listed PDU should have 20A overcurrent protection to the
connected receptacle, in addition to any overcurrent protection internal
to the PDU. A cord with a 20A ampacity may overheat significantly if it
faults internally in such a way as to cause more than 20A, but less than
30A (or whatever overcurrent protection is in the branch circuit), to
flow; there are numerous ways cords can fault in this manner. You could
easily get a situation where the cord is partially faulted internally
but the PDU's breaker doesn't detect it because the fault shunts current
ahead of that breaker; again, not a dead short but still an overcurrent
fault. I've seen this type of fault before, where the cord itself was
shunting a few amps prior to the PDU input breaker (in this particular
case the cord was damaged by lightning, even though the equipment to
which it was connected still had power).
But the other condition, where a 20A breaker is feeding a 30A PDU, could
result in dropping power to the PDU but is not unsafe.
I know that I wouldn't approve (in the NEC-speak sense of that word) of
the use of any of these adapters or similar kludges in my data centers,
as the insurance liability issues are potentially much more costly than
just buying the right PDU or running a branch circuit with the correct
overcurrent protection in the first place.
It also depends a bit on exactly how the PDU is listed. You can look up
the listing's details in the UL White Book (download link:
http://www.ul.com/global/documents/offerings/perspectives/regulators/2013_WB_LINKED_FINAL.pdf
).
But the final say rests with the authority having jurisdiction, AHJ in
NEC-speak.
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