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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