Bob,
There are a few references to the 125% rule (or 80% rule) in the National
Electrical Code (NFPA 70):
1. NEC Article 384-16(c) for Panelboards- "Continuous Load. The total load
on any overcurrent device located in a panelboard shall not exceed 80 percent
of its rating where, in normal operation, the load will continue for three
hours or more.
Exception: An assembly, including the overcurrent device, shall be permitted
to be used for continuous operation at 100 percent of its rating where it is
listed for this purpose."
2. NEC Article 210-22 for Branch Circuit - "Other Loads. The rating of the
branch-circuit overcurrent device serving continuous loads, such as store
lighting and similar loads, shall be not less than the noncontinuous load
plus 125 percent of the of the continuous load. The minimum branch-circuit
conductor size, without the application of any adjustment or correction
factors, shall have an allowable ampacity equal to or greater than the
non-continuous load plus 125 percent of the continuous load.
Exception: Circuits supplied by an assembly, together with its overcurrent
devices, that is listed for continuous operation at 100 percent of its
rating."
Hope this helps.
Tin
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From: bris...@mail.dec.com (Bob Brister)
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Reply-to: bris...@mail.dec.com (Bob Brister)
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Date: 97-04-25 09:06:53 EDT
Hello,
Are there requirements or guidelines around what a power supply input
breaker should be rated for a given nominal input current assuming the
power cord is rated appropriately? Looking through the harmonized 950
standard I did not come across anything.
I seem to recall seeing a 125% figure once but don't remember where that
was. I would assume that the circuit breaker would be some percent
higher than the nominal input current rating to prevent nuisance trips
and to take into account surge current or perhaps this is addressed by
the trip characteristics of the breaker.
To give an example, if the unit has an input current rating of 16amps,
is there a rule which states the input circuit breaker must fall within
some rating range.
Regards,
Bob Brister
DIGITAL
bris...@mail.dec.com
