Lauren -
I'm not familiar with NFPA79, so I'll answer only with regard to NFPA70 (the
US NEC).
The 125% issue in the NEC is for two items: attachment plugs sizing and
circuit breaker sizing (1/1.25 = 0.80), both relative to load current.
Attachment plugs for equipment that contain motors must be rated for not
less than 125% of the steady state operating current. General purpose
circuit breakers need to be sized at 80% of steady state operating current
for all branch circuits, unless marked for 100% load rating when used as a
switch for lighting circuits (IIRC, this marking is "SWD").
Fuses, on the other hand, are useable up to 100% of their marked rating.
Wiring need must be sized according to the applicable load current and
overcurrent protection provided according to the wire size and load current.
The above, of course, can be used outside these ratings, if under
engineering supervision and acceptable to the Authority Having Jurisdiction.
Use at lesser currents than rated is just fine for wiring, fuses, circuit
breakers and attachment plugs.
It's difficult to comment further, without knowing more detail about the
product, since speculation can get far afield from your specific case.
BTW, I hope your client is aware of Sections 400-7 and 400-8 of the NEC, as
well as the notes to Table 400-5(b). I would have normally expected to use
the tables in Article 310 for this.
Regards,
Peter L. Tarver, PE
ptar...@nortelnetworks.com
-----Original Message-----
From: Crane, Lauren
Colleagues,
I have an internal client who is proposing a design. In this design, a
resistive heating load is being supplied by conductors that are far below
the 125% of load recommended/required by the NEC or NFPA-79 (ref 1994 sect
15.5.3). The client argues that it is okay that the supply conductors are
far below because they are fused at a level that will protect them.
.26 ohm load
driven at 120 volts max
Full Load Amps calculates to 462 amps. 125% = 577 amps. 60degC wire size
should be 700 mcm per NFPA-79 1994 table 11.
Client proposes using 2/0 90degC wire because drive circuit is an SCR
controller fused to 50 amps feeding a 4:1 step down transformer. They
rationalized max current (given 50A fuses) is 200 amps on the secondary. 2/0
90degC wire is rated to 300 amps per NEC table 400-5b.
It seems clear to me that the codes say the wire should be sized to the
load. But the codes don't seem to say that wires SHOULD NOT be sized to
thier over current protection (regardless of the load).
Does anyone have a pro or con rational for undersizing the conductors to a
load provided they are protected correctly by a fuse or breaker?
Thanks in advance.
Lauren Crane (my own opinions, not my employeer's)
* Product Design Safety & Compliance Manager
* Ion Beam and Thermal Processing Systems
* Axcelis Technologies Inc.
* 108 Cherry Hill Dr.
* Beverly, MA 01915
* 978.921-9745 lauren.cr...@axcelis.com
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
