My experience is with air conditioners which may not be as complicated as your
equipment. I have had products that include up to five motors, some with VFD
and others with electronically commutation. These products have also included
electric heaters and other electric loads. I agree that the correct marking
can become complicated, but I can give some guidance.
UL 1995 for heating and cooling equipment has marking requirements based off
of NFPA 70. The requirement for UL 1995 is marking that represents the worst
case load, but that can be based on the normal operation. If you have two
motor loads that cannot be on at the same time, you only include the FLA for
the larger of the two motors. The guidance from UL 1995 can generally be
extended to other equipment.
Please note that you must determine the worst case operating condition, even
if it would be a very rare occurrence. The NFPA 70 required marking is used
to determine the wire, disconnect and overcurrent protection sizes. These
devices must be sized appropriately for all possible conditions. If your
equipment has multiple supply connections, a separate calculation is required
for each.
You have to use your best judgment where computer controls are used to limit
internal components to ratings of less than full power. Generally, unless
your software has been tested thoroughly for safety, you cannot depend on it
to limit a device to 50% power. This type of testing is a long and arduous
process and is best avoided. Hardware interlocks are much easier to test and
certify. Software can also be more easily changed. An operating mode that
was not originally anticipated could be programmed into the system. At that
point, the required wire size or overcurrent protection may change, yet it
would be unlikely that a maintenance department would rewire the equipment
accordingly unless expressly instructed to do so. Would your programmers
recognize that the ratings need to change? This is yet another reason to
depend on hardware only for safety and not to depend on proper operation of
software.
Ted Eckert
The opinions expressed are strictly my own.
lauren_cr...@amat.com wrote:
For industrial machinery, the NEC (NF{PA 70) requires an FLA marking
on the
machine nameplate, and that the value "shall not be less than the sum of the
full-load currents required for all motors and other equipment that may be in
operation at the same time under normal conditions of use."
Large, complex machinery may have many, many loads fed from its internal
power distribution system (e.g., controllers, power supplies, robots, etc...).
Each of these sub-systems is usually a stand-alone commercial product with a
nameplate of its own indicating its own FLA rating.
More often than not, such large complex machinery is not operated with
all of
its sub-systems running at full power (e.g., all set to '10'). In typical
operation it may be that many are set at 8, most at 5 and a few at 2 (for
example).
Does the 'may' in 'may be in operation at the same time' require that
one
consider what *could* happen even if it is not how the machine would ever,
realistically, be operated, or is it acceptable to take some empirical
measurements for a 'typical' operating scenario and set the nameplate FLA
based on that?
Is there any commonly accepted position paper for US electrical
inspectors on
this point?
The issue of course is whoever installs the machine has to design their
infrastructure to support the nameplate FLA. If a machine nameplate FLA must
be determined by the simple addition of sub-system FLA's, then there is
possibility for great waste (unneeded current carrying capability). If a
machine nameplate FLA can be set by some more empirical method (e.g., current
draw during intended use), then sensible infrastructure is possible.
==============
60204-1:2005
The question converts into the EU also since '204 has very similar
criteria
for the full-load current marking of the nameplate.
"The full-load current shown on the nameplate shall be not less than the
running currents for all motors and other equipment that can be in operation
at the same time under normal conditions."
Thanks for any insight/advice.
Lauren Crane (Mr.)
Product Regulatory Analyst
Corporate Product EHS Lead
Applied Materials Inc.
Austin, TX 512 272-6540 [#922 26540]
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