RESIDENTIAL BY DAVID SHAPIRO
The inspector’s basic question, and
mine, was whether luminaire manufacturers require unnecessarily high conductor
temperature ratings in the same way drug
companies stamp conservative use-by
dates on their products. You might wonder what the manufacturer has to lose.
Nationally Recognized Testing Laboratories simply evaluate luminaires for the
ability to pass tests based on the product
standard when installed in accordance
with the manufacturers’ specifications.
This means an electrician or inspector
would be caught between accepting only
that which complies with manufacturers’
instructions and using common sense,
the accuracy of which is limited. Any
alternative probably means digging up
the product standard. What jurisdiction
has product standards on hand, and what
inspector has enough time to research
them? So the careful person follows the
label. Still, the down-to-earth person,
driven by common sense and informal
field experience, may say, “It stands to
reason that the light-emitting diode (LED)
fixture is safer than the old one, so it’s
bound to be OK to install here.”
Marc Ledbetter of the Pacific North-
west National Laboratory consults
with the Department of Energy on
solid-state lighting issues. He said
waveform nonlinearity of LED drivers’
current draw could be responsible for
significantly higher levels of heating
than you would anticipate from an LED
luminaire’s alternating current wattage
rating. This would be dependent on the
electronic architecture, which might,
NEMA SSL 7A, phase-cut dimming for
While interesting, this was not
enough of an answer. Since Ledbetter is
not an expert in the safety standards for
luminaire design, I looked further.
National Fire Protection Association
(NFPA) members have the benefit of
consulting with their engineers on code
issues. Not all instructions are essential
to following product listing in keeping
with National Electrical Code (NEC)
Section 410.6. I asked the NFPA’s Chris
Coash if the 90°C insulation requirement
is part of the actual listing instructions or
merely a suggestion? His answer was an
It still left me wondering. Fortunately,
two manufacturers’ representatives took
the time to discuss the 90°C rating on mod-
ern luminaires. If there’s any chance that
you will make up your own mind about
whether to obey these listing instruc-
tions, you must have the facts. Next month,
I will cover the basis of the temperature
marking. It applies to all luminaires.
First, though, some questions and
answers that may make some readers
sweat. You might have noticed the 90°C
marking is found on both incandescent
and solid-state lamps. Why the latter? Though LEDs offer vastly greater
lumen-per-watt (efficacy) ratings than
incandescents, they can get plenty hot—
at least in spots.
How is this possible? As a contrast,
consider some old, familiar lampholders, addressed by NEC sections 410.90
to 410.102. Have you ever run into, for
example, a keyless porcelain that needed
to be equipped with a special heat sink?
No, but LEDs usually have them.
Have you ever bought, say, a pull-chain porcelain that had a pad of
fiberglass between lampholder and outlet
box? I’m guessing no, and next month I’ll
explain why this is. I will also explain the
surprising reason the heat sink and insulating pad (or reflector, or insulator and
reflector) appeared in other types of light.
Let’s start by looking at how hot LED
luminaires can get. I consulted with Jeff
Shaner, a former electrician who now is
director of design engineering at Acuity
Brands. He acknowledged the lumens per
watt for an LED luminaire are commonly
five times that of an incandescent lamp.
Because LED sources develop less
heat for each lumen of light than incandescents, he said, LED luminaires in the
sub-600-lumen range commonly do not
require a specially designed heat sink.
LED luminaires that are significantly
brighter use heat sinks to balance temperatures that can exceed a localized
100°C at the printed circuit board. That’s
right: more than 100°C.
The heat sink and other design features may even out the temperature, so
the building wire feeding the luminaire is
nowhere near that hot. However, they may
not try very hard.
Filling Those Old Sockets
Should you rely on fixture ratings?
MY JULY COLUMN ENDED WITH the statement that ignoring luminaires’
insulation-temperature requirements can be dangerous. In the project I discussed,
the old, low-temperature insulation, if used with the proposed luminaires, could
have been at risk.
SHAP IRO, author of “Old Electrical Wiring: Evaluating, Repairing, and Upgrading Dated
Systems,” is a contractor, consultant, inspector and writer/editor based in Colmar Manor, Md.
He also is affiliated with IAEI. He can be reached at