SENSITRON
SEMICONDUCTOR
TECHNICAL DATA
DATASHEET 4310, Rev B
• 221 West Industry Court
Deer Park, NY 11729-4681
Phone (631) 586 7600 Fax (631) 242 9798
•
• World Wide Web - http://www.sensitron.com • E-Mail Address - sales@sensitron.com •
SPDXXD28 SERIES
Wire Size
MIL-W-5088L has a chart the shows wire size as a function of wire temperature and current. This chart is for a
single copper wire in free air. For an ambient temperature of 70
oC, the chart allows an 18-gauge wire to handle
25 Amps continuously at a wire temperature of 200
oC – a wire temperature rise of 130 oC. For a wire
temperature limited to 150
oC, the chart requires a 16-gauge wire and for a wire temperature of 105 oC, the chart
requires a 14-gauge wire.
Amendment 1 of MIL-W-5088L has a table for copper wire in a bundle, group or harness with conditions on the
number of wires, percent of total harness capacity, etc. This table shows that a 12 gauge wire is necessary for
200
oC operation, 10 gauge for 150 oC and 8 gauge for 105 oC.
MIL-W-5088L has various figures showing derating for harnesses as a function of the number of current carrying
conductors and for altitude. MIL-W-5088L only specifies wire for DC or RMS AC conditions, not for transient or
overload conditions.
For transient or overload conditions, the transient or overload happens so quickly that heat is not transferred
from the wire to the surroundings. The heat caused by the I
2R heating of the wire causes the temperature to
rise at a linear rate controlled by the heat capacity of the wire. The equation for this linear rise in temperature,
with respect to time, can be solved as: I
2t = constant. Every wire has an I2t rating that’s dependent on the
temperature rise allowed and the diameter of the wire. If the I
2t rating of the SSPC or circuit breaker is less than
the I
2t rating of the wire, then the SSPC or circuit breaker can protect the wire. The maximum I2t rating for the
SPD25D28 is 7.45 Amp
2-Seconds. Every wire size in the paragraphs above has an I2t rating that exceeds the
SPD25D28 I
2t rating for the temperature rises stated. Therefore, to select a wire size, it’s simply a matter of
determining the maximum temperature rise of the application and deciding whether or not the wire will be in a
bundle and use the information above.
Application Connections
The SPDXXD28 Series may be configured as a high-side or low-side switch and may be used in positive or
negative supply applications. Figure 5 shows the connections as a high-side switch with a positive power
supply.
Figure 5 – High-Side Switch, Positive Supply
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