ASHRAE Journal - February 2015 - 52

TECHNICAL FEATURE

Time (Seconds)

ground faults. Fuses and circuit breakers respond
FIGURE 1 Motor starting/60 amp breaker time-current coordination plot.
to the combination of current and time to open
the circuit before excessive current flow would
100
cause a fire.
The general rule in Section 240.4 says that con10
5
ductors must be protected in accordance with
their ampacities. Under the general rule, our #4
4
3
copper conductor with a 60°C (140°F) ampacity of
1
70 amps calls for a 70 amp breaker.
2
Suppose ambient temperature or some other
0.1
derating factor reduced the ampacity of the #4
1
copper conductor to 63 amps. Section 240.6(A)
lists standard fuse and circuit breaker ratings.
0.01
0.1
1
10
100
The available sizes of interest for this probCurrent (Per Unit)
lem are 60 amps and 70 amps. 60 amps looks
Motor Starting Current
Breaker Minimum
Breaker Maximum
like a good choice because it is less than the
adjusted conductor ampacity so would keep
the #4 conductor from overheating. But 60 amps does
onds as the motor accelerates the load (in our case a fan
not meet the minimum circuit ampacity of 60.1 amps.
or compressor). Once the motor reaches operating speed,
Enter Section 240.4(B). For overcurrent devices rated
the current levels out (in this example) at 60% of circuit
800 amps or less, "The next higher standard overcurnominal load.
rent device rating (above the ampacity of the conduc* The red and green lines represent circuit breaker
tors being protected) shall be permitted to be used"
performance. All mechanical devices have some tolerprovided certain conditions not relevant to this discus- ance. The red line 3 is the fastest the circuit breaker can
be expected to operate; the green line 4 is the slowest.
sion are met. At first glance, it looks like a 70 amp fuse
* As current increases, the length of time the circuit
or circuit breaker is the size to use, but one more check
breaker will stay closed decreases. That's why circuit
is needed.
breakers and fuses are called "inverse time" devices.
If the circuit for this 10 ton (35 kW) rooftop unit had to
In the case of the red line, the breaker can hold roughbe protected by a 60 amp breaker, there's a good chance
ly 200 amps for 100 seconds. The circuit breaker lines
the breaker would trip almost every time a compressor
are nearly vertical beyond about 100 seconds. In the
started. To understand why, look at Figure 1.
* The graph is a set of time-current curves plotted part of the breaker curve not shown in this extract, the
on log-log axes. Unlike most graphs that engineers use, lines become vertical at rated current at about 5,000
the independent variable, time, is on the y-axis. For this seconds (83 minutes). That means the breaker can
analysis, it helps to read the graph vertically instead of hold rated current virtually indefinitely without tripping.
from left to right.
* Notice what happens while the motor accelerates
* The blue line shows motor current as a function
of time. Starting at the bottom (time less than 0.01 sec- and draws locked rotor current 5 . At 6 seconds, the
onds), the motor draws a momentary "inrush" current motor current line crosses to the right of the red circuit
of up to 20 times the nominal circuit current 1 . (Nomi- breaker line. That means the combination of time and
nal circuit current includes the other rooftop unit loads current the motor draws exceeds the combination the
in addition to the compressor). Besides being very short circuit breaker will hold. So the circuit breaker trips. To
duration, the role of the inrush current is primarily to coordinate the circuit breaker with the motor starting
magnetize the motor windings, so it does not draw much current, the circuit breaker curve has to be entirely to the
real power. The inrush current rapidly drops off to a little right of the motor starting curve.
over four times nominal current 2 . The motor draws
While time-current curves are available for almost
this so called "locked rotor current" for around 10 sec- every circuit breaker and fuse, time-current curves for
52

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2015



ASHRAE Journal - February 2015

Table of Contents for the Digital Edition of ASHRAE Journal - February 2015

Contents
ASHRAE Journal - February 2015 - Cover1
ASHRAE Journal - February 2015 - Cover2
ASHRAE Journal - February 2015 - 1
ASHRAE Journal - February 2015 - 2
ASHRAE Journal - February 2015 - Contents
ASHRAE Journal - February 2015 - 4
ASHRAE Journal - February 2015 - 5
ASHRAE Journal - February 2015 - 6
ASHRAE Journal - February 2015 - 7
ASHRAE Journal - February 2015 - 8
ASHRAE Journal - February 2015 - 9
ASHRAE Journal - February 2015 - 10
ASHRAE Journal - February 2015 - 11
ASHRAE Journal - February 2015 - 12
ASHRAE Journal - February 2015 - 13
ASHRAE Journal - February 2015 - 14
ASHRAE Journal - February 2015 - 15
ASHRAE Journal - February 2015 - 16
ASHRAE Journal - February 2015 - 17
ASHRAE Journal - February 2015 - 18
ASHRAE Journal - February 2015 - 19
ASHRAE Journal - February 2015 - 20
ASHRAE Journal - February 2015 - 21
ASHRAE Journal - February 2015 - 22
ASHRAE Journal - February 2015 - 23
ASHRAE Journal - February 2015 - 24
ASHRAE Journal - February 2015 - 25
ASHRAE Journal - February 2015 - 26
ASHRAE Journal - February 2015 - 27
ASHRAE Journal - February 2015 - 28
ASHRAE Journal - February 2015 - 29
ASHRAE Journal - February 2015 - 30
ASHRAE Journal - February 2015 - 31
ASHRAE Journal - February 2015 - 32
ASHRAE Journal - February 2015 - 33
ASHRAE Journal - February 2015 - 34
ASHRAE Journal - February 2015 - 35
ASHRAE Journal - February 2015 - 36
ASHRAE Journal - February 2015 - 37
ASHRAE Journal - February 2015 - 38
ASHRAE Journal - February 2015 - 39
ASHRAE Journal - February 2015 - 40
ASHRAE Journal - February 2015 - 41
ASHRAE Journal - February 2015 - 42
ASHRAE Journal - February 2015 - 43
ASHRAE Journal - February 2015 - 44
ASHRAE Journal - February 2015 - 45
ASHRAE Journal - February 2015 - 46
ASHRAE Journal - February 2015 - 47
ASHRAE Journal - February 2015 - 48
ASHRAE Journal - February 2015 - S1
ASHRAE Journal - February 2015 - S2
ASHRAE Journal - February 2015 - S3
ASHRAE Journal - February 2015 - S4
ASHRAE Journal - February 2015 - S5
ASHRAE Journal - February 2015 - S6
ASHRAE Journal - February 2015 - S7
ASHRAE Journal - February 2015 - S8
ASHRAE Journal - February 2015 - S9
ASHRAE Journal - February 2015 - S10
ASHRAE Journal - February 2015 - S11
ASHRAE Journal - February 2015 - S12
ASHRAE Journal - February 2015 - S13
ASHRAE Journal - February 2015 - S14
ASHRAE Journal - February 2015 - S15
ASHRAE Journal - February 2015 - S16
ASHRAE Journal - February 2015 - 49
ASHRAE Journal - February 2015 - 50
ASHRAE Journal - February 2015 - 51
ASHRAE Journal - February 2015 - 52
ASHRAE Journal - February 2015 - 53
ASHRAE Journal - February 2015 - 54
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ASHRAE Journal - February 2015 - 80
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