TM 5-811-1/AFJMAN 32-1080
Table 5-1. Three-Phase Medium-Voltage Circuit Loading Check Values.
Voltage
Wire
size
13.8kV
4.16kV
25kV
35kV
AWC
Megavolt-ampere-mileb
or
Maximum
amperes
(Maximum megawatt load)
Type of line
Material
kcmil
480
0.8(3.1)
8.8(10.3)
29.2(18.7)
57.3(26.2)
4/0
360
6.2( 7.7)
22.4(14.0)
0.6(2.3)
44.1(19.6)
2/0
310
0-5(2.0)
5.9( 6.7)
19.5(12.1)
38.1(16.9)
1/0
Copper
1
270
0.5(l.7)
5.1( 5.8)
16.5(10.5)
32.3(14.7)
2
230
0.4(1.5)
4.3( 4.9)
14.1( 8.8)
27.6(12.5)
4
180
3.0( 3.8)
9.8( 7.0)
19.2( 9.8)
0.3(1.2)
6
2.0( 2.8)
6.7( 5.1)
130
0.2(0.8)
13.1( 7.1)
linesc
336.4
0.8(3.4)
Aerial
530
9.3(11.4)
30.4(20.7)
59.8(28.9)
340
0.5(2.2)
5.9( 7.3)
19.4(13.3)
38.0(18.5)
4/0
ACSR
300
0.5(1.9)
5.1( 6.5)
16.7(11.7)
32.7(16.7)
3/0
270
0.5(1.8)
4.4( 5.8)
14.4(10.5)
28.3(14.7)
2/0
230
0.3(1.5)
3.7( 4.9)
12.2( 9.0)
23.9(12.5)
1/0
2
200
0.2(l.2)
2.7( 3.9)
8.9( 7.0)
17.5( 9.8)
Copper
500
465
26.0(10.0)
82.7(18.1)
230
9.9( 4.9)
32.4( 9.0)
2/0
linesd
Underground
350
305
14.5( 6.6)
47.2(10.8)
Aluminum
230
9.8( 4.9)
31.8( 9.0)
4/0
155
5.4( 3.3)
17.5( 6.0)
1/O
a
For balanced loads.
b
For loads having a 90 percent power factor thin value represents a 3 percent voltage drop. The values given
parenthesis represent the megawatt capacity of the wire and values were computed using the method shown on
figure 5-1.
c
Baaed on values from Standard Handbook for Electrical Engineers (Eleventh Edition) for 60 Hz circuits
approximately 75 percent loaded and operating at a temperature of 50 C and an equivalent spacing of 5 feet.
d
For conductors operating at 40 C.
(2) Overvoltage prevention. Destructive over-
covers an entire feeder because of circuit length
voltages do not occur when group-operated switch-
and impedance, automatic circuit reclosers or re-
ing is provided. Pole-mounted transformers are
closing circuit breakers may be necessary remote
switched so close to transformer terminals that
from the station. This application of automatic
circuit reclosers will be provided only to protect
there is not sufficient line capacitance for reso-
nance. Clearing of one fuse in a line should not be
the aerial portions of the feeder, and only when
a problem, because the transformer is loaded at
reclosing is also provided on that feeder at the
that time. Very rarely are aerial lines long enough
main electric supply station.
c. Connections to transformers. Normal trans-
to provide enough capacitance to cause ferroreson-
former circuit design will eliminate most circuit
ante.
(3) Occurrence. Ferroresonance may occur on
elements which produce the destructive voltages
long underground circuits which are single-pole
which can arise from ferroresonance.
(1) Overvoltage. Ferroresonant overvoltages oc-
switched. Since the occurrence of ferroresonance
cannot be reliably predicted, group-operated
cur when the ratio of the distributed line-to-ground
switches are required for, and should be integral
capacitive reactance (Xc) in series with the trans-
to, ground-mounted transformers.
former magnetizing (no-load) inductive reactance
d. Transition points. Transition points between
(Xm) is nearly equal and the effective resistance in
aerial and underground sections (riser poles) will
the circuit is minimal (almost no load connected to
the transformer). At such times the circuit may be
be provided with primary fuse cutouts and surge
arresters for protection of the underground cables
near resonance resulting in a total circuit imped-
and cable-supplied equipment. When the under-
ance close to zero. Under these conditions, a high
ground service supplies two or more transformers,
current will flow and cause correspondingly high
some of which may be remotely located, fuse
voltages.
5-6
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