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Equipment Damage Curves Conductors

 
     
  The information presented in this application guide is for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc.

Purpose

The purpose of this guide is to provide basic information about conductor damage curves and characteristic landmarks necessary for plotting on time-current curves (TCC), for the purpose of equipment overcurrent protection.

Ampacity

The rated continuous current carrying capacity of the conductor at a referenced ambient temperature, allowable temperature rise, geometry and installation. For bare overhead conductors an ambient air temperature of 40°C is typical. For underground insulated power cables an ambient earth temperature of 20°C is typical. Temperature correction factors are then given to adjust the base ampacity for other ambient temperature levels.
If a cable is loaded continuously above rated ampacity the insulation temperature design limits will be exceeded. This will lead to loss of insulation life, not instantaneous failure.
If a bare overhead conductor is loaded continuously above rated ampacity the mechanical strength of the conductor is reduced. This will lead to a loss of mechanical life, not instantaneous failure.

Table 1 summarizes typical allowable conductor temperature limits under short circuit, emergency overload and normal operating conditions.
 
  Table 1 Typical conductor operating temperature limits  
 
Type Insulation Voltage Short Circuit Emergency Normal
0.01 < t < 10 sec. 10 sec. < t < ~1-6 hrs t > ~1-6 hrs
Al or Cu TW

600V

150ºC 85ºC 60ºC
Al or Cu THWN 600V 150ºC 90ºC 75ºC
Al or Cu THWN 600V 150ºC 105ºC 90ºC
Al or Cu XLP

5-15kV

250ºC 130ºC 90ºC
Al or Cu EPR

5-15kV

250ºC 130ºC 90ºC
AAC Air All 340ºC 150ºC 100ºC
ACSR Air All 645ºC 150ºC 100ºC
 
  The ampacity landmark is located in the top decade of a TCC at 1000 seconds.

Emergency Overload Limit Curve

Conductor overcurrent operating limit that if exceeded will reduce the insulation life of a cable or reduce the mechanical life of a bare overhead conductor beyond an acceptable design loss of life limit.

Cable limit curves are based on the thermal inertia of the conductor, insulation and surrounding material. As a result, it can take from 1 to 6 hours for the temperature of a cable to stabilize after a change in load current. Therefore, under these emergency operating conditions, currents much greater than the rated ampacity can be supported. Tables 2 and 3 provide factors and percent overload capability for various installations.
 
  Table 2 Cable K factors  
 
Cable Size K Factors
Air UG Duct Direct Buried
No Conduit

Conduit

< #2 AWG 0.33 0.67 1.00 1.25
#2 - 4/0 AWG 1.00 1.50 2.50 3.00
> 4/0 AWG 1.50 2.50 4.00 6.00
 
  Table 3 Emergency overload current at 40°C ambient  
 
Time Percent Overload
Seconds K=0.5 K=1 K=1.5 K=2.5 K=4 K=6
  EPR-XLP TN = 90°C TE = 130°C
10 1136 1602 1963 2533 3200 3916
100 374 518 629 807 1018 1244
1000 160 195 226 277 339 407
10000 126 128 132 140 152 168
18000 126 127 128 131 137 147
  THH TN = 90°C TE = 105°C
10 725 1020 1248 1610 2033 2487
100 250 338 407 518 651 794
1000 127 146 163 192 229 270
10000 111 112 114 118 124 131
18000 111 111 112 113 116 121
  THW TN = 75°C TE = 95°C
10 987 1390 1703 2197 2275 3396
100 329 452 548 702 884 1080
1000 148 117 202 245 298 357
10000 121 123 125 132 142 154
18000 121 121 122 125 130 137
 
  Similar methods exist to determine the limit curve for bare overhead conductor applications, but are not covered in this guide.

Emergency overload curves are typically not shown on a TCC. However, when shown, are plotted in the upper 2 decades of the TCC.

Short Circuit Damage Curve

Curve that describes the conductor short circuit current operating limit, which if exceeded, will damage the conductor insulation. The curve is calculated assuming all heat is absorbed by the conductor metal, with no heat transmitted from the conductor to the insulation.

Separate equations are given for copper and aluminum cables. Both equations relate conductor temperature rise to conductor size, fault current magnitude and fault duration.
Insulated copper conductors

t = 0.0297 log10 [(T2+234) / (T1+234)] (A/I)2 (1)

Insulated aluminum conductors

t = 0.0125 log10 [(T2+228) / (T1+228)] (A/I)2 (2)

For bare conductors the short circuit damage temperature limit is much higher than those listed for insulated conductors. In this case the curve describes the conductor short circuit current operating limit at which the maximum acceptable loss in conductor mechanical strength is reached. Therefore, if this limit is exceeded, the conductor will be damaged.

For bare stranded aluminum conductors the upper temperature limit is 340ºC (300º rise over a 40ºC ambient). For bare stranded ACSR conductors the upper temperature limit is 645ºC (605º rise over a 40ºC ambient).

Bare stranded aluminum conductors

t = (0.0671A/I)2 (3)

Bare stranded ACSR conductors

t = (0.0862A/I)2 (4)

where:

A = conductor area – circular mils
I = short circuit current – RMS amperes
t = time of short circuit – 0.01 to 10 seconds
T1 = rated insulation operating temperature limit
T2 = rated maximum insulation short circuit temperature limit

Example 1

Plot the conductor landmarks for 3-1/C, 500kCM, THWN copper cables installed in metallic conduit on a 480V distribution system.

Solution

FLA from NEC table 310.16 is 380A

Emergency overload points calculated from Tables 2 and 3