HOW MUCH WILL IT STAND?
What are the limits of your cable system? Southwire's Power Cable Manual can help you know.
- Published in Southwire's Power Cable Update newsletter in November 1997
- Reprint permission granted
When you're designing a power cable system, you're concerned about more than just normal operations. You also need to know the outer limits of emergency and short-circuit loads. Some sections in Southwire's Power Cable Manual (PCM) can help you find answers.
How Much In Emergencies?
ICEA specifications allow emergency overloads for a total of 100 hours per year and no more than five such periods in the life of the cable. Table 6-7 in the PCM lists emergency overload uprating factors for 60OV- and medium voltage cables in a range of insulations and ampacity rating temperatures. Here's an example: You have a 15kV cable with a single 500kcmil copper conductor and XLPE insulation rated at 90'C. The NEC allows an ampacity of 685 Amperes at an ambient temperature of 40°C (Table 310-69).
The ICEA emergency overload uprating factor for this combination is 1.27. That gives an overload limit of just under 870 Amperes, subject to the ICEA operating time limitations.
What About Short Circuits?
The allowable short circuit current in a conductor is determined by conductor size, insulation type and the time the current flows. ICEA publication P-32-382, Short Circuit Characteristics of Insulate Cable, discusses the relationships.
The PCM contains sets of curves showing allowable short circuit currents. The sample here shows allowable currents for copper conductors and thermoset insulations.
PCM Table 6-7: Medium-Voltage Emergency Overload Uprating Factors |
Insulation |
Operating Temp. |
Overload Temp. |
Uprating Factor for Ambient Temp.* |
| |
|
|
20° C |
30° C |
40° C |
50° C |
PE |
75° C |
95° C |
1.10 |
1.13 |
1.17 |
1.24 |
XLPE |
90° C |
130° C |
1.18 |
1.22 |
1.27 |
1.33 |
EPR |
90° C |
130° C |
1.18 |
1.22 |
1.27 |
1.33 |
*Temperature at which ampacity is calculated
PCM Table 6-1: Conductor Short Circuit Factors, FC |
Insulation |
Copper |
Aluminum |
Thermoset (XLP, EPR 90° operating, 250° C max. |
.0719 |
.0467 |
Thermoplastic (PVC, PE) 75° C operating, 150° C max. |
.0529 |
.0344 |
|
