Short Circuit Currents - Shielding Currents for Electrical Wire and Cable

Shield short circuit current formula

For short-circuit shield ampacity with a known cable shield area (or an area that can be calculated from formulas given below), the following simplified formula may be used.

..........	Where	I = Amperes
		A = Shield Area in CM
		N = Number of Cycles
		T₁= Initial Temp. 65°C
		K = See Table Below

T₂MAX C	VALUES OF K
T₂MAX C	Cu	Al	Pb
150 200 250 350	0.193 0.288 0.372 0.517	0.083 0.123 0.159 0.221	0.0063 0.0094 0.0121 —

NOTE: Use 200 for thermoplastic and 350 for thermosetting jackets.

For Overlapped Tape (1) A = 4bd_m x
For Tubular Sheath A = 4bd_m
For Wire Braid A = Nd_s²
For Spaced Tape A = 1.273 Wb

Where:	b = Shield thickness, mils	...........	W = tape width, mils
	dm = mean diameter, mils		A = area in CM
	ds = wire diameter, mils		L = lap of tape, mils
	N = number of wires

(1) Substituting various overlap values gives the following:

OVERLAP	..........	AREA
1/2 W (50%		4bdm
1/4 W (25%)		3.27bdm
1/5 W (20%)		3.16bdm
1/8 W (12.5%)		3.02bdm

Permissible short circuit currents for copper shielding tape* amperes
Shield Diam. Inches	Effective Shield Area Circular Mils	Short Circuit Time in Number of Cycles (60 per sec.)
Shield Diam. Inches	Effective Shield Area Circular Mils	1	2	4	8	16	30	60
1/2... 3/4... 1... 1 1/4... 1 1/2... 1 3/4... 2... 2 1/4... 2 1/2... 2 3/4... 3..	7,484...... 11,264...... 15,044...... 18,824...... 22,604...... 26,384...... 30,164...... 33.944...... 37.724...... 41,504...... 45,284......	4,016 6,044 8,073 10,101 12,130 14,158 16,187 18,215 20,243 22,272 24,300	2,840 4,274 5,708 7,143 8,577 10,011 11,446 12,880 14,314 15,749 17,183	2,008 3,022 4,036 5,051 6,065 7,079 8,093 9,107 10,122 11,136 12,150	1,420 2,137 2,854 3,571 4,289 5,006 5,723 6,440 7,157 7,874 8,591	1,004 1,511 2,018 2,525 3,032 3,540 4,047 4,554 5,061 5,568 6,075	733 1,104 1,474 1,844 2,215 2,585 2,955 3,.326 3,696 4,066 4,437	518 780 1,042 1,304 1,566 1,828 2,090 2,352 2,613 2,875 3,137

*Values are derived from the Conductor Currents formula and T₂= 200°C, T₁= 65°C, 5 mil copper tape with 12.5% overlap.

Short Circuit Currents Shielding Currents

Shield short circuit current formula

Permissible short circuit currents for copper shielding tape* amperes

Short Circuit Currents
Shielding Currents

Permissible short circuit currents
for copper shielding tape* amperes