Lecture 12 Voltage Drop and Short Circuit Calculation

Shop Practice with Electrical Code: Voltage Drop and Short Circuit Calculation Residential Single Phase System Jayson Bryan E. Mutuc, REE, RME BSEE-PUP MSEE, Major in Power Systems -MIT (candidate)

Voltage Drop 2.10.2.1 (a) (1) FPN No. 4: To provide reasonable efficiency of operation of electrical equipment, branch-circuit conductors should be sized to prevent a voltage drop not to exceed three percent. In addition, the maximum total voltage drop on both feeders and branch circuits should not exceed five percent.

Voltage Drop Calculation Voltage Drop Calculation: IR %VD = x100% Voltage Supply (Vs )

where: I – line current, amperes R – line ac resistance, ohms

Purpose of Short Circuit Calculation The purpose of short circuit calculation is to know the rating the interrupting capacity of the overcurrent protective device In circuit breaker there are 3 important parameters when it comes to residential design, ampere trip, ampere frame and interrupting rating Ampere trip – the current rating the circuit breaker set to trip Ampere frame – insulation housing for maximum long time current available in physical Interrupting Rating – the highest current at rated voltage that a device is intended to interrupt under standard test conditions.

Analogies to Short Circuit Condition Normal Current Operation

Short Circuit Condition with Inadequate Interrupting Rating

Analogies to Short Circuit Condition Short Circuit with Adequate Interrupting Rating

Calculation of Short Circuit Currents Basic Short Circuit Calculation Procedure Step 1: Determine transformer full-load amperes from either: a) Name plate b) Table c) Formula Step 2: Find the transformer multiplier Note: most % Z of xformer is multiplied by .9

Formula 𝟏 ∅ 𝐭𝐫𝐚𝐧𝐬𝐟. 𝐈𝐅𝐋𝐀 =

𝐊𝐕𝐀 𝐱 𝟏𝟎𝟎 𝐄𝐋−𝐋

EL-L = Line to Line Voltage

𝟏𝟎𝟎 𝐌𝐮𝐥𝐭𝐢𝐩𝐥𝐢𝐞𝐫 = 𝐓𝐫𝐚𝐧𝐬𝐟. % 𝐙

Calculation of Short Circuit Currents Step 3: Determine transformer let-through short-circuit current Step 4: Calculate “f” factor Note: IL-N = 1.5 x IL-L at transformer terminals

Formula 𝐈𝐋−𝐋 = 𝐭𝐫𝐚𝐧𝐬.𝐅𝐋𝐀 𝐱 𝐦𝐮𝐥𝐭𝐢𝐩𝐥𝐢𝐞𝐫 1Φ line-to-line

𝐟=

𝟐 𝐱 𝐋 𝐱 𝐈𝐋−𝐋 𝐂 𝐱 𝐧 𝐱 𝐄𝐋−𝐋

(L-L) faults 1 Φ line-to-neutral

𝐟=

𝟐 𝐱 𝐋 𝐱 𝐈𝐋−𝑵 𝐂 𝐱 𝐧 𝐱 𝐄𝐋−𝑵

(L-N) faults L – length (feet) of conduit to the fault. C – conduit constant (table B)

Subscript L-L means Line to Line

n – number of conductors per phase

Subscript L-N means Line to Neutral

I – available short circuit current in amperes at beginning of the circuit.

Calculation of Short Circuit Currents Step 5: Calculate “M” or take Step 6: Compute the available short circuit current (RMS symmetrical)

Formula

𝟏 𝐌= 𝟏+𝐟

𝐈𝐒𝐂𝐀 = 𝐈(𝐋−𝐋 𝐨𝐫 𝐋−𝐍) 𝐱 𝐌

Short Circuit Sample Computation All faults are line to line single phase fault

Fault #1 Step 1 IFLA =

KVA x 100 EL−L

=

50 x 1000 240

= 208.33 A

Step 2 Multiplier =

100 Transf.% Z

=

100 .9(2)

= 55.55

Step 3 IL−L = trans.FLA x multiplier = 208.33 x 55.55 = 11572.92 A Single phase short circuit current at Transformer Secondary Step 4 2 x L x IL−L 2 𝑥20 𝑥 11572.92 f= = C x n x EL−L 22185 𝑥 2 𝑥 240

= 0.04347125185

Short Circuit Sample Computation Step 5

1 1 M= = = 0.958 1+f 1 + 0.043

All faults are line to line single phase fault

Step 6 ISCA = I(L−L or L−N) x M = 11572.92 x 0.958 = 11090.7863 A Fault #2 (Use ISCA at Fault #1 to calculate) Step 4 2 x L x IL−L 2 𝑥20 𝑥 11090.7863 f= = C x n x EL−L 5907 𝑥 2 𝑥 240 = 0.156 Step 5 M=

1 1 = = 0.864 1+f 1 + 0.156

Step 6 ISCA = I(L−L or L−N) x M = 11090.7863 x 0.864 = 9590.257 A

Use Circuit Breaker with 10 KAIC rating

Table A. Short – Currents Available from Various Size Transformers

Voltage and Phase

KVA

25

120/240 1ph**

Full Load Amps

104

% Impedance (nameplate) 1.5

Table B. “C” Values for Conductors mm2(AWG)

Three Single Conductors 600V

2.0 (14 AWG)

389

3.5 (12 AWG)

617

5.5 (10 AWG)

981

8.0 (8 AWG)

1557

12175

14 (6 AWG)

2425 3806

Short Circuits Amps

37.5

156

1.5

18018

22 (4 AWG)

50

208

1.5

23706

30 (2 AWG)

5907

75

313

1.5

34639

38 (1 AWG)

7293

100

417

1.6

42472

50 (1/0 AWG)

8925

1.6

666644

10755

696

60 (2/0 AWG)

167

80 (3/0 AWG)

12844

100 (4/0 AWG)

15082

125 (250 MCM)

16483

150 (300 MCM)

18177

175 (325 MCM)

19704

200 (400 MCM)

20566

250 (500 MCM)

22185

300 (600 MCM)

22965

375 (725 MCM)

24137

500 (1000 MCM)

25278