---
title: "Phase order and star load (6+j8)"
subject: "Basic Electrical Engineering"
chapter: 8
key: "semester_1/basic_electrical_engineering:phase-order-star-load-6j8"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/phase-order-star-load-6j8
site: BCT Engineering
---

# Phase order and star load (6+j8)

Basic Electrical Engineering · Chapter 8: Three-Phase Circuit Analysis

- Asked in: 2074 Chaitra, 2+6 marks
- Video: [Numerical on 3 Phase Star connection (Unit 2 AC circuit) BEE](https://www.youtube.com/watch?v=-PvMcIDHNDU) — Gautam Varde
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/phase-order-star-load-6j8
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-8

## Question

Define phase order and explain its significance. A three phase balanced star connected load with (6+j8) ohm per phase is supplied by 400V, 50 HZ three phase source. Find the line and phase currents and the total power dissipated in the load.

## Short answer

Phase order is the sequence (e.g., RYB) in which voltages reach peak. Significance: determines motor rotation, needed for parallel operation and protection. For star load Z=6+j8 Ω, 400 V: phase and line currents are 23.09 A; total power is 9.6 kW.

## Answer

#### Phase order (phase sequence)
Phase order is the order in which the three phase voltages (or currents) reach their positive maximum values, for example R → Y → B (RYB, positive sequence) or R → B → Y (RBY, negative sequence).

**Significance:**
- It decides the direction of rotation of three-phase induction and synchronous motors. Interchanging any two supply lines reverses the sequence and the motor rotation.
- It is essential for connecting generators in parallel and for connecting a new supply to an existing system.
- It decides the readings of the two wattmeters and the line currents in unbalanced loads.
- It is needed for correct phasor diagrams and for protection relays and phase-sequence meters.

#### Numerical
Given Z_ph = 6 + j8 = 10∠53.13° Ω, star connected, V_L = 400 V.

- Phase voltage V_ph = 400/√3 = 230.9 V
- Phase current I_ph = V_ph/Z = 230.9/10 = **23.09 A**
- Line current I_L = I_ph = **23.09 A** (star connection)
- Power factor = 6/10 = 0.6 lagging
- Total power P = 3 I_ph² R = 3 × 23.09² × 6 = **9600 W = 9.6 kW**

(Check: √3 V_L I_L cos φ = 1.732 × 400 × 23.09 × 0.6 = 9600 W.)

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does phase order matter for a three-phase induction motor?
   A. It sets the supply frequency
   B. It decides the direction of rotation
   C. It sets the motor power rating
   D. It has no effect
   Answer: B. Swapping any two lines reverses the sequence and the rotation direction. (Key point: Significance)

2. For a star load (6+j8) Ω on a 400 V line supply, the phase current is:
   A. 40 A
   B. 230.9/10 = 23.09 A
   C. 400/14 = 28.6 A
   D. 23.09 x √3 = 40 A
   Answer: B. V_ph = 400/√3 = 230.9 V and |Z| = 10 Ω, so I_ph = 23.09 A. (Key point: Phase current)

3. What is the line current?
   A. √3 x 23.09 A
   B. Equal to the phase current, 23.09 A
   C. 23.09/√3 A
   D. Zero
   Answer: B. In star I_L = I_ph. (Key point: Line current in star)

4. What is the total power?
   A. 13.9 kW
   B. 12.8 kW
   C. 9.6 kW
   D. 16 kW
   Answer: C. P = 3 I² R = 3 x 23.09² x 6 = 9600 W (pf = 0.6). (Key point: Total power)

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