electrical system design_branch ckt

7/25/2019 Electrical System Design_branch Ckt

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RESONANZ TG Inc.

making innovative solutions...CENTER FOR PROFESSIONAL DEVELOPMENT

IN ENGINEERING & TECHNOLGY

TRAINING WORKSHOP IN

ELECTRICAL SYSTEM DESIGN

Motor Circu i t Calculat ion


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RESONANZ TG Inc. making innovative solutions...

About the Speaker: Gener G. Restubog

Registered Electrical Engineer

2009 President IIEE Northern Laguna Chapter

Education M. of Engg Elect Engg Adamson University, Manila

M.Sci in Elect Engg (Units Earned), University of the Phils Diliman QC

B.Sci in Elect Engg, Central Colleges of the Phils, QC

Current Job/Affiliations

Sr Partner & Head, Training & Consulting Resonanz TG Inc

Consultant - Tricore Solutions, Inc Engineering Faculty De Lasalle UniversityDasmarinas

Engineering Faculty Technological Inst of the Phils - Quezon City

Engineering Faculty University of Perpetual Help System Laguna

Former Electrical Design Manager GHD Pty Limited

Former Facility Design Manager Intel Technology Phils

Former Designer Proj Manager Trans-Asia Phils. Framegroup Mgt System

Former Electrical Design Engineer Trans-Asia Phils, Kinhill-Tan Spore, TCGI Engrs Former Engineering Faculty Member Lyceum Univ-Laguna, PLM, MFI, CCP

Experience

Over 24 years experience in engineering design, consultancy, construction, andteaching of electrical engineering


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Design CalculationMotor Branch Ckts & Feeders


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AC Motors

Branch Circuits & Feeders

Source: NEC Handbook: W.M. Earley


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Motor Branch Ckts & Feeders

-Design Calculation

Motor Circuit Conductor

Single Motor, PEC 4.30.2.2

Conductors that supply a single motor used in a continuous duty application shall have

an ampacity of not less than 125 percent of the motor full-load current rating, as

determined by 4.30.1.6(A)(1), or not less than specified in 4.30.2.2 (A) through (F).

Several Motors or a Motor(s) and Other Load(s). PEC 4.30.2.4

Conductors supplying several motors, or a motor(s) and other load(s), shall have an

ampacity not less than the sum of each of the following:

125 percent of the full-load current rating of the highest rated motor, plus the

sum of the full-load current ratings of all the other motors in the group, as

determined by 4.30.1. 6(A) plus the sum of other loads


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-Design Calculation

Motor and Branch-Circuit Overload Protection

Continuous-Duty Motors - More Than 1 Horsepower

A separate overload device that is responsive to motor current. This device shall be

selected to trip or shall be rated at no more than the following percent of the motor

nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater 125%Motors with a marked temperature rise 40C or less 125%

All other motors 115%

Larger Than 1500 Horsepower.

For motors larger than 1500 hp, a protective device having embedded temperature

detectors that cause current to the motor to be interrupted when the motor attains a

temperature rise greater than marked on the nameplate in an ambient temperature of

40C.


7/23RESONANZ TG Inc. making innovative solutions...


-Design Calculation

Motor and Branch-Circuit Short Circuit and Ground Fault Protection

PEC Section 4.30.4.2

In Accordance with Table 4.30.4.2. A protective device that has a rating or

setting not exceeding the value calculated according to the values given in

Table 4.30.4.2 shall be used. (With exceptions)




-Design CalculationTable 4.30.4.2 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and

Ground-Fault Protective Devices




-Design Calculation

Motor Feeder Short Circuit and Ground Fault Protection

Rating or Setting Motor Load and Other Load(s). Where a feeder supplies

a motor load and other load(s), the feeder protective device shall have a

rating not less than that required for the sum of the other load(s) plus the

following (PEC Section 4.30.5.3)

(1) For a single motor, the rating permitted by 4.30.4.2

(2) For a single hermetic refrigerant motor-compressor, the rating permitted

by 4.40.3.2

ICB

=< 175% - 225% of motor rated-load current or branch-circuit

selection current, whichever is greater




-Design Calculation

Motor Feeder Short Circuit and Ground Fault Protection

(3) For two or more motors, the rating permitted by 4.30.5.2

a) ICB= PEC Section 4.30.2.4)

Exception: Where the feeder overcurrent device provides the overcurrent

protection for a motor control center, the provisions of 4.30.8.3 shall apply.



Example 1

A three-phase, 460 volts, 60Hz, 25-Hp squirrel cage induction motor is to be

connected for full voltage starting. It has a full load current of 31.6 amperes, a

service factor of 1.15 and a Code Letter F in its nameplate. The ambienttemperature of the place of installation is 40C.

Total Load : (See Table 4.30.14.4)

Full load current of 3-phase, 25-Hp motor = 34 Amperes

Note : The full load current value used to determine the ampacity of conductors formotors shall be based on Table 4.30.14.4


-Design Calculation Single Motor



Table 4.30.14.4 Full-Load Current , Three Phase AC Motors


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AC Motors

Namplate Rating


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Branch Circuit Conductors : [See Section 4.30.2.2(a)]

125% of Full Load Current:

34 Amperes x 1.25 = 42 Amperes

Use 3-14mm THW wires

Note : Ampacity of 14mm THW at 40C ambient is 55 Amperes x 0.88 = 45

Amperes. See Table 3.10.1.16 for ampacity correction factors.

Motor and Branch Circuit Overload Protection : [See Section 4.30.3.2(a)]

125% of Rated Full Load Current :

31.6 Amperes x 1.25 = 39 Amperes (Maximum)




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Branch Circuit Short-Circuit and ground Fault Protection:

(See Section 4.30.4.2)

The branch circuit maybe protected by any of the four common types of

protective devices, namely: Nontime delay fuse, Time delay fuse,

Instantaneous trip breaker and Inverse time breaker.

For the protection of a squirrel cage breaker with Code Letter F (See Table4.30.4.2) the protective device can be any of the following:

Nontime Delay (NTD) Fuses: ( See Table 4.30.4.2)


34 Amperes x 3 = 102 Amperes Use 100 Amperes NTD fuse.

Note : If the 100-A fuse is not sufficient for the starting current of the motor, see

Section 4.30.4.2(c)(1) Exception No. 2a.




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Time Delay (TD) Fuses : (See Table 4.30.4.2)


34 Amperes x 1.75 =59 Amperes

Use 60 Amperes TD fuse ( See section 4.30.4.2(c) Exception No. 2b).

Instantaneous Trip (IT) Breaker : (See Table 4.30.4.2)

175% of Full Load Current:34 Amperes x 7 =238 Amperes

Use 250 Amperes IT circuit breaker (See Section 4.30.4.2(c) Exception

No.1)

Inverse Time Delay (ITD) Breaker : (See Table 4.30.4.2)

250% of Full load Current :34 Amperes x 2.5 = 85 Amperes

Use 90 Amperes ITD circuit breaker.


-Design Calculation


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Example 2

Three squirrel cage induction motor: 25-Hp, 20-Hp and 10-Hp, all rated at 460

volts, 3-phase, 60-Hz are to be served by a feeder circuit. Ambient

temperature does not exceed 30C.

Total Load :( see Table 4.30.14.4)

One 3-phase, 460 volts, 25-Hp motor = 34 Amperes



25% of Largest Motor ( see Section 4.30.2.4)

34 Amperes x 0.25) = 8.5 Amperes

Net Computed Current = 83.5 Amperes

Feeder Conductors : ( See Table 3.10.1.16)

Use 3-22mm THW wires.


-Design Calculation Multiple Motors


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Feeder Circuit Protection : [See Section 4.30.4.3(c)]

Largest Protective Device:

The 25-Hp motor being the largest motor, has the largest protective device.

Nontime Delay (NTD) Fuses:

Largest NTD fuse = 100 Amperes

Full load current of 20-Hp motor = 27 Amperes


Total Computed Current = 141 Amperes

Use 150 Amperes NTD fuse.


-Design Calculation Multiple Motors


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Time Delay (TD) Fuses:

Largest TD fuse = 60 Amperes



Total Computed Current = 101 Amperes

Use 150 Amperes TD fuse.

Inverse Time Delay (ITD) Breaker:

Largest NTD fuse = 90 Amperes


Full load current of 10-Hp motor = 14 AmperesTotal Computed Current = 131 Amperes

Use 125 Amperes NTD breaker.Note : Only fuses and/or inverse time circuit breakers are allowed for this type of

installation. [(See Section 4.30.4.3(c)(1)].


-Design Calculation


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-Design Calculation


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-Design Calculation


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References:

1. IEEE Std 241-1990 Recommended Practice for Electric PowerSystem in Commercial Buildings

2. IEEE Std 141-1993 IEEE Recommended Practice for ElectricPower Distribution for Industrial Plants

3. Philippine Electrical Code 2009

4. National Electrical Code 2011

5. Eaton Cuttler-Hammer Catalogue

6. Designing Electrical System, James Stallcup

7. NEC Handbook, McPartland

8. NEC Handbook, W.M Earley

electrical system design_branch ckt

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