Steps of Calculating Cable and Fuse Size for Motor Circuit

Calculating Cable and Fuse size for Motor Circuit

Steps of Calculating Cable and Fuse Size for Motor Circuit- The following factors should be kept in mind while making a selection of accurate sizes of cables as well as fuses for 20 horsepower DC and single-phase AC motors, whereas 50 horsepower three-phase motors.

(1). A cable size less than 7/ .029” should not be used for motor circuits (i.e., 7/.029” is that minimum cable size, which should be used for motor circuits)

(2). If the distance between a distribution board and motor is greater than 100 feet (33 meters), necessary calculations should be carried out in order to ascertain voltage drops.

(3). The rating of sub-circuit fuses for a particular motor circuit depends on the factor whether the fuses are of re-wirable type, or HRC type, and what type of starter has been used along with the motor. Thus, in order to determine an accurate fuse, corresponding table 7.1, must compulsorily be kept in view.

(4). The sub-circuit fuse rating should not be twice as high above the rating of a circuit cable, or inversely the minimum cable rating, must be equal to half of the fuse rating.

(5). To learn the cable size of any motor, temperature, cable’s insulation, and moto’s starting current must also be kept in mind. 

Steps for Calculating Cables and Fuse Sizes for Motors

The arrangement of mathematical procedure to determine a circuit’s size and rating of sub-circuit fuses, is as below;

(1). First of all, determine (through a motor’s nameplate, table, or calculation) the motor’s full load current keeping in view the type of motor, type of a starter to start a motor, and type of a fuse.

(2). In order to determine the current rating of a sub-circuit, multiply motor’s full load current with the given factor (which depends on the type of fuse and the type of starter being used) in table 7.1.

(3). In order to find out cable’s minimum current rating, halve the value of this received fuse current rating (i.e., divide the fuse current rating by 2). For example, if the value of fuse current rating is 30 amperes, the rating of cable current will be 15 amperes (i.e., 3- / 2 = 15 amperes). Then according to the cable’s current rating tables, and keeping in view temperature and batching etc., such a size of cable should be selected, which has a current rating slightly higher than the minimum known rating of the cable.

(5). If the length of a cable is above 100 ft. (33 meters), then in such a condition, then voltage drops occurring within a cable must also be determined through calculation. If value of the determined voltage drop is higher than the permissible value (i.e., 2.5 %), then size of the cable is increased until voltage drops occurring on motor terminals, drops down below 2.5 % extreme value.

Table 7.1 Motor Circuit Fuse Rating 

Table 7.2 Sizes of Available Circuit Breakers

Table 7.3 Sizes of H.R.C Fuses (Amperes)

Example1: Calculating Cable and Fuse size for 1 H.P single-phase Motor

1 H.P single-phase split phase induction motor of continuous rating; Full load current stated on nameplate = 5.8 amp; Declared supply voltage = 240 volts; Direct-on starting used; Average temperature in installation = 95°F; Supply to motor to be by two single core cables run in conduit from multi-way distribution fuse board; Approximate distance between distribution fuse board and motor = 40 ft.; Sub-circuit protection to be by rewireable fuses as H.R.C fuses not available.

Find; Size of cable for motor and rating of sub-circuit fuse.

Solution;

Full load current of motor = 5.8 amp

Declared supply voltage = 240 V

Method of starting = Direct on

Average ambient temperature = 95°F or 35°C

Length of cable = 40 ft

Size of cable for motor =?

Rating of sub-circuit fuse =?

Now motor is single-phase, and direct-on starting is used. According to table 7.1, the rewireable fuse rating is 4 times the full load current of the motor.

Rating of rewireable fuse = Circuit fuse rating x F.L current of motor = 4 x 5.8 = 23. 2 A

From table sizes of fuse elements (Given in data tables) nearest fuse rating above this is 24 amp (22 S.W.G)

Minimum current rating of cable = 24 / 2 12 amp.

At Ambient Temperature 30°C

Now look up table 12 under column 3 & 4, selected the size of single – phase core cable.

Nearest current rating of cable = 13 amp.

Cable size = 3 / .029”

At Ambient Temperature 35°C

The same cable will be derated by multiplying the approximate rating factor which is 0.97 at 35° (for coarse excess protection)

So, rated current of this cable = Current rating at 30°C x Rating factor = 13 x 0.97 = 12.6 A

Since, load current is less than this rated current, so 3/ .029” cable is suitable. However, we know that no size smaller than 7/.029” may be used for motor circuits. So, 7/.029” cable is selected for use.

Answer; Two single core PVC cable = 7 / .029”

Rating of sub–circuit rewireable fuse = 24 amp.

Example2: Calculating Cable and Fuse size for 125 H.P three-phase Motor

125 H.P three–phase split phase induction motor of continuous rating; Full load current state on nameplate= 18 amp; Declared supply voltage = 440 volts; Star delta starting used; Average summer temperature in installation = 95°F (35°C); Supply to motor to be by three single PVC cables run in conduit from multi–way distribution fuse board; Approximate length between distribution fuse board and motor = 300 ft.; Motor sub–circuit to have coarse excess current protection by H.R.C fuses.

Find the size of the cable for motor and the rating of the fuse.

Solution;

Full load current of motor = 18 amp

Declared supply voltage = 440 V

Method of starting = Start Delta

Ambient temp. = 95°F (35°C)

Length of three single–core cable run in conduit = 300 ft.

Size of cable for motor =?

Rating of fuses =?

Motor sub–circuit protection = Coarse excess current by HRC fuses

From the Table 7.1

Rating of HRC fuse = circuit fuse rating x F.L current of motor = 2 x 18 = 36 A

The nearest HRC fuse rating = 40 A (see table for size of HRC fuse)

Minimum current rating of cable = 40 / 2 = 20 A

At Ambient Temperature 30°C

According to Table 12 under column 5 & 6

Nearest current rating of cable = 23 Amp.

Cable size = 7 / .037” (0.007inch²)

At Ambient Temperature 35°C

The same cable will be derated by multiplying the approximate rating factor which is 0.97 at 35°C (for coarse excess protection)

Now rated current of this cable = Current rating at 30°C x Rating factor

= 23 x .97= 22.31 V

Now load current is less than this rated current, so 7 /.036inch cable is suitable. Voltage drops calculation will have to be made because length between DFB and motor is over 100 ft. i.e., 300 ft.

Maximum permissible voltage drop = 2.5 / 100 x 440 = 11 V

For 22.31 A voltage drop per 100 ft. = 5.3 V

For 18 A voltage drop per 300 ft. = 18 / 22.31 x 300 / 100 x 5.3 = 12.83 V

This voltage drop in the 7/.036 cable is greater than maximum permissible voltage drop, so the calculation must be repeated for the next large size of the cable which is 7/.044”.

Then next large size of cable = 7/.044”

Current rating of this cable = 28 A

For 28 A voltage drop per 100 ft. = 4.1 V

For 18 A voltage drop per 300 ft. = 18 / 28 x 300 / 100 x 4.1 = 7.9 V

This now within the acceptable limit

Answer; Three single core 7/.044” cables 660 / 1100 V grade

Rating of sub–circuit fuses should be 40 amps. 

Example3: Calculating cable and circuit breaker size for Motor

A 3–phase, 50 A load is to be supplied through a distance of 60 m by a four–core, PVC insulated, sheathed cable stretched by a stay wire between pole and building. Ambient temperature is being 20°C and protection being close excess current. Which size of cable and circuit breaker would you suggest if recommended voltage drop is 9 volt?

Solution;

Load current (I) = 50 A

Length of cable (L) = 60 m = 60 x 3.28 = 196 .8 ft

Ambient temp. = 20°C

Sze of cable =?

Size of circuit breaker =?

Recommended voltage drops= 9 A

Four–core PVC insulated sheathed cable stretched by a stray wire between pole and building and protection is close excess current.

At Ambient Temp. 30°C

According to Pakistan Cables and table 12 under column 9 & 10

The nearest current rating = 60 amp.

Size of PVC cable single core = 7 /.064” (0.0225 inch²)

At Ambient Temperature 20°C

Ambient temperature of 20°Cis not given in the table, we will take 25°C. the same cable will be uprated by multiplying the appropriate rating factor which is 1.06 (as protection is close excess current protection)

So, current rating of this cable – 60 x 1.06 = 63.6 A

For 63. For 63.6 A, voltage drop of 7 / .064” PVC cable for 100 ft. = 4.3 V (in case of coarse excess protection)

(Conversion in close excess protection) = 4.3 x 1.33 = 5. 719 V

For 50 A voltage drop of 7 / .064” cable for 196. 8 ft. 

= Actual load current / Rated current of cable x length of circuit run / 100 x Voltage drop / 100

= 50 / 63.6 x 196.8 / 100 x 5.719 = 8.848 V

This voltage drop is within recommended limit

Thus, the suitable PVC cable 7 / .064 “single core is selected.

Size of circuit breaker = 1.2 x load current = 1.2 x 50 = 60 Amp.

If circuit breaker of 60 Amp. is not available, then we will use the circuit breaker of 63 A.  

Answer;

Suitable size of cable = PVC 7 / .064” single core

Size of circuit breaker = 60 A Fuji Company

Size of circuit breaker = 63 A siemens Company

Factor for conversion close excess current protection = 1.33

Example4: Calculating Cable and circuit breaker size for Motor

35 H.P phase slip ring induction motor of continuous rating; Declared supply voltage = 440 volts; Rotor resistance starting to be used; Average temperature in installation = 40°C. Supply to motor by three core unarmored aluminum conductor cables run in a duct in the ground from a combination switch fuse board; Approximate length between switch fuse board and motor is 20 ft; Motor sub-circuit to have close excess current protection provided by a circuit breaker.

Find; Size of the cable to motor and rating of circuit breaker.

Solution;

Full load current = 44 amp.

Declared supply voltage = 440 volt

Average summer temperature = 40°C

Length of PVC cable run in duct= 20 ft.

Motor is 3 – Phase slip – ring type & rotor resistance starting to be used. Motor sub-circuit is provided close-excess current protection by a circuit breaker.

Size of cable to motor =?

Rating of circuit breaker =?

Rating of circuit breaker = 1.2 x full load current

= 1.22 x 44 = 53 Amp.

The nearest standard rating of a circuit breaker to this is 60 A. We know that the minimum rating of a three-core cable should be 1.3 times the full load current of a motor, when circuit breakers are used for protection. 

So, rating of three-core PVC cable = 1.3 x 44 = 58 Amp.

As three-core PVC cable insulated aluminum conductor cable is to be run in a duct, we should refer to table 19, column 5 & 6.

At Ambient Temp. 30°C

Nearest cable current rating = 75 Amp.

Cable size = 0.06in² (19.064″)

At Ambient Temp. 40°C

The same cable will be derated by multiplying the appropriate rating factor which is 0.87 (for close circuit protection)

So, rating of this cable = current at 30°C x rating factor

= 75 x 0.87 = 65.25 Amp.

Since load current is less than this rated current, so 19 / .064″ cable is suitable.

Since the length of the circuit is less than 100 ft. (i.e., 20 ft. only), there is no need for volt drop calculation.

Answer;

(1). 0.06 sq inch (19.064′) three core PVC cable   voltage grade 660 / 1100 V

(2).

Rating of circuit breaker = 60 A (Fuji company)

Example5: Calculating Cable and Fuse size for Motor Circuit

A 10 H.P three-phase squirrel cage induction motor of continuous rating using star-delta starting is connected through 400 V supply for three single core PVC cables run in conduit 250 feet away from multi-way distribution fuse board. Its full load current is 19 A, Average summer temperature is installation is 95°F (35°C), Motor sub-circuit to have coarse excess current protection by HRC fuses. Calculate;

(i). Size of the cable for the motor (ii). Rating of the fuses.

Solution;

Motor load = 10 H.P = 10 x 746 = 7460 W

Supply voltage = 400 V (3 – phase)

Length of cable = 250 ft

Motor full load current = 19 A

Temp. factor for 35°C = 0.97

At Ambient Temp. 30°C

According to Pakistan Cables & Table 12 under column 5 & 6,

The nearest current rating = 23 Amp.

Size of PVC cable single core = 7 / .036”

Voltage drops for 100 ft. run = 5.3 V

At Ambient Temp. 35°C

Current rating of cable for 35°C = 0.97 x 23 = 22.31 A

This cable is suitable because its full load current rating is less than its current rating for 35°C.

Allowable voltage drops = 2.5 / 100 x 400 = 10 V

Actual voltage drops for 250 ft. = 19 / 23 x 250 / 100 x 5.3 = 10.94 V

This cable is not suitable because its voltage drop is more than the allowable voltage drop. Now we select the next larger size of cable, which is 7 / 0044 inch, having current rating of 28 A and voltage drop for 100 ft. run is 4.1 V. Now calculate actual voltage drop of this cable for 250 ft. again.

Actual voltage drops for 250 ft. = 19 / 28 x 250 / 100 x 4.1 = 6.95 V

This drop is less than the allowable voltage drops, so the 7 / .044” cable is suitable.

(ii). We know that full load current of the motor = 19 A

As shown from the table 7.1, the approximate HRC fuse rating is 2 times the full load current of the motor. Thus, the rating of the fuse = 2 x 19 = 38 A. So, the rating of fuses should be at least 40 A because it is the nearest HRC fuse rating.

Example6: Calculating Cable and Fuse size for Motor Circuit

Make a sketch for installation of a motor fed from D.B, keeping in view the sequence of supply control, what will be the size of the cable & circuit breaker if the motor is installed 60 yards away from D.F.B it takes 15 A at 400 V, cable run is in open air (unenclosed) and ambient temperature is 30°C.

Solution;

Length of cable = 60 yards = 60 x 3 = 180 meters

Load current = 15 A

Supply voltage = 400 V

Ambient temp. = 30°C

Size of cable in open air =?

Size of circuit breaker =?

According to Pakistan Cables & Tables 12 under column 9 and 10,

The nearest cable current rating = 18 A

Size of cable = 3 / 0.036” (0.003 sq. in)

We know that 7 / .029” is smallest cable size that should be used for motor circuits

Current rating of 7 / .029” cable = 23 A

This current rating is greater than load current, so size of PVC cable 7 / .029” is suitable, if voltage drop in cable is ignored. It should not be done because the distance between DFB & motor is greater than 100 ft. (i.e., 180 ft.)

Maximum permissible voltage drop = 2.5 % of supply voltage

= 2.5 / 100 x 400 = 10 V

For 23 A voltage drop per 100 ft. = 8.4 Volt

For 15 A voltage drop per 180 ft. = 15 / 23 x 180 / 100 x 8.4 = 9.86 Volt

This voltage drop is within acceptable limits.

The suitable PVC cable size = 7 / .029” (0.0045 sq. in)

Single-core voltage grade = 660 / 1100 V

Size of circuit breaker = 1.2 x load current = 1.2 x 15 = 18 A

Circuit breaker of such rating is not available, so the nearest rating of circuit breaker is 20 A (Fuji Co.)

Example7: Calculating Cable and Fuse size for 10 lathe machines

A workshop is equipped with 10 lathe machines. All machines work eight hours per day continuously. Find out the size of the main cable for D.B. to switch gear, the size of circuit breaker & and rewireable fuse, if 4 single core cables run in conduit to a distance of 50 yards. Ambient temp. is 25°C. Motor nameplate data is;

V = 380 V, I = 2.83 A, N = 1500 r.p.m., connection = Y (star)

Solution;

Length of cable = 50 yards = 50 x 3 = 150 ft.

Ambient temp. = 25°C

Supply voltage (V) = 380 V

Load current of a motor (I) = 2.83 A

Load current of 10 motors = 2.83 x 10 = 28.3 A

Size of 4 single core cable run in conduit =?

Size of current breaker & rewireable fuse =?

At Ambient Temp. 30°C

According to Pakistan Cables & Tables 12, under column 5 & 6,

The nearest cable current = 28 A (As temp. is 25°C)

Size of PVC cable = 7 / .044” (0.01 sq. in)

At Ambient Temp. 25°C

The same cable will be uprated by multiplying the appropriate rating factor which is 1.02 (for coarse excess protection)

So current rating of this cable = 28 x 1.02 = 28.56 A

Since this rated current is greater than the load current, so 7 / .044” PVC cable is suitable if voltage drop is ignored. It should not be done because the distance between DFB & load is greater than 100 ft.

Maximum permissible voltage drop = 2.5 % of supply voltage

= 2.5 / 100 x 300 = 9.5 V

For 28.5 amp., voltage drop per 100 ft. = 4.1 volt (according to table)

For 28.3 amp., voltage drop per 150 ft. = 28.3 / 28 .56 x 150 /100 x 4.1 = 6.09 volt

This voltage drop is within the acceptable limit

The suitable PVC cable size = 7 / .044” (0.01 sq. in)

Size of circuit breaker = 1.2 x load current

= 1.2 x 28.3 = 33.96 A

Circuit breaker of such rating is not available, so the nearest rating of circuit breaker is 40 amp.

Size of Rewireable Fuse

Refer table 7.1 “Motor circuit fuse rating”

Suppose Y/  starter is used as motor is Y connected)

Size of rewireable fuse = Sub – circuit fuse rating x full load current

= 2.5 x 28. 3 = 70 .75 amp.

From sizes of fuse elements table (given in data tables)

Nearest size of fuse wire = 78 amp. (15 S.W.G)

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