# CABLE SIZE

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• #10621

I am working in a Design office as a junior electrical engineer , i would like to know the exact method for cable calculation???

the power that will be supplied by the utility is ac power 220/380 volt

Any help ???

#12466

Cable sizing depends on the regulations for your location and on four factors 1. Length 2. Cross-sectional area 3. Temperature 4. Conductor material.

The cable must satisfy the following conditions: Ib ≤ In ≤ Iz, Iz = In / (Ca x Cg)
Where Ib = design current. In = protective devise rating. Iz = current carrying capacity of cable under specific conditions.
Ca = cable ambient temperature factor. Cg = cable grouping factor.

The volt drop for the run of cable then must be checked = mV x A x m
Where mV = volt drop per meter (for a perticular size of cable). A = Ib. m = length of run (meters).

The max overload must then be checked = I2 ≤ 1.45 (this is an Irish figure) x Iz
Where I2 = the current that will operate a protective device (obtained from the manufacturer)

The cable withstand energy for short-circuits must also be checked = I^2 x t ≤ K^2 x S^2
where I = fault current. t = time for protective device to operate. K = conductor materrial factor.
S = cable cross sectional area.

#12467

haha Ahmed looks like I have a lot to learn as well. Jay can you give me an example of cable calculation other than just the equations. I think this would help more.

Darwin

Cable and Wire Connectors

#12469

Put your email up and I’ll send you on a fully worked out example.

#12471

It will be better to define the Standard that your are using in your country, if you use the ” IEC… “, please find below the method to define the Cable's Section.

1- Calculate the Load's Current that we called ” Ib “.

2- Accordingly to :

– The methode oh installation of cable ” Barried  or  Unbarried ” and the type of installation for each one.

– The number of the circuits laid togother.

– The ambiant temperature.

we will define the ” Correction Factors – k1, k2, … ” from the table of ” IEC 60364-5-52 “

3- We calculate the Total Correction Factor ” k ” by multiplying all correction factors ” k = k1 x k2 x … “

4- we calculate the nominal current for the cable in the working condition ” I’z ” by the formula : I’z = Ib / k “

5- From the ” IEC 60364-5-52 ” tables that mentioned to the Nominal Current of each section in the Standard Conditions ” Iz ” depending on :

– The ” methode oh installation “

– The kind of cable ” Cu  or  Alu    ;   Single Core   or  Multi Cores    ;   PVC  or  XLPE “

we will define the suitable cable's section where ” Iz ≥ I’z “.

5- Then we calculate the Voltage Drop by the Formula : ” Δ V = 1.73 x I ( R . Cosφ  +  X . Sinφ ) “, where : R = ρ . L / S    ;    X = λ . L

6- This value of the Voltage Drop should be smaller or equal than the value defined by ” IEC… ” ( 3% for Lighting   ;   5% for Machines ), if it's not the case, then the Cable's Section should be oversized.

7- In the 1st & 2nd levels of the network where the values of the short circuit currents are important, it will be better to be sure that the cable can withstand the short circuit current during the tripping time of the protection device, where we use the formula : ” I² . t ≤ K² . S² “,

if it's not the case, then the section of cable should be oversized.

#12472

@6, if your going to specify a percentage for volt drop you should let them know that this volt drop is the total from the intake point to the final circuit not just from a sub-board.

#12480

jaymez83

please can you give a solved example .

appreciate ur help

#12481

Spir Georges GHALI

thank you very much for ur help

can you send me any book or post any example that might help ???

#12482
#12486

jaymez83 said:

@6, if your going to specify a percentage for volt drop you should let them know that this volt drop is the total from the intake point to the final circuit not just from a sub-board.

Dear Jaymez83 ;

I think that we do a changing of infortmation at the engineering level, so, that means an engineer should know that the ” total Voltage Drop ” for any point is from the Source to that point.

by the way, I want to drow your attention that when calculating the Voltage Drop it's better that we don't use the formula ” mV x A x m ” for the following reasons :

– The value ” mV ” is for a specific value of Cos φ that is not the case, and also for Sin φ

– I'm not sure if ” mV ” contains ” X . Sin φ ” as for a long cable the value of ” X ” is important.

#12487

MY EMAIL

a_sayos20@hotmail.com

I will send you an exemple.

#12488
#12489

Spir Georges GHALI said:

MY EMAIL

a_sayos20@hotmail.com

I will send you an exemple.

i will be waiting for your email

thank you

#12499

Dear Georges Ghali,

I just wanted to ask if i want to size the main cable of the main distribution board ,should I consider the load current Ib   as the connected load current or the demanded current????

Spir Georges GHALI said:

It will be better to define the Standard that your are using in your country, if you use the ” IEC… “, please find below the method to define the Cable's Section.

1- Calculate the Load's Current that we called ” Ib “.

2- Accordingly to :

– The methode oh installation of cable ” Barried  or  Unbarried ” and the type of installation for each one.

– The number of the circuits laid togother.

– The ambiant temperature.

we will define the ” Correction Factors – k1, k2, … ” from the table of ” IEC 60364-5-52 “

3- We calculate the Total Correction Factor ” k ” by multiplying all correction factors ” k = k1 x k2 x … “

4- we calculate the nominal current for the cable in the working condition ” I’z ” by the formula : I’z = Ib / k “

5- From the ” IEC 60364-5-52 ” tables that mentioned to the Nominal Current of each section in the Standard Conditions ” Iz ” depending on :

– The ” methode oh installation “

– The kind of cable ” Cu  or  Alu    ;   Single Core   or  Multi Cores    ;   PVC  or  XLPE “

we will define the suitable cable's section where ” Iz ≥ I’z “.

5- Then we calculate the Voltage Drop by the Formula : ” Δ V = 1.73 x I ( R . Cosφ  +  X . Sinφ ) “, where : R = ρ . L / S    ;    X = λ . L

6- This value of the Voltage Drop should be smaller or equal than the value defined by ” IEC… ” ( 3% for Lighting   ;   5% for Machines ), if it's not the case, then the Cable's Section should be oversized.

7- In the 1st & 2nd levels of the network where the values of the short circuit currents are important, it will be better to be sure that the cable can withstand the short circuit current during the tripping time of the protection device, where we use the formula : ” I² . t ≤ K² . S² “,

if it's not the case, then the section of cable should be oversized.

I just wanted to ask you, if i want to size a cable for a panel (main cable of the main distribution board)

do i have to consider the current Ib as the currnet for connected load or demanded load ??

Spir Georges GHALI said:

It will be better to define the Standard that your are using in your country, if you use the ” IEC… “, please find below the method to define the Cable's Section.

1- Calculate the Load's Current that we called ” Ib “.

2- Accordingly to :

– The methode oh installation of cable ” Barried  or  Unbarried ” and the type of installation for each one.

– The number of the circuits laid togother.

– The ambiant temperature.

we will define the ” Correction Factors – k1, k2, … ” from the table of ” IEC 60364-5-52 “

3- We calculate the Total Correction Factor ” k ” by multiplying all correction factors ” k = k1 x k2 x … “

4- we calculate the nominal current for the cable in the working condition ” I’z ” by the formula : I’z = Ib / k “

5- From the ” IEC 60364-5-52 ” tables that mentioned to the Nominal Current of each section in the Standard Conditions ” Iz ” depending on :

– The ” methode oh installation “

– The kind of cable ” Cu  or  Alu    ;   Single Core   or  Multi Cores    ;   PVC  or  XLPE “

we will define the suitable cable's section where ” Iz ≥ I’z “.

5- Then we calculate the Voltage Drop by the Formula : ” Δ V = 1.73 x I ( R . Cosφ  +  X . Sinφ ) “, where : R = ρ . L / S    ;    X = λ . L

6- This value of the Voltage Drop should be smaller or equal than the value defined by ” IEC… ” ( 3% for Lighting   ;   5% for Machines ), if it's not the case, then the Cable's Section should be oversized.

7- In the 1st & 2nd levels of the network where the values of the short circuit currents are important, it will be better to be sure that the cable can withstand the short circuit current during the tripping time of the protection device, where we use the formula : ” I² . t ≤ K² . S² “,

if it's not the case, then the section of cable should be oversized.

#12517

Kindly send me a copy of cable size calculation methods.

thanks

mali

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