How to calculate cable size, switchgears and fuses for feeders and equipment?

 

Visit this link, it might help.

http://www.tlc-direct.co.uk/Book/Contents.htm

 

In most places, you start with an electrical engineer or master electrician, who knows how to do this. But, there really isn't any rocket science involved. You simply sum up the expected and anticipated loads and size the wires for the current. In most places, the safety factors, allowable temperature rise, conduit fill and the like are regulated so you do your calculations and apply the regulations. In my part of the world, you need to have permits and inspections and even UL approvals to discourage those who don't know how from doing this. The regulators discourage even those who do know how, and provide employment for electricians with safety as the cover story. There is a push in my home state to require that you be licensed to do anything involving wire.

Regards

cww

 

Whenever amps go running down wires they give off surface heat. The amount of heat given off depends on the wire resistance. Bigger wires have less resistance, and hence give of less heat.

The amount of heat given off leads to a temperature increase, whitch in turn heats the wire insulation. If the insulation gets too hot, it melts, leading to cable failure. (If the installation has been designed properly, this will blow a fuse. If the design was wrong, it can start a fire.) The insulation melt-point depends on the material selcted. This is why different cable types can take different amounts of current for the same conductor size.

Sizing of switchgear is the topic of many textbooks. Some of them are very thick.

Fuses are sized so they let enough amps flow down the circuit to service the load, but not enough amps to overheat the cable insulation... which would cause it to fail as mentioned above.

So. First you need to know how many amps the load needs. Next, make sure the fuse size will allow the load to be met. Next, make sure the cable can carry more amps than the fuse.

But... a little knowledge is a dangerous thing. The above is the basic idea only. Cable selection is a mutli-factor task, always best left to an expert. Remember, fuses always blow at the most inconvenient time, (just after the replacement-fuse store has closed for the weekend) and electric shock can be painful.

 

Anonymous, following is a list of many factors that must be considered:

o Allowable voltage-drop for normal operating load.
o Allowable voltage-drop for future operating load as determined by motor's Service Factor.
o Allowable voltage-drop for starting condition.
o Type of starting, i.e., full-voltage, reduced-voltage, part-winding, delta-wye, VFD, etc.
o Short-circuit withstand capability for an in-close fault (@ motor starter.)
o Ground or earth-fault capability for a far-end fault (@ motor terminal box.) Ground or earth-return impedance must be included.
o Plastic or rubber sheath resistance to chemical attack.
o Metallic screen or shield material withstand capability for excessive heating.
o Derating factor(s) covering installation, i.e., free-air, direct-burial (dry, wet) in conduit (duct bank, steel, plastic.)
o Derating factor(s) covering conductor material (Cu, Al.)
o Derating factor(s) covering construction, round or sector shaped, 3x1/c, 1x3/c, 3-1/2 core, 4-core, overall sheath and/or shield.
o Derating factor(s) covering insulation type and voltage level
o Derating factor(s) covering termination type (bolt, lug, clamp.)
o If direct burial consider subsidence concerns.

If you need additional info, please feel free to contact me.

Regards, Phil Corso, PE ([email protected])

 

Anonymous, I forgot to include a very important factor:

o Allowable voltage-drop for motors to be re-accelerated following a voltage interruption.

Regards, Phil Corso, PE ([email protected])

 

If you are working to NEC regulations I recommend this inexpensive (but good) software package.

Follow link to get download http://www.mc-group.ca/

Good luck.

Dave

 

This is a great site... It's really helpful. Thanks for sharing.

I was also looking for transformer & battery size calculations for a turnkey project. And also what are the paramaters I need to consider for calculations? Appreciate if you could help in these subjects...

Regards,
Ram

 

Hi,
Its very happy to share our technical clarifications and doubts here.

For you clarification of Transfomer Sizing calculation could be followed as
1. Calculate Total Connected Load(KW)(
2. Calc the Total I/P in KW ( Considereing 85% efficiency)
3. Calc Total I/P in KVA (Considering 0.92 Power Factor)
4. Required KVA by mutiplying the Total I/P in KVA by 1.2 times.

Calculation Example
1. Say Total Connected load in KW=1100
2. Therefore Total I/P in kw = 1100/0.86 = 1279
3. Total I/P in KVA = 1390
4. Total KVA required = 1390*1.2=1668

Therefore by considering the future connection you can select 2000 KVA transformer for the above said example.

With Regards
Shahul KM
Elec & Instrumentation

 

the calculation explained above is right basically

but actually transformer sizing also depends upon which type of load is connected
if the connected load is motor so its starting current is too high and trasformer has to withstand this starting load

so also consider the 5 to 6 times of full load current of largest connected motor
in our company we are doing this type of calculation.........
if anyone need so i can explain in detail next time
and one more thing i m just GET here, i may be wrong ok? i am giving my id


Regards,
Abhishek
[email protected]

 

If you want to work to IEC standards you can use the following free cable sizing tool:

http://www.myelectrical.com/pages/cables/cableSizing.aspx

Steven

 

Dear Abhishek
You are right about the Tranformer sizing considerations. I feel you are much more experience as compared to me , I have just started my job in this field. Kindly can anyone send me some sample Cable and Trasformer sizing calculations. I'll appreciate that so much.

Thanks and Best Regards,
Sai

Email: [email protected]

 

How to calculate cable size:

cable size[in mm2]=(18*meters*amp)/v*1000
Meter = total length of the cable (both positive and negative)
Amp = total current flow through the cable
v = acceptable voltage drop in volt

 

hey pal...who ever u are...dat site was immense help...thnx...

 

check this link
http://www.link512.com/2008/11/calculation-of-cable-size.html

 

www.elek.com.au/cablecalc.htm

 

Dear sir,

im having small clarification in your transformer sizing calculation.
that is why you put 85% efficiency with input kw?
why you used 0.92 power factor?

Regards,
K.Dinesh Raj.
Electrical designer.

 

actually when we give some load to electrical equipment it takes some energy as loss that means the electrical parts like coil and core takes some energy some of energy wasted in these parts so if we give some input the output will be low when compare to input. commonly the losses will be in these ranges so that approx we take 85% in total load in calculation. efficiency of 85%. output/input is called efficiency.
and 0.92% is the power factor between voltage and current. it varies depends upon the load.commonly we have to maintain this power factor. if u have any doubt on this just mail my id.

vijayakanth.boopathi [at] gmail.com

 

> cable size[in mm2]=(18*meters*amp)/v*1000 <

What does 18 stand for?

 

you need to follow whatever code book is being used in your area. in Canada we have a Canadian code, amended province by province. there are many changes made in the provincial books, and every rule must be followed. if not, problems can arise. for instance some types of wire are not suitable for underground use, or to be pulled into conduit or to be run free air or not. wire with certain insulation needs to be pulled into machines only, not into conduit (tew f/i cannot be pulled into conduit.)

if all of the rules are not followed you may need to pull out the wire and run it again.

what country/province/state are you in?

rick