Do We Need to Consider Short Circuit Calculation in Lighting Wire?

Yep. Here's one. Follow British Standard 7671 to the letter.

For the most part following the recommend standards used by various government agencies around the world for wire sizing, will be your best bet. As suggest by others here. If you begin to look at the true question here and begin to calculated the wire sizes as you suggest. We would never get to the end of what is correct based on your assumption's. Wire sizes would go out to infinity trying to catch up to the size needed to protect itself form its self.

Requirement for short circuit calculations is there but you have to realize that formula for calculation of earth conductor size does not implies on power circuit cable sizing.

Codes require that for power circuit you have to consider short circuit at the load end (device terminals) thus the impedance of the entire circuit loop (wiring) is essentially taken into account for assessment of current in the event of short circuit.

If you are capable to do the calculations yourself, you will realize that even 1.5 mm sq. is correct use in light circuits due to high impedance which limits the current flow.

....subject to an appropriately-selected circuit protection device being installed upstream of it!

Installations designed by engineers will include SC calculations for sizing earth wire and CB. But in small premises like a home or a shop wired by traditional wiremen,no calculations are done. He follows standards/practices learnt from his boss who also would have been another wireman. When a testing and certifying engineer visits the premises the wireman may be in trouble if engineer checks thoroughly. It also depends on which part of the globe the premises is. In UK it is mandatory for the utility to give the fault level to the owner of the premises but in some undeveloped third world the utility won't give it. The fault level depends on whether the premises is located near the generators or transformer or far away from it. In north america they may have pole mounted single phase transformers(even 3 in canada) in each pole but in british practice in many countries a transformer will feed many homes, in some areas more than 100.

18 milliseconds for an MCCB to trip seems high when one is talking about short-circuit. MCCBs are mostly current-limiting, and will clear the fault in a much shorter time.

_{t =.018sec for MCCB as per IEEE 242}

i wonder how a standard can specify the clearing time of any device.....must study more...thanks for the input.

current limiting MCCB is usually used in high short circuit level, but in my example I only have 9kA short circuit in lighting panel.

Current limiting MCCB is I think 2.5 times the cost of ordinary MCCB.

Actually the formula I have used here is not only for earthing. This is for power cables I have just simplify the long formula as per 70 degree Celsius, PVC insulation.

Yes! there is an impedance to reduce the fault but what if the fault contribution is very high?

In one of our project (University Building) we have 1.6MVA,13.8kV-0.38/0.22kV, %Z=6% transformer. This is connected to MDB and they are located on ground floor, While the lighting Panel located on the first floor is connected to MDB.

If we know short circuit hand calculation we can tell the the fault on lighting panel is extremely high as per my project example. I will not specifically calculate and show here, but my point is that we can not just assume that the short circuit level in lighting circuit is always small. It will depend to the electrical sys. that's why we have power sys. study.

Now back to my original query, assuming I have existing electrical system that will give a 9kA fault on lighting panel. Why most engineers do not consider or even check the short circuit thermal capability of lighting wires? Is there any code will tell us that short circuit consideration for lighting wires can be neglected?

Thanks to those who replied,

The high price of current-limiting MCCBs is a commercial thing, not really related to the actual cost. Be that as it may, you will have to pay that if you want one, so not an easy choice. i agree with you.

Why not use a MCB ? They are generally of at least 10kA capacity. Very quick in operating, as low as 2 milliseconds...

Please read more here

With such low times, the let-through energy is so low that even 1.5 mm² wire should be ok....

For lighting circuits 1 or 1.5 mm^2 wires also are used depending on the load connected.

Lighting circuits/sub DBs could be protected by fuse or mcb as well.

#10: good engineers use lighting transformer. Yes, absolutely!

An absolutely guesstimated 9kA short current produces sufficient magnetic forces to rip even big cables from their terminals. I saw it in a breaker panel for welders.

On the other hand, a dedicated transformer inherently limits max. current in the circuit.

Fusing/CB on a lighting circuit should be about 8 Amps vs 15 Amps on a power circuit.

This is designed to take into account the smaller wire size.

Jonald,

You are right. We generally check for fault levels for all Dbs falling within a certain distance from the transformer. If the fault current is 9KA then the incoming cable for the DB has to be minimum 10 Sq.mm. copper. If the fault current at the end of an outgoing circuit is more than capacity of 2.5 Sq.mm wire then the following options are available( Generally a few meters of circuit run will drop the fault current to within the capacity of 2.5sq.mm wire):

1) Go for a current limiting breaker in the incomer to the DB.

2) Provide an HRC fuse in the incoming to the DB. It will perform the same function of current limiting.

3) Increase the conductor size suitably.

You may also follow advice given by Sridhar by going for high performance MCBs.

Mr. Learner42,

If we only place current limiting breaker on the incomer then the tripping time of incomer will be faster than the branch circuit of lighting wire if branch circuit is not current limiting. Out of coordination.

Thanks.

Jonald,

If you want to go for proper coordination then you have to go for fast acting MCB on the outgoing circuit or back it with an HRC fuse.

Please take a look at this thread : http://cr4.globalspec.com/thread/71318

There are some good posts on Cascading (called 'Series rating' in USA) which uses current-limiting breakers (or fuses) as the incomer and normal CBs as outgoing....

Thanks Learner42..

In my conclusion with this discussion.

We should always design our electrical system so that it will reduce the fault and eventually can use of 1.5mm2 lighting wire, like providing dedicated lighting transformer.

If high fault is inevitable then we can use current limiting device on every branch circuit.

Therefore it is a must to check the short circuit capability even it is just a lighting wire, and provide a solution if necessary.

Thanks to all,

Before you log off this thread, please learn about that version of Co-ordination called Cascading or Series Rating. Americans, who are (at least used to be ) very rich, use this system often. It is the relatively poorer countries (especially India) who are aghast at this, and insist on Total Coordination using current-limiting MCCBs. Substantially more expensive than Cascading. A point to ponder.

Sridhar,

Total coordination need not be more expensive if intelligently and correctly applied. Firstly, the situations where current limiting will be needed on final lighting DBs will be very few and these too can be avoided by bringing the supply from subDbs rather than mounting a lighting DB on the main switchboard. Secondly HRC fuses can be used for current limiting in those rare cases where it is necessary.

Thanks for the link discussion on Cascading. Actually discrimination using energy base is very complex as what shown on ECT 167 I thingk it would take a lot of time and additional references for me to understand this publication.

i know what you mean it can be formidable....

Since you are on building lighting, shouldn't be looking at AFCI ? Here is one of a large number of videos which bring home the aspect of electrical fires.... http://www.youtube.com/watch?v=jdoPmVO3MsM

Most sparks over here in the mother country use 1.5mm² for house wiring.

According to BS7671/IEC/NEC what should be the minimum size of cpc (groundwire) for each of the lights,fans and socket(receptacle)circuits?. Some(C.Eng) say according to BS it should be at least 2.5, while many so-called consultants(C.Eng) say it should be the same as phase/neutral wire(1,1,5,2.5 etc)

As per BS practice the size of earth wire(E) is related to the phase/neutral wire(S) as under: If S<16,E=S,If 16<S<35,E=16 and if S>35,S=S/2 but NEC says E is related to the OC setting of main breaker.

Consider instead, the actual cable.

What happens to the cable when the SC current flows. If left flowing long enough the insulation would melt - then the conductors would burn out. Then you would have a fire.

But in cases where the current is flowing for a few milliseconds probably nothing happens except a temporary rise in acceptable temperature which would not do any harm - even to 1.5mm2 cables.

The practical problem is not one of short circuit in fault conditions, but one of inadvertently overloading a small lighting cable with larger bulbs - but not enough to trip out.

Then the gradual heat builds up to harmful levels over time - the insulation softens and melts (into adjacent cables ?) - and you get a short circuit.

Which is where we came in!

Sir,

Try to graph 1.5mm2 cable and 9kA fault in a loglog paper with breaker tripping time of 0.018sec. If the fault hits the thermal curve of your cable before breaker trips then this is not coordinated, cable will be burned. That's why they are suggesting to use a current limiting device to coordinate the protection of cables.

Aside from loglog paper check again the formula I've shown. Thanks.

<...they...>

Who?

Are "they" standing up in front of "the beak" when it all goes wrong?

Wire size should be calculated according to total load + whatever your local code dictates.

Short circuit calculations dictates Fuse size and + (or minus) whatever your local code dictates.

From your posting I assume that you are talking strictly house lighting.

Wangito

Bear in mind that there is a physical limitation to the size of wire that wiring devices will accept. Many will not accept anything larger than the 2.5mm2 wire.

Although the size of phase/neutral as well as cpc or ground wire for final circuits varies from country to country the size should be sufficient to carry the load current and the PD should trip before damage occurs by overload or short circuit or earth fault. Also I believe that the wire or conductor should consist of more than a single strand as a single strand can break while handling carelessly.

Hi Jonald

The way I see this problem and that you are in the Philippines depends on a few factors that have to be taken into consideration.

1. To calculate the perspective short circuit current from the transformer that is suppling your property.

2. Find out what sort of protection supplies your property from the supply. Current Ratting and Perspective Short Circuit Current Ratting.

3. The main supply size (from the network to you property) and the length of the length of this main.

If your property is just a domestic supply you will probably find that the perspective short circuit current probably has a maximum value of 6kA and hopefully this is protected by the main supply fuse supplied by the power authority. Just to be on the safe side you could replace your main switch with a circuit breaker rated at 10kA this will protect the main supply. Next you have to look at the sub circuits, discrimination is the answer, you can now de-rate your circuits to 6kA as the main circuit breaker will look after any problems up to 10kA.

As long as the sub circuits have 6kA MC B's it doesn't matter if the fault current exceeds this as the main MCB will handle the fault and protect the wiring.

The other thing that should be taken into consideration is the current supply required to the sub circuit, if you only need 6 amps, then size your circuit breaker accordingly.

This way if you do have a major short, as the current rises it would trip the sub circuit first and if this did fail it would trip your main mcb or the main supply fuse.

I have seen many houses in the Philippines that do not have 2.5mm wiring usually they have 1.0mm single strand cable and very poor wiring within the whole household.

But I hope this is of some help

Best Regards

Joe

Recently ABB has started manufacturing 50kA high performance MCB up tp 125A suitable as main breaker in load centre/consumer unit in houses and shops where SC level is more than 10kA.

instead of increasing the MCCB or MCB fault clearing capacity it is better design strategy to reduce the system prospective fault level . . .

that way the installation costs are kept under control thereby reducing chances / need to cut corners whilst designing or installing . . .

safety aspects of building systems with lower fault levels are also worthy of consideration . . .

Reducing system fault level is not in the hands of the consumer or consultant. What he can do is to use suitably rated main CB.

like i said before . . . a good designer (consultant) can consider use of lighting transformers . . .

If an EF occurs at the primary of the lighting transformer, still you need a CB with suitable(high) rating to feed the primary.

of course you mean the primary side wiring for the lighting transformer? the side which will definitely not be part of the scope of any discussion on the size of wire feeding the lamp circuit my dear pnaban . . . . . . . .

Size of wire should withstand fault current and MCB should disconnect it before wire gets overheated or burnt. Whether to use a lighting transformer with highly rated MCB in primary or use a highly rated MCB directly in load centre could be determined by economical consideration.

we decide based on priorities broadly listed as:

1. SAFETY

2. ENERGY SAVINGS

3. ENVIRONMENTAL CONCERNS

4. LIFE CYCLE COSTS

however i will limit my posts to the subject of how to limit priorities to selection of wiring connected to the lamp . . . i have little else to prove having already stated the base fact that limiting fault level is a high safety priority for any designer . . . .

i promise to not flood mailboxes with notifications on unnecessary posts on this thread henceforth.

Since you are working in Saudi Arabia, you must have read at least once the complete NEC-70, and then you need not have to ask this question because of the following:

As per NEC 210-23(a); 15 & 20Amps Branch Circuits; A 15 or 20Amps branch circuit shall be permitted to supply lighting units.

Again NEC Table 210-24(Summary of Branch-Circuit Requirement) says that circuit wire size 14AWG (2.5mm²) is recommended the Overcurrent protection of 15Amps CB. Maximum load on this circuit shall not be more than 15Amps.

When a code is prepared that time all possibilities things must have been considered well.

How do you relate the 9kA to 2.5mm2 branch circuits?

Please take a small example of lighting circuit having connected with many fixtures in parallel on 127V (Phase to neutral) circuit.

Let us assume the distance from first Lighting fixture on the circuit to CB of the Lighting Panel board be 5meters.

Now assume the short circuit takes place at fist fixture itself.

Short Circuit current will be 127/ (2*5*9.5/1000) = 1.336KAmps after neglecting the Source impedance. The Resistance of the 2.5mm2 cable per km is 9.5Ohm/km.

CB will clear this fault 1.336KA very comfortably without damaging the conductor insulation.

That is why no short circuit calculation is required on 14AWG (2.4mm2) or 12AWG (4mm2) branch circuits. Only you have to see the branch circuit current shall not exceed provided it percentage voltage drop shall meet as per NEC 215-2(b)-code otherwise you have to reduce branch circuit current less than 15amps(by connecting less number of Lighting fixtures) to meet both branch maximum allowable current as well as allowable percentage voltage drop.

In fact, single pole CB are rated 10KA AIC & 22KA AIC available for 15Amps CB as per NEC. This rating only for the direct fault on the CB terminal load side at that time the short circuit current depend on the upstream loop impedance. Upstream short circuit impedance will be much & much less than above example short circuit impedance (2*5*9.5/1000 = 0.05Ohm). Short circuit current may go from 10KA to 22KA.

Mr Mushtaq,

Thanks for your comment. Yes NEC 210-23(a) and table 210-23(a) is correct but you need to be careful as there are to many exception in this code, you may also consider NFPA 70 (NEC 2008) Table240.92(B). In addition you may study IEEE 141 & 242 or IEC 60909-1 to 4 so you may have additional info in short circuit hand calculation. You may also study cascading with enhance discrimination in LV breaker this will help a lot.

All code you have indicated are for sub-feeder, feeder cable and main feeder cables where short circuit current will be very high.

Branch short circuit current is small( as I calculated for you above) and upstream CBs feel as if it were a over load current on them. So NEC is perfect.

Yes NEC 210-23(a) and table 210-23(a) is correct but you need to be careful as there are to many exception in this code, you may also consider NFPA 70 (NEC 2008) Table240.92(B).

For hand calculation of short circuit, study thoroughly the code especially IEEE 141. It shows a calculation even for single phase short circuit. Much better if you can consult somebody there so that he may explain this to you very well. We have different types of calculation and it would take time to explain this here in forum.

thank you and good luck,

when so much calculation is done the consultant's fees go through the roof and the project never gets completed . . . therefore some "ready reckoners" and "tabulated charts" are created by reputed trade and industry associations . . . to be used by people with sufficient background knowledge and applying factors of safety . . .

it is not always possible or expedient to calculate everything from first principals and we rely on experience and knowledge shared by experts in such tabulations . . .

validate the same on a sample basis as you go along . . .

Eng. Jonald & Eng.Soebfatehi

Dear Eng. Jonald

You raised the question whether to calculate short circuit calculation for lighting circuit. I gave answer with NEC code and an example. I have been working as an Electrical consultant for long time and designed more than 50 different projects.

Now you want to accept NEC and you want to linger with exception in this code. Please tell me, is there any exception in the code I gave to you? If not then why you prolong the discussion by bringing IEEE. You know that NEC is for buildings code whereas IEEE is power Systems code.

Eng. Soebfatehi

Please do not blame the consultants. There are one of the pillars in the project in electrical system design section. No consultant uses Stone Age tabular charts which will give general case answer but not particular case when computers are at their disposal. If engineer cannot run program for design of % Voltage drop and Short circuit calculation then he is considered not to be called as a consultant.

I advise you please collect some programs in short circuit and run my example. You will feel happy and excellent

I feel to advise you as followings:

It is not enough to justify you own comments but try to understand when somebody gives answer with international standards. If you do not want to agree, run one of the below mention programs and you will get the same answer for my example. I have been working as consultant; I shall be able to design, voltage drop, short circuit, illumination communication system calculation for the project in Saudi Arabia. As a consultant, I must be thorough in NEC code.

We have been working in this field of power system analysis design such as Load flow, Short circuit analysis & Transient stability since 1982.

I have many suppliers programs with me and installed on computer. For Example:

1. Ecodial Program(Schneider France)

2. LS-LG program( Korea)

3. ABB program

4. MELSHORT2-Mitsubishi Electrical program(Japan)

5. GE Program(USA)

6. Elite Short Circuit Program(USA)

7. My Own program written in MS VB6

8. Electrical load flow program in MS VB6 and tested IEEE 30 Bus System

I am also a programmer since 1975 when input to computer was done by card ( Only two computers were available in my country that time) not by personal computer's Monitor and Key board.

I did computer Power system analysis studies(Load flow, short circuit and Transient study) of interconnection between Jabil Ali and Industrial Area in Dubai.

Similarly, I did Power system analysis studies of Eastern & Western region SCECOs inter connection in Saudi Arabia when I was doing MS KFUPM in Saudi Arabia.

I can do by hand short circuit calculation for small area electrical network and same thing I explained in my previous other Post.

Please let us close the topic here itself & let us not waste each other time as well as CR4 forum.

Thanks to you all