de-rated cable size Vs breaker size

Cable is sized based on the load connected.

The breaker,cable etc are rated and selected based on the load.Supposing the the load is 1400 Amps.

Give over load allowance of 10%.i.e 1540 Amps

Select the breaker and the cable for thermal rating of 1540 Amps.

Since Breakers of ratings 1000,1250,1600,2000 Amps are standard.In this case you have to choose the 1600 Amps breaker.

Regarding the setting of the breaker, supposing you wanted to isolate the circuit when the over load takes place at 5%, then over load trip current would be 1400*1.05=1470 Amps.

This current settings on breaker would be 1470/1600=0.918

Therefore you have to set the breaker at 0.918 setting.

I hope this is clear.

Can you please expalin where do you get a factor 1.45.

Protective devices have 3 values of interest here:

In: its nominal rating (e.g. 32A)

I1: its non-fusing/non-tripping current. For a Type B MCB this is 1.13In I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is Ib, then this should not be greater than the rating of the protective device:

Ib ≤ In

If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:

In ≤ Iz

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:

1.45Iz ≥ I2

So Ib ≤ In ≤ Iz

and

I2 ≤ 1.45Iz.

So this is the reason for 1.45What is your take on this?

Dear sir

In your approach you are fixing the device rating first then fix the cable rating based on the protective settings.

I have taken the load as basis then selected the breaker and the cable.In any case current settings of the protective device should least of the currents.

supposing load current is 940 amps,cable rating 1050 amps and the breaker rating is 1025 amps then the protective setting is to be for 940 amps. plus 5 % over load,this setting is to be for instantaneous operation.Supposing your are allowing over load of say 10 % then the cable,breaker to be selected with over loaded capacity.

Now you have selected the breaker rating as 1600 amps.then comes load and the cable, if the load is 1400 amps and it can take the over load of say 10%, then select cable for 1540 amps then set the breaker for 1540 amps.supposing you select the cable for lower rating, then set the breaker for lower amps between the two.

Electrical engineering do not assume reverse engineering, you cannot decide the breaker then go back to cables and load. Start with load, ruch current, intermittent ruch current, power factor, material of cable; is it copper or aluminum, then listen to reply 2

You wrote..."If we know the breaker size, how do we calculate the minimum cable size?"

You can't.

What are you trying to do?... Protect the cable?... To fuse this size of cable, you will need at least 25kA for at least the breaker trip time. Not very smart to put it mildly...and certainly a very vague question.

Wangito.

Since we typically size breakers to allow for load growth, conductors are typically (well for the last 30 yrs anyway) sized so they will carry the load that may be powered by the breaker in the future.

If the breaker trip setting has restricted access you can size the conductors on the trip setting. Otherwise you must size the conductors on the maximum setting possible.

Did I miss something here , the MINIMUM size of the cable is absolutely determined by the breaker feeding it. Having said that there are a lot of factors that can cause the cable to be engineered larger like, voltage loss over length , conductor quantity enclosed,future use ,and wire and lug temperature ratings.

By Code the cable must be able to be fully protected by its source device.

No offence meant but deliverable short circuit current determines a lot about the breaker and about cable bracing and physical protection but not its size, at least not in any normal applications short of some utility busing etc.

JonRR,

You said: the MINIMUM size of the cable is absolutely determined by the breaker feeding it. Be careful. The original post asked a question without giving specifics of the type of load, jurisdiction, etc. Yet you gave an absolute answer!

Each jurisdiction can have its own rules regarding cable sizing. Also, its own exceptions to these rules. I do not know about other countries, but in the USA the NEC §240 has many cases where the cable size can be significantly smaller than the breaker ampacity may otherwise require. If you have access to the book, look at §240.4(G), and §240.21.

Thanks--JMM

If you're sizing cable per the American "National Electrical code" the cable amp rating must match the circuit breaker size unless it's used as a motor circuit to permit motor starting.

Guest,

You are correct that motor circuits are an exception, but they are not the only one. Consider also feeder tap rules and welders as two others. Look at the citations in post #7 for these as well as others.

Regards--JMM

Well, I have seen many value addition on the topic. Thanks to everyone. Let me sum-up.

1. In case of motor feeder breaker the cable ampacity should be corresponding to the load.

2. In case of feeder breaker e.g PMCC to MCC feeder, the cable ampacity should be corresponding to the down stream switchgear bus rating.

Sir,

The topic does sound confusing. Perhaps I have added to this confusion; if so, I apologize.

1. As many have said, the proper design method is to start with the load and then work towards the supply, to find the size cable needed for this load and then the size over-current device (fuse or CB) needed for it.

2. Some types of loads (such as motors) have a high starting current --"inrush", which often is greater than can be carried by a fuse or CB sized only for the required cable size and the running current of the motor. In these cases, the codes permit the fuse or CB to be increased in size by certain amounts (these vary---based on how the motor(s) are started, their inrush current characteristics, the time needed for them to get up to speed, etc.).

3. With feeders supplying both motor and general loads, the codes have rules on how to calculate the proper feeder and CB sizes.

4. In some installations it is impractical or very difficult to locate the CB where the cable receives its power (such as feeder taps and service entrances), so the smaller cable is permitted to be protected at its load end (with certain restrictions on length-vs.-size).

5. All these rules are based on practical experience for over 120 years, and occasionally get changed and improved as experience, accidents, new equipment, and other factors may require.

In all cases, the National Electrical Code in the USA (I don't know about others) is a minimum standard, so its requirements can always be exceeded (such as an 800-amp load being supplied from a 1200-amp feeder instead of a 1000-amp feeder to allow for future load growth). You can therefore, take the load, size the minimum feeder required and the minimum CB required, increase the feeder size to allow for voltage drops as needed, and then increase them more (along with the bus size of your downstream panelboard or equipment) to allow for a margin of safety or future growth. Doing this may cost you more at first but probably will save an even larger amount over time, and may be a good investment. Remember also, as one earlier post stated, that with the larger-frame adjustable trip CB's feeding loads which don't fit the exceptions, your feeder cables must be sized for the highest possible setting, not the one which is in use (and even here, an exception exists if the ability to change this setting is physically restricted to qualified people only...).

Yes, all this stuff is easily confusing. Just try to remember the purposes--protecting equipment, protecting property, and protecting people. As you keep these purposes in mind, the rules and their exceptions begin to fit together.

If you want to post a specific question (with details of the type of loads, size of loads, and distances) I or others could give you our interpretation of what the code(s) would require or permit for this particular case.

All this can be a little like asking if the speed limit is 40 km/hr (or 30 mph . . .) and the answer would be "it depends", because if the visibility is poor because of dense fog, the safe speed (and thus the legal speed limit) may be as low as 5 km/hr! But if you state the specific conditions, then a good answer can be given.

Thanks--John M.

<...calculate the minimum cable size?...>

Simply apply the principles contained in the applicable national standard, such as British Standard 7671, in the same way that qualified Electricians do as part of their everyday business.