Spare Feeder Sizing

British Standard 7671 gives the protocol for cable sizing based upon the projected distance from the panel to the load. For practical purposes, the feeder connection from the bars to the upstream side of the fuses is designated a "fault-free-zone", and it is sized the same as the remainder of the feeder.

I think you didnt get my question correctly. I didnt ask any thing about cable sizing

We usually dont connect anything thing to spare feeder. It may be considered for future use. I asked what should be rating of the spare feeder.

Suppose my panel has 2 nos of 250 Amps outgoing,1 no 200 Amps outgoing,3 Nos of,100 Amps out going& 2 nos of 63 Amps outgoing.

so from the above what should be the spare feeder rating
should it be 250 Amps..?

is there any standard procedure or rule applicable....?

This poster, songsaah, is a DANGER TO HIMSELF AND OTHERS.

If he really is handling 400AMPS, he will kill somebody.

Sir,

Asking a question doesnt kill any one i thought the experts in this forum will come up with answer .If you know how to size it please share..

Thanks and Regards

No!

Others may help you, and that's fine with me.

If you are working with 400AMPS, and asking these questions, then I think you are lethal.

If you are a student, read the text book.

Get a copy of BS6731 or NEC code book.

Fine.

I don't know how many spare feeders you have considered. If there is only one consider it as 250A. If there are two spare feeders then you may choose one of 250A and the other of 63A so that it can be used as replacement feeder for the highest and the lowest rated feeder.

However, for motor feeders you need to consider rating of all the components i.e circuit breaker/ SFU, contactor and overload relay. Hence the spare motor feeder may not really serve the purpose unless you change the components to match with your requirement.

If you intend to use the spare feeder for future load increase, please add this load while calculating the maximum demand of the panel.

It depends upon the cable size from the bars through the cubicle all the way to the load, as already indicated.

check this link

Feeder sizing

Read your local electrical standards and codes of practice, all the information is in there.

Spare feeders? Are you asking about breakers?

If these are spares why put any breaker in at all and waste money. Breakers are sized to the wire that they are intended to protect. Since you have no knowledge of their future use you can't determine what breaker size to use. If these open and have to be filled they make covers. If not, the smallest or lease expensive breaker possible.

Why do so many people think that everything can be calculated?

I use my noggin. If I am asked to supply spares, I generally look at what is in there and give them at least one of everything. Most of the time I am responding to a spec that says something like "Provide a minimum of 20% spare feeders". So since I can't provide 20% of a single breaker, I give them at least one of each size, then multiples rounded up to at least exceed 20% on anything else.

Then when they see the exorbitant cost and scream about it, I point out the folly of their wording and make them decide what they really want...

• "Why do so many people think that everything can be calculated?"

Something just occurred to me, prompting me to respond to myself.

Self,

What if the frequent use of the term "calculate" I am seeing is merely a translation error and people are really asking "How does one DETERMINE..." It could be that translation software is being used and it is equating "determine" with "calculate". That would explain a LOT of the postings I see in this forum...

So if I apply that to the OP, we get:

• "Most of the panel will have spare feeders with some ampere rating.
  Could you please tell me how it is determined...?"

Makes a lot more sense to me now.

If you are working with others in the company, and they have input for future installations, ask them what is to come. That will give you at least a fighting chance to anticipate future loads

I refer you to my answer on your earlier question "I think 20% is on the low side."

The only people who can give you the guidance you need are your own company managers and directors who know better than anyone the future direction that the company will take. Ask them what they think the requirements will be at the end of the panel's lifespan. They will probably give you an over estimate but as the implications only relate to the in-feed cable, main isolator and bus bars cost that should not be too bad. As suggested elsewhere I would not fit additional breakers until there is an actual requirement. Provided the panel is capable of up-rating that is all that counts.

(Note If they say that no spares are needed, it means that they have no plans for further investment in the company. In such circumstances I would consider looking for alternative employment. No company in this day and age can stand still and survive.)

The number and rating of the spare feeder depends on the power requirement of future loads and this could vary from case to case.If a future growth is planned, it is not only enough to have suitably sized spare feeders but it is also very important to see that the power source(s) has the capacity to supply the future loads and the main feeder(s) are sized accordingly. So rather than assuming a random rating for the spare feeders one should size the spare feeder based on future growth expected and look at other factors as well.