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Swapnendu Sil
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Electrical

12/27/2015 12:29 AM

I want to know the answer with description ,what is the incomin cable or wire size for 1)63amp 4p mcb,2)32amp industrial socket.Will the size be same for cable and wire .If so why ,if not so then why?(i faced these question in a interview).
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SolarEagle
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#1

Re: Electrical

12/27/2015 1:00 AM

A wire is a single conductor while cable is a group of two or more conductors....

http://www.cerrowire.com/ampacity-charts

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Crabtree
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#2

Re: Electrical

12/27/2015 1:41 AM

It depends upon the method of installation, its length, the ambient temperature, the cable construction and a host of other factors, Mildred, the likes of which you will find in your local electrical standards. I mean, what is it made of? Copper? Aluminium? Cream cheese?

The question doesn't have a single answer. The question is there for you to demonstrate the thought process you would use to determine one.

Did you get the job? =-O

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Swapnendu Sil
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#3
In reply to #2

Re: Electrical

12/27/2015 3:46 AM

No i didnt get the job ,i could not give the answer & i was confused,generally 4c 16 sqmm al. or 35 sqmm is ok (from site expireance).i came to know that from the thumb rule it can be determine but dont know how exactly.Can anybody tell me in brief on that .
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Tornado
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#4
In reply to #3

Re: Electrical

12/27/2015 4:34 AM

It's not really by rule(s) of thumb, nor by calculation. Instead, you assess the conditions of installation that Crabtree mentioned, look up current carrying capacity of the wire(s) or cable(s) in the proper table, and select sufficient conductor(s). You might think of it as a decision tree rather than a calculation. Better luck next time.

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TonyS
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#6
In reply to #3

Re: Electrical

12/27/2015 8:20 AM

Under BS7671 you would need the cable type, anticipated load and length, to this you apply correction factors for installation method. You are aiming for a volt drop of no more than 6% for power 3% for lighting.

NEC, just look it up in the tables, they are way oversized so no need for correction factors.

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Crabtree
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#7
In reply to #3

Re: Electrical

12/27/2015 2:14 PM

I'm not surprised. If you are using rules of thumb rather than national standards, then you aren't going to get the right answer.

You don't have to remember the answer. You have to demonstrate how to get there, which someone else clearly did.

I'm off now. This is my stop. Cheerio!

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Codemaster
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#10
In reply to #3

Re: Electrical

12/28/2015 10:10 AM

I think you'd have been better to tell the interviewer you couldn't give an off-the-cuff answer, but you knew the principles and would know where to find out. Then if you got the job, brush up on it quick!

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Mr. small
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#5

Re: Electrical

12/27/2015 6:16 AM

Get subscription in CR4, you will learn a lot from here.

Don't ever get into the battle without preps.

See this NEC guidelines, this is useful.

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TonyS
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#8
In reply to #5

Re: Electrical

12/27/2015 11:30 PM

If he's using mm² for cable sizing referring to NEC codes will be of no use what so ever!

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Tornado
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#9
In reply to #8

Re: Electrical

12/28/2015 4:26 AM

I think the NEC now has cross-references, but my edition is too old to tell.

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TonyS
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#15
In reply to #9

Re: Electrical

12/29/2015 9:24 AM

2014 isn't cross referenced, I have to use separate tables.

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ozzb
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#11

Re: Electrical

12/28/2015 12:16 PM

Going to guess what you asking and say no that the wires are not the same. Even though the amperage of the two circuit would be the same. The socket is the limiting factor. You don't run 64 amps to a 32 amp socket.

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JIMRAT
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#12

Re: Electrical

12/28/2015 5:30 PM

Everything CRABTREE said. Just to explain for you some of what happens.

Electrical cable running in air with no insulation can be allowed to get hot, very hot! Like 500 deg. So the cable size can be small. ( the conductors heat up as resistance increases, small conductor = high resistance ) BUT when using insulated conductors twisted around other conductors that are designated for N or E (ground) or other phases you don't want the insulation to melt. Sooo, you need to keep the cable cool. To do this you need bigger conductors, but if you go too big you waste money, they don't fit the terminals and other problems.

Now here is the hard bit; if you run the cable in conduit, or underground, or in a concrete slab, or next to other cables, or in a hot country you will need a different size. For each of these conditions! Also length of the cable will increase resistance and cause the voltage to drop. To compensate for this you will need to go larger than the size you determined already based on whether it is in conduit etc. NOBODY has all of this info in their head. That is why all posters have said look up the standard in your country.

There is a lot to learn and no shortcuts. Start with the standard and a mentor, work hard and watch the experts. Ask questions and reference the answers to the standard.

Jim

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PWSlack
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#13
In reply to #12

Re: Electrical

12/29/2015 4:28 AM

GA. KaBoom!

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JIMRAT
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#14
In reply to #13

Re: Electrical

12/29/2015 4:43 AM

Thanks, never got a kaboom before. (insert smiley here)

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Users who posted comments:

Codemaster (1); Crabtree (2); JIMRAT (2); Mr. small (1); ozzb (1); PWSlack (1); SolarEagle (1); Swapnendu Sil (1); TonyS (3); Tornado (2)

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