circuit breaker size?

Discussion in 'General Electronics Chat' started by montanacsi, Jun 18, 2008.

  1. montanacsi

    Thread Starter New Member

    Jun 18, 2008
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    hello everyone!
    i have an incoming supply of 440Vac and i had rated my cable size as 95mm2 which has a cable current capacity of 276Amps. can't i have my circuit breaker sized as 300A or 400A?i can not possibly have an exact circuit breaker size of 276Amps right?thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It would not be smart (or legal) to have a breaker that tripped past the melting point of the insulation on the conductor. 200 amps gives enough overhead for safety. That still gives you 88,000 watts to play with. Check your local codes for conductor ampacity and maximum continuous current allowed.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Received via PM, posted for posterity.
    Hello montanacsi,
    It is best to post your question in the most appropriate forum, rather than via private messages. You will get an answer more quickly by posting in the forums, and you will also get the benefit of having the answers reviewed by many knowledgeable people, and if there are errors or perhaps better solutions, people will be quick to point them out.

    Additionally, those who are learning may benefit from the resulting discussion.

    But to answer your question:
    Your circuit breaker must be sized no larger than the "weakest link" in your circuit. If your wiring is rated for 276 Amperes, you must use the next size SMALLER breaker. If a 275 Ampere breaker were available, you could use it. My bet is that you will wind up with a 250 Ampere or smaller breaker.

    I am NOT an electrician!
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    I am an electrician, but I am NOT familiar with codes in Singapore.

    Those codes I am familiar with all require a protection device rated lower than the conductors and devices int he circuit. The breaker must protect the circuit, not the other way around!
     
  5. montanacsi

    Thread Starter New Member

    Jun 18, 2008
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    Thanks to both of you for enlightening me!

    But I should say that Sgt. Wookie is so harsh for a newbie like me. What if at that time, I am still learning how to post in a thread?Is that such a big crime?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    He is not being harsh, he is simply being concise.;)
     
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
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    I would select the breaker according to the current consumption of the sum of the load devices, not the ampacity of the conductor.
    The conductor itself must had been chosen well above the maximum current expected, with a safety margin.

    Miguel
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    The breaker is not for protecting load. The load has its own protection. The breaker is for protecting the branch circuit, ie the conductors and the outlet and any switches. If the load device draws more current than the branch circuit can safely supply, the breaker will trip to prevent over-heating of the conductors et al.

    An example would be an 18 Amp hair dryer on a 15 Amp circuit. Inrush current from the hair dryer starting would trip the breaker. If we replaced the 15 Amp breaker with a 20 Amp breaker we risk overheating in the conductor and damage to the building. We also risk harm to the occupants of the building.

    If Montanacsi has a load needing 300 or 400 A, then larger conductors are needed. As the conductors can only safely handle 270 A, the breaker must be rated at that value or less in order to protect lives and property.

    I hope this helps to clear things up.:)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I do not understand how my reply to you could be interpreted as "harsh". You asked a question, and I answered it to the best of my ability, along with pointing out that the best way to get an answer quickly and accurately is to make a new thread in an appropriate forum, not by private message.

    Private message storage on these forums is limited to a total of 50 incoming and outgoing messages. I have a number of private messages that I don't wish to purge. My inbox is therefore very limited.

    My intent was to give you a brief introduction to how the forums work best, and how to get the most rapid reply in a manner that both you and the general reading population would gain the most benefit.

    If you think that I'm harsh, you should have met my Drill Instructors. :eek:
     
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