The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 4 replies
  • Subscribers 31 subscribers
  • Views 1725 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Electrical Load Assesment - Total demand

Former Community Member
Former Community Member

Hi everyone,

Can you guys make some light into my head, please?

I have gotten very confused on how to do a load assessment for, let's say a building, that would have single-phase consumers as well as three-phase consumers. For this purpose and for easy understanding let's say we have sockets on single phases: L1 - 10A, L2 -12A, L3 -10A, and a three phase consumer of 30A. Can somebody show me a calc for getting the total demand in KVA please?

  • 0

    Calculate the KVA demand of each load and add them together.

    For example 10 amps=2.3 KVA presuming a 230 volt supply.

    A 3 phase 30 amp load will be about  7KVA on each phase or 21 KVA in total. In a real world example with numerous intermittent or variable  loads, the actual maximum demand may be less than that obtained by simple addition since it is unlikely that all loads will be used at the same time.

    On the other hand,  with only a few loads, simultaneous operation should normally be allowed for.

    Also remember that three phase supplies usually presume a balanced load.

    12 KVA on one phase, 12 KVA on the second phase and 40 KVA on the third phase does NOT mean that a 64 KVA supply will suffice. A 120 KVA supply would be needed (three times the load on the highest loaded phase)

    The LOAD would be 64 KVA, relevant when assessing running costs for example.

    But the SUPPLY needs to be 120 KVA.

  • Former Community Member
    0 Former Community Member

    Thank you so much, got confused at work since doing so i have been told I am wrong. Honestly drove me mad. Any chance you could explain why 120kVA (three times the load of the heviest phase) would be necessary if the phases are not balanced please? 

  • 0
    A three phase supply is generally provided on the assumption of a balanced load. So a 120 Kva supply can supply up to 40 Kva on each phase.
    Providing a three phase supply of less than 120 Kva for a load with 40 Kva on one phase will result in overloading one phase.

    Or consider it in amps. A three phase cable, or transformer, or alternator rated for say "150 amps" means not exceeding 150 amps on any phase. It does not mean "an average of 150 amps" it means no more than 150 amps.

    On small supplies to say a dwelling, unbalance is virtually inevitable as many single phase domestic appliances are used largely at random.

    On a large supply to say a big factory or a large apartment block, then care should be taken to balance the loading as equally as possible.
  • Former Community Member
    0 Former Community Member
    Thank you so much for your explanations.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines