Load Panel 240VAC Supply and Source Calculations

Discussion in 'General Electronics Chat' started by demartinoj, Jul 31, 2009.

  1. demartinoj

    Thread Starter Member

    Jan 25, 2009
    10
    0
    Can someone explain how the calculations work for sourcing a load panel for a specofied load.
    For example: If I need to hook up two devices that each require 120VAC 10A ( 20A total)to a load panel that gets its supply from a 240VAC source that has two seperate 120VAC phases. I need to have 20A to supply my devices at 120VAC. If I put one device on one phase and the other device on the second phase what amperage does the 240 VAC source need to supply and how do you specify that. Is it 240 VAC 10A or 240 VAC 20A to supply the 120 VAC 20A since the VA calculation is the same. 120x20=2400VA and 240x10=2400VA. This is confusing to me. Can someone explain?
     
    Last edited: Aug 1, 2009
  2. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    The attached photo shows how my house is wired; the transformer sits way up on a pole. The two secondary circuits are AB and BC; these provide the 120 VAC. The voltage with respect to ground on A is 180 degrees out of phase with the voltage on C with respect to ground. The ground wire from D to E is just to ensure that the neutral B is at ground potential and doesn't normally carry any current when things are wired and grounded properly.

    Suppose you draw 10 A from the AB leg. That's going to be 1200 VA. Similarly, if you draw 10 from the BC leg, that's 1200 VA for the BC leg. This means a total of 2400 VA. The A leg has 10 A in it, as does the C leg. In this balanced condition, the neutral leg has no current. If you look at the just the circuit containing A and C, it's 240 VAC with 10 A of current, or 2400 VA.

    You electricians, please correct me if I've made a mistake. I intend to use substantially the same words in a document on AC safety that I'm working on.
     
  3. demartinoj

    Thread Starter Member

    Jan 25, 2009
    10
    0
    Thank you someonesdad that makes sense to me. So to state my power requirements at 240 VAC I can state it as 120 VAC 20A , 240 VAC 10A or simply give it in VA, 2400VA
    Correct?
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You are right.
     
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