NEC 2008 ANNEX D Example D1(b) One-Family Dwelling

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In general, that's a tough one. What is larger - 1 HP 120V motor or a 1.5 HP 240V motor? The first has higher amp values, but the second is larger HP.

In your case, I'd just add 25% to the value of 3 amps and add it as a 120V load (add a 90 VA load, or .75A to line 1 and neutral). Are you doing you calculation just in VA or via each leg and neutral?
 
In this example they seem to be taking 25% of the largest current on any and each conductor.

25% of 12 amps ungrounded L1 for A/C.
25% of 12 amps grounded for A/C.
25% of 8 amps ungrounded L2 for disposer.
 
They have determined that the voltage requirements in this example is 1- phase 240,1-phase 120 and 1-phase 115,,and resultant amps are listed for each load ,they are loaded on the phase conductors as shown for the best balance on both line and neutral.Total this up and then multiply the largest load by 1.25% and add it to your total,use this largest line amp total column to size your feeder.If you can determine that something cannot run while certain other units are running then you can reduce the amp total by that units load.You can down size the neutral if you want or go full size,depending on the economics .

dick
 
dicklaxt said:
They have determined that the voltage requirements in this example is 1- phase 240,1-phase 120 and 1-phase 115,,and resultant amps are listed for each load ,they are loaded on the phase conductors as shown for the best balance on both line and neutral.Total this up and then multiply the largest load by 1.25% and add it to your total,use this largest line amp total column to size your feeder.If you can determine that something cannot run while certain other units are running then you can reduce the amp total by that units load.You can down size the neutral if you want or go full size,depending on the economics .

dick
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But did you notice that only two of the three wires would have 25% added to them if only the largest amperage was used, rather than the largest on each conductor. In the example, they added 25% to all three wires. That means they added the largest motor current on each, which is comprised of two separate motors
 
Volta said:
But did you notice that only two of the three wires would have 25% added to them if only the largest amperage was used, rather than the largest on each conductor. In the example, they added 25% to all three wires. That means they added the largest motor current on each, which is comprised of two separate motors
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I agree, IMO you explain this well.
 
No, I did not notice that but right you are,,,,,,,,it would be the higher of the two in this case and better to go worse case,,,,,,,,,,,but I wonder why that was not mentioned in 430.24, it used the largest flat out with no regard to phases that I can see.Do you suppose an error is in the midst one place or the other?

dick
 
It is possible, but now that I've seen it I can only think that the wisest course of action is to count an additional 25% of the largest motor current draw on each and every circuit conductor. That could effectively be the loads of 3 separate motors is some instances on a 3-phase premises wiring system.
 
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