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How to calculate ampacity of 3 phase 4 wire singles installed as quads spaced 2De not trefoils?

DHRobertson

Designing data center installations involving multiple transformers and generators. 

IEC60364 and BS7671 give ampacities for multiple trefoils but not quads

IMO, quads must have a lower ampacity than that given for trefoils spaced 2De on cable ladders

I note multicore 3 core and 4 core (and 5 core including earth) have the same ampacity

For singles, ONLY 3 cables are listed as trefoils with no mention of quads

What is the recommended solution for unbalanced parallel 3 phase 4 wire circuits with 100% neutral and smaller earths.

For example 6(4*240)/3*120G XLPE on cable ladder at 40C ambient

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  • 0

    I note multicore 3 core and 4 core (and 5 core including earth) have the same ampacity

    That seems to be the general rule - just count the loaded conductors - and of course in conventional 3-phase circuits the most onerous is when all three phases are fully loaded - and so N carries zero current, so still just 3 cores carrying current. In other situations any N current (due to imbalance) is compensated by an equal reduction in current in one or more of the line conductors, so the overall heating effect isn't increased.

    Granted the presence of an extra (unloaded) conductor will make some difference - it's solid presence may restrict heat loss by convention, but at the same time it may provide a extra path to take heat away by conduction and provide a larger surface area for overall heat loss. All cable ratings are approximations, and in the great scheme of things, unloaded cores/cables seem to make a small difference.

    If it's a datacentre, with presumably lots of switch-mode PSUs, my worry would be more about currents not cancelling out in the N and you actually have 4 loaded cores rather than the 3 that usual 3-phase theory predicts.

       - Andy.

  • 0 in reply to AJJewsbury

    Thanks for the response. My concern is that Table 4D1A Ref F ONLY refers to 3 cables, 3 Phase. Ref A, B, C refers to 3 or 4 cables. Table 4D2A Ref A, B, C, E refers to 3 or 4 cables. What ampacity should one use 4 for say single quads, separated by 2De on a cable ladder. I accept your comment about harmonics and, in fact, PF may be leading or lagging 0.95. I Dont think Ref F trefoil can be correct. I see Ref F 3 cables touching flat has a higher ampacity!

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  • 0 in reply to AJJewsbury

    Thanks for the response. My concern is that Table 4D1A Ref F ONLY refers to 3 cables, 3 Phase. Ref A, B, C refers to 3 or 4 cables. Table 4D2A Ref A, B, C, E refers to 3 or 4 cables. What ampacity should one use 4 for say single quads, separated by 2De on a cable ladder. I accept your comment about harmonics and, in fact, PF may be leading or lagging 0.95. I Dont think Ref F trefoil can be correct. I see Ref F 3 cables touching flat has a higher ampacity!

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