Question on 3 phase load calculation and 125% rules

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brycenesbitt

Senior Member
Location
United States
Could someone look over this three phase load calculation?

This is a subpanel load calculation for an existing multifamily building with 2000A service and several subpanels.
The house panel has a 600A 208Y/120 3 phase 4 wire feed. The original plan set reads:

1735106490183.png

My question is the EVCS calculation. The 7700 VA is counted at 125%. Then the overall load is raised by 125%.
Is the load calculation correctly done? Is the EVSE continuous load portion double counted?
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And what rule allowed a 600A breaker on a 605A calculated load?
The actual EV loads are listed in the same document as 22 KAIC, with all loads as "Lighting":
1735106791653.png
1735106895296.png
 
125% factor is used for determining conductor ampacity and overcurrent protection for continuous loads.

Much of art 220 load calculations is about where can you apply demand factors and none of the results where adjustments are used are considered to be continuous loads.

I don't know without further investigating if that 125% in item 6 is correct or not, but even if it is there the net load of everything is not considered a continuous load and not subject to an additional 125% increase.
 
It sure looks like the engineer was using that last 125% multiplier for spare capacity, which is why a 600A breaker was used when the calculated value in that last step was 605A.

Is the house load on its own meter that you could pull a demand load from? That approach would most likely give you more available capacity than is shown in the calculation.
 
brycenesbitt said:
Then you'd suggest the existing panel load calculates out to 483 amps?
Click to expand...
I'm not so familiar with the details of a multi-family load calculation, so I haven't checked the inputs to the calculation. But it looks like the EVCS is the only continuous load. A few comments:

First, the calculation shows (4) EVSEs @ 7700 VA each @ 125%, for 4*7700*1.25 = 38,500. 7700VA is surely a rounding of 7680 VA, which is 240V * 32A, so they are 32A EVSEs. Installed L-L on a 208Y/120V system, they are only 6660 VA each. If you are under the 2023 NEC, the 220.57 would require you to bump that up to 7200VA (which requirement make no sense to me).

Assuming 2020 NEC, that makes the calculation shown 5200VA lower, so 169,097VA, or 469A. But the way I would put it is that the load is 162,437 VA (no 125% factor for continuous loads), or 451A, of which roughly 32 + 32*sqrt(3) = 87A is continuous. [With 4 EVSEs connected L-L on 3 phase, two of the line conductors will see 2 EVSEs from one of the other lines and 1 from the third line.] So if the relevant breaker is not 100% rated, then it would need to be rated at least 451A + 25% * 87A = 473A.

Cheers, Wayne
 
brycenesbitt said:
The project goal is to install quantity x19 1.92 KVA single phase 120V EVSE instead.
Click to expand...

Seems like it would be just fine. Comes out to 126.7A when you had 135A in the original calc.

For peace of mind, perhaps you could get the meter data for the panel and verify that no interval exceeded the equivalent of 460A. I bet it's way lower.
 
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