When computing current-related parameters on panels, Revit makes certain conservative assumptions that may make the current on unbalanced panel phases slightly larger than expected.
To calculate the Total Connected load (VA): Apparent Load Phase A + Apparent Load Phase B + Apparent Load Phase C = Total Connected
To calculate the current per phase:
For a 3-phase, 4-wire system, when the per-phase load on a panel is balanced, each phase's current can be computed from the quotient of the load and the L-N current. However, if the per-phase load on the panel is not balanced (for example, if there are only loads on 2 of the 3 phases), then Revit will assume conservatively, as though it is a L-L load.
For example, in this unbalanced state, two 120v (L-N) 900VA loads on separate phases will compute the same as a single 208v (L-L) 1800 VA load, resulting in 8.65 A (1800VA/208V) instead of 7.5 A (900VA/120V).
This helps ensure that if there are line-to-line loads, Revit won't artificially compute a smaller than expected current on the panel's phases. The result is that, on unbalanced panels, the per-circuit current may be slightly higher than expected, compared to computing the current as the quotient of the per-phase load and the L-G voltage.
The table below contains a current calculation example. A three-phase panel has per-phase loads of 3456VA, 3060VA, and 2160VA. Revit will compute that all three phases share 18.0A in common (2160 VA / 120V). The remaining loads are 1296VA (3456VA-2160VA) and 900VA (3060VA-2160VA). Those remaining phases share 8.65A (900VA * 2 / 208V). Finally, the remaining 396VA (1296VA-900VA) results in 3.3A (396VA/120V).
The phase with the smallest load has 18.0A, the phase with the largest load has 29.95 A (18.0A + 8.65A + 3.3A), and the remaining phase has 26.65A (18.0A + 8.65A).
The procedure for a 3-phase, 3-wire system is computed similarly. However, instead of using the L-G voltage, the quotient uses the L-L voltage / sqrt(3), so slightly different values will be computed.
To calculate the Total Connected Current (A) for a 3-phase panel: Total Connected Load / (L-L Voltage) (sqrt(3)) = Total Connected Current
To calculate the Total Connected Current (A) for a 1-phase/3-wire panel: Total Connected Load / (L-L Voltage) = Total Connected Current
To calculate the Total Connected Current (A) for a 1-phase/2-wire panel: Total Connected Load / (L-G Voltage) = Total Connected Current
where:
L-G = line to ground
L-L = line to line
sqrt(3) = 1.732
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Example:
L-G/L-L = 120/208 volts 3-phase/4-wire panel A = 3456 VA B = 3060 VA C = 2160 VA panel total load = 8676 VA |
A 3456 VA
2160/120 = 18.0 A
1296 VA 900/(208/2) = 8.65 A
396 VA 396/120 = 3.3 A
Current/phase = 29.95 A |
B 3060 VA
18.0 A
900 VA 8.65 A
26.65 A |
C 2160 VA
18.0 A
18.0 A |
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Total Connected Current = 8676/(208/1.732) = 24.082 A |
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