Thursday, June 14, 2018

Electrical Calculations in Revit - Why kVA Totals Can Vary from Feeder to Panel

Because my firm created the electrical portion of the MEP Productivity Pack (MEPPP,) which CTC sells to the world, we have seen a number of problems and fielded many questions from customers over the past eight years (with CTC providing the lead on all support). This has helped to expand our knowledge of Revit and improve the MEPPP product.

One question that comes up from time to time is why the kVA Totals for Panels are sometimes different than the kVA Loads on Feeders. An example of this can be seen  in the following image.

What is happening is Revit is inconsistent with how it tabulates the loads.  Both are...
correct, however, depending on who you ask.

Revit's Calculation Methods:

Sum true load and reactive load:
In the image below (click to enlarge.) the yellow highlight shows where Revit is following 3-phase / power theory:  summing the real and reactive components independently, and determining the resulting apparent power value.  This method is correct by definition.  Loads (apparent power) getting ‘smaller’ on circuits should be expected when there are varying power factor.  For example, a circuit with two loads of 100 VA, one with 1.0 PF, and the other with 0.5 PF, will have a resulting load of 173 VA, and 0.866 PF.

The purple highlight shows where Revit is computing by summing the VA, ignoring the PF on the circuit.  This is what many engineers do, even though it defies the theory.
In short, circuits follow the theory.  Panels sum VA.  

The way Revit performs this calculations will work just fine 99% of the time, and any of those particularly tricky situations where loads are very, very close to the limit would warrant some extra attention from the designer anyway.

Sum appearent load:Starting in Revit 2017, we have the option of summing just the apparent load on circuits and panels. The following can be found in Manage (tab) -> Electrical Settings (dialog) -> Load Calculations:

EPP Overview

Revit is set up to use a designated power factor in content and calculations.  We have built the MEPPP content to calculate using this method:

Lighting Fixtures:

  1. Assign Real Power parameter.
  2. Assign Power Factor (ballast or driver).
  3. Assign Voltage.
  • Revit will calculate Max Current (amperes) and Apparent Power (volt-amperes) for each circuit based on (3) user inputs above.  Lighting is always single phase.
  • Our MEPPP lighting fixture content is built with default values already assigned with Advance ballast/driver information.

Electrical Fixtures: (motors, receptacles, equipment connections, etc.)

  1. Assign Max Current parameter.
  2. Assign Power Factor.
  3. Assign Voltage.
  4. Assign Phase (1 or 3).
  • Revit will calculate Real Power (watts) and Apparent Power (volt-amperes) for each circuit based in (4) user inputs above.  EPP knows that 3 phase uses √3 in the calculation formula that we created.
  • Max Current (amperes) parameter:  Our MEPPP electrical fixture content is built with default ampacities and power factors
    • Motors:  Ampacities per NEC tables, PF default 0.8, calculated values for VA & W
    • Receptacles:  1.5 A and 0.8 PF = calculated values of 180 VA, 225 W
    • Receptacles for Computers:   4.1 A and 0.9 PF = calculated values of 492 VA and 547 W.
    • Other connections:  User needs to enter Max Current, default PF is 0.8.

Panel Schedules:

  1. Revit will use Apparent Power (volt-amperes) in panel schedules.
  2. Revit will sum all of the connected Apparent Power (volt-amperes) to determine Total Connected Load.
  3. Revit will calculated Total Demand Load based on the Load Classifications demand factor calculations of the connected loads, using connected circuits Apparent Power (volt-amperes).  We built the content to use demand calculations per the NEC.

Panel Feeders:

  1. Revit does NOT use the Total Connected Load apparent power (volt-amperes) reported in the panelboard schedule for feeder connected load.  It recalculates feeder apparent power for the feeder based on the real power and reactive power on the panel circuits with an internal calculation.  It does this for every upstream panel or switchboard in a project.  This is the value that it reports in the “LOAD” column of the upstream OCP assigned to the feeder.
  2. This creates what appears to be conflicting information.  The Total Connected Load on an individual panelboard will be greater than the “LOAD” for the circuit indicated in the upstream distribution panel or switchboard where the feeder is connected.  This happens whenever the downstream panel has branch circuits that do not all have the same power factor.

Circuit Schedule Analysis

On a related note, for circuits, Revit provides the True Load and Apparent Load values. We created a calculated parameter for Reactive Load to complete the power triangle. In the MEPPP starter project, we provide a circuit analysis schedule shown below, which is helpful for coordination and design review.

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