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Calculation Basis & Code Reference Sheet

ElectroBIM makes use of several calculation bases and NEC references. This article briefly explains each basis and reference used in the software and provides links to additional resources, providing a single source you can use to evaluate ElectroBIM's bases or clarify its calculations for reviewers.

Voltage Drop​

Voltage drop is calculated for distribution equipment feeders and for branch circuits using the following formula:

Three−Phase % Voltage Drop=Load in Amps×Effective Z×Wire Length1,000Line−to−Neutral Voltage×100Three-Phase\>\%\>Voltage\>Drop = {{Load\>in\>Amps \times Effective\>Z \times {Wire\>Length \over 1,000}} \over Line-to-Neutral\>Voltage} \times 100 Single−Phase % Voltage Drop=Load in Amps×Effective Z×Wire Length1,000×2Voltage×100Single-Phase\>\%\>Voltage\>Drop = {{Load\>in\>Amps \times Effective\>Z \times {Wire\>Length \over 1,000} \times 2} \over Voltage} \times 100 Effective Z=(Resistance×Power Factor)+(Reactance×sin⁡(arccos⁡(Power Factor)))Parallel RunsEffective\>Z = {{\Big(Resistance \times Power\>Factor\Big) + \Big(Reactance \times \sin\big(\arccos(Power\>Factor)\big)\Big)} \over Parallel\>Runs}

(See NEC Table 9 Note 2)

The variables for the above formulas are as follows:

  • ResistanceResistance and ReactanceReactance are based upon NEC Table 9 by default and can be modified.
  • Transformer impedance values are for dry-type indoor transformers by default and can be modified.
  • Power FactorPower\>Factor can be based upon Revit settings for individual pieces of equipment, or fixed at 0.850.85.
  • When applicable, the Effective ZEffective\>Z is divided by the number of Parallel RunsParallel\>Runs for the feeder or branch circuit.
  • Line−to−Neutral VoltageLine-to-Neutral\>Voltage is used for three-phase calculations based upon NEC Table 9 Note 2: "Multiplying current by effective impedance gives a good approximation for line-to-neutral voltage drop."
  • VoltageVoltage is the line-to-neutral voltage for single-pole circuits and line-to-line voltage for two-pole circuits.
  • For distribution equipment, Load in AmpsLoad\>in\>Amps uses the calculated load as determined by Revit's load calculations. For branch circuits, the total connected load is used.
  • Wire LengthWire\>Length is the calculated or fixed length of the feeder or branch circuit.

Voltage Drop Through Transformers​

The voltage drop through transformers can be included, ignored, or reset to 0%.

Additional Voltage Drop Resources​

More information about ElectroBIM voltage drop calculations is available at the following links:

Fault / Short Circuit Analysis​

Fault calculations are based upon IEEE Std 242-2001: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems and calculate the three-phase line-to-line fault.

Fault=VoltageResistance2+Reactance2Fault = {Voltage \over \sqrt{Resistance^{2} + Reactance^{2}}}

The variables for the above formulas are as follows:

  • ResistanceResistance and ReactanceReactance are based upon NEC Table 9 by default and can be modified.
  • Transformer impedance values are for dry-type indoor transformers by default and can be modified.
  • Motor contributions are included for devices that have been designated as motors in Revit.
    • The circuit length from the motor to the panel is accounted for.
    • Motor current is equal to Rated Current×4Rated\>Current \times 4 and the X/R ratio of motors is 5 by default and can be modified.

If the system has more than one source, such as multiple utility transformers or a utility transformer and generator, the fault calculation will use the worst-case scenario.

Additional Fault Analysis Resources​

More information about ElectroBIM fault calculations is available at the following links:

Arc-Flash​

Arc-flash calculations are based upon IEEE Std 1584-2018 and use information from ElectroBIM's fault calculations, selective coordination features, and device information as specified in the standard.

Additional Arc-Flash Resources​

More information about ElectroBIM arc-flash calculations is available at the following links:

Wire, Breaker, & Conduit Sizing​

ElectroBIM provides automated wire, breaker, and conduit sizing based upon various NEC references. This automation can also be manually overridden as needed.

Wire Sizing​

ElectroBIM's automated feeder and wire sizing uses the following NEC references:

  • NEC 250.102 for sizing service grounds being fed from transformers.
  • NEC 250.122 for sizing equipment grounds being fed from non-transformers.
  • NEC 310.15 for derating wires due to double neutrals or ambient temperature.
  • NEC 310.16 for default wire sizes, which can be modified.
  • NEC Table 9 for default resistance and reactance values, which can be modified.

Breaker Sizing​

ElectroBIM's automated breaker sizing uses the following NEC references:

  • NEC 430.52 for sizing based upon the protected device's FLA, as defined by the user.
  • NEC 440.22, 440.32, & 440.33 for sizing based upon the protected device's MOCP and/or BCSC, as defined by the user.
  • NEC 440.35 for sizing based upon the protected device's MCA, as defined by the user.

Conduit Sizing​

ElectroBIM automatically sizes conduits to not exceed 40% fill per NEC Table 1. Each conduit is assumed to house a single feeder or branch circuit.

Additional Sizing Resources​

More information about ElectroBIM's automated sizing is available at the following links: