Voltage Drop

What is Voltage Drop?

  • The circuit conductors themselves have an electrical impedance; thus, if they have significant length (circuit distance), the voltage magnitude at the end of a circuit may be less than at the source
  • The National Electrical Code (NEC) requirement for voltage drop: None, but they do recommend a maximum of 3% voltage drop for branch circuits and 2% for feeders (total 5%)
  • Energy codes may set maximum % voltage drop limitations also, as excessive voltage drop means energy losses in the form of “i-squared R” losses. This power, available at the source, is lost (in the form of heat) by the time it reaches the utilization equipment (inefficient).
  • Rule-of-thumb: 1.5x the source voltage = max number of feet before voltage drop is likely a concern
    • Example: for a 480V circuit, you will want to confirm voltage drop at around 700FT; especially if it is single-phase
    • Due diligence: review all conditions to determine if voltage drop is a concern – do not rely solely on the rule-of-thumb

Fundamentals

Voltage drop is based on a fundamental electrical concept:

Ohm’s Law

Voltage (V) = Current (I) x Resistance (R)

 

With Direct-Current (DC) circuits, resistance is the only circuit element present (no reactance: inductance or capacitance). These values are available in NEC Chapter 9, Table 8. For AC circuits, especially over 1/0 AWG, utilize NEC Chapter 9, Table 9 – this accounts for the skin effect.

DC Voltage Drop

This basic circuit diagram shows the voltage proportionally over each circuit element. This means, in addition to the utilization equipment, the wire used in circuits will also, proportionally, consume voltage.  In general, the system components that contribute to voltage drop are: transformers, busways, and circuit conductors in conduit. Switches, circuit breakers, transfer switches, etc. are negligible impedance contributions to the overall circuit and are typically ignored for voltage drop calculations.

When utilizing NEC tables for calculations it it is important to note the parameters given. For instance, NEC Chapter 9 tables would not be appropriate for medium-voltage voltage drop calcs, as higher voltage circuits require thicker insulation, which increases conductor spacing.  The geometric mean distance (equivalent conductor spacing) impacts the impedance of the conductors; this is the case with busways also. Consult manufacturer literature for impedance data when your application is outside the parameters listed in the NEC tables.

Load Dependence

Problem: Building A serves Building B via an 800A, 208/120V, 3-Phase feeder. Building B is 1500 feet away. An electrician goes to Building B to check the voltage of a 120V receptacle, fully expecting the voltage to be low due to voltage drop. After putting on 00 Class safety gloves, the electrician uses a multi-meter to check voltage at the receptacle: 118V.  This is only a 1.7% voltage drop (far less then the recommended maximum of 5%). WHY THOUGH? 1500FT is very far away for a 120V circuit.

Answer: Building B electrical loading (apparent power, VA) is very low. If Building B were to fully occupy and run all of the internal systems running near the capacity of the 800A feeder,  voltage drop would be an issue.  

Commonly, engineering specifications will set limits on branch circuit conductor lengths before they need to be up-sized. 

Example Specification

#12AWG minimum, #10AWG minimum for homeruns

208/120V: Increase conductor size for every 100 feet

480/277V: Increase conductor size for every 200 feet

Increase conduit size as required for any up-sizing

The circuit one-way lengths given in engineering specifications are often appropriate to mitigate voltage drop concerns. To determine the length demarcations, a load is needed. Engineering judgement (typically) assumes general purpose lighting and branch circuits will not be loaded to 100%. Considerations should assume a reasonable loading (based on the application) when reviewing voltage drop limitations. Each project must look at the criteria and confirm the demand current make sense. There is good opportunity for Value Engineering (VE) if branch circuit length criteria may be updated to allow greater distances before up-sizing.

One approach to consider when using conductors that have dual temperature ratings, such as THHN/THWN, is to use the lower temperature rating (60C) as the average  and (75C) as the maximum circuit loading. Further considerations may need to be made for demand currents over 100A.

On three-phase circuits, attention is needed to review the load balance between the phases. Consider single-phase voltage drop of individual phases if loading may be un-even and up-size all three phases to match the worst case  up-size of the three phases. 

Summary: the connected load may or may not be the load you use to consider voltage drop.

Equipment Operational Concerns

  • Voltage drop is (generally) not a safety concern, but instead a concern for proper operation of utilization equipment
  • Utilization equipment operating at lower than the nameplate voltage range may shorten equipment life
  • Not to be confused with voltage drop, but voltage dip, due to motor and transformer in-rush, may cause momentary voltage variation
    • Rule-of-thumb: 5KV per 1HP for loading during start (locked rotor, or 5-6x FLC) is considered for flicker considerations
    • Ideally, the design will segregate motors on their own feeders to limit impacts to other utilization equipment
      • Serving motors from their own separately derived system helps also
      • Consider reduced voltage starting controller options

Computer Power Supply Unit (PSU) Nameplate

The lower voltage limit shown is 100V at 60Hz. That is over 16% voltage drop from 120V. Limiting the voltage drop to 5% provides headroom for both the computer and other branch circuit appliances to spontaneously demand higher loading. When a cord-and-plug personal fan, on the same circuit, is plugged in and turned on high you’ll have at least 11% headroom (13.2V) before the voltage at the PSU is less than 100V.

 Opportunities to Reduce Voltage Drop

  • 1st: The best mitigation technique is to locate the source close to large loads or centralized in the facility
    • Example: If your campus has a data center, it is most cost effective to have the electrical distribution in close vicinity to such a high-density load
  • 2nd: Consider your distance and choose your voltage wisely. Using a higher voltage, if it makes sense for the application, may be appropriate.
  • 3rd: Transformation is your friend, as transformers have taps to increase voltage typically up to 2.5%.
    • Transform to the utilization voltage as close to the load as practical
  • 4th: Up-size circuit conductors to reduce losses
    • Make note on the drawings when the design up-sizes conductors for voltage drop considerations. Doing so will reduce confusion for the bidding/installing contractors.
    • If calculations are borderline based on the NEC reference values, consider looking at the manufacturer published data

Basic Voltage Drop Calculation (Disregarding Skin-Effect)

For smaller circuits (under 1/0 AWG ), DC resistance values are often “close-enough” for the sake of Alternating-Current (AC) voltage drop checks. The following formulas are used:

Option 1: Calculate the minimum circular mil area of the conductor

Single-Phase: cmil = (2 x K x L x I ) / Vd-Max

Three-Phase: cmil = (1.732 x K x L x I ) / Vd-Max

Where: K = resistivity of the material (12.9 = CU and 21.2 = AL); L = one-way length of the circuit; I = current; Vd-Max = maximum voltage drop

Note: The K values given are based on conductors 1 mil wide by 1 foot long.

Option 2: Calculate the maximum resistance of the conductor

Single-Phase: R = [Vd-Max / (2 x L x I )]  x 1000

 Three-Phase: R = [Vd-Max / (1.732 x L x I )] x 1000

Where: R = conductor 1 foot resistance (NEC Ch 9, Table 8); Vd-Max = maximum voltage drop;  L = one-way length of the circuit; I = current

Note: The factor of 2 is utilized for single-phase circuits to represent the to-from of the conductors and root(3) for three-phase circuits. Basic Voltage Drop Calculation (Disregarding Skin-Effect)

The factor of 2 is utilized for single-phase circuits to represent the to-from of the conductors and root(3) for three-phase circuits.

Example (Disregarding Skin-Effect):

  • (6) duplex receptacles connected to a 120V AC copper branch circuit (single-phase). Per NEC, at 180VA per receptacle, the total load is 1,080VA (9A).
  • What is the conductor size required for the homerun conductors if the circuit is 1200 feet long (one-way length)?
    • Assume NEC loading of 9A will be the demand current
    • Max voltage drop of 3% (branch circuit): 120V x 0.03 = 3.6V
    • Option 1:
      • (2 x 12.9 x 1200 x 9 ) / 3.6 = 77,400 cmil
      • Using NEC Ch 9, Table 8: #1 AWG
    • Option 2:
      • [3.6 / (2 x 1200 x 9) ] x 1000 = 0.167 ohms per 1000 feet
      • Using NEC Ch 9, Table 8: #1 AWG

Voltage Drop Calculation (Skin-Effect Considered)

For circuits 1/0 AWG and over, DC resistance values are not sufficient. Reactance must be considered. Except for long distance transmission/distribution circuits, capacitance is negligible. Thankfully, there method to calculate voltage drop on these circuits is not overly complex – simply calculate the effective impedance and utilize Ohm’s Law.

Effective Impedance

Ze = R x PF x [ XL x sin[arcos(PF)] ]

Where: Ze = effective impedance; R = conductor 1 foot resistance (NEC Ch 9, Table 9);

PF = power factor; XL = conductor 1 foot inductance (NEC Ch 9, Table 9)

The factor of 2 is utilized for single-phase circuits to represent the to-from of the conductors and root(3) (1.732) for three-phase circuits.

Example (Considering Skin-Effect):

  • 130A demand load for a 208V AC copper feeder (three-phase)
  • What voltage drop if the circuit is 1200 feet long (one-way length) using 1/0 AWG copper conductors?
    • Assume 0.9 power factor and EMT conduit
    • Max voltage drop of 2% (feeder circuit): 208V x 0.02 = 4.16V
    • NEC Ch 9, Table 9: R = 0.12 per 1000 feet; XL = 0.055 per 1000 feet
      • Ze = 0.132 per 1000 feet
    • Ohm’s Law: Vd = 130 x 1.732 x 0.000132 x 1200 = 35.7 V (17.14% Voltage Drop)
      • As this is beyond the 2% maximum, one should re-work the equations and solve for the minimum conductor Ze

Tools for Voltage Drop Calculations

Although performing calculation by-hand is useful to confirm your understanding or back-check work, manual calculations are not practical at scale. Further, calculations are revisited as electrical designs progress and change overtime resulting in continuous re-work of the voltage drop calculations. In lieu of pencil and paper case-by-case calculations, many elect to utilize design tools for calculating voltage drop, such as:

  • Spreadsheet calculators using VLOOKUP commands or VBA Programming
  • Engineering departments often develop standard voltage drop length charts
  • Power system calculation software if load data is inputted by the user
    • Example: SKM, E-Tap, EasyPower, Revit, AmpCalc, etc.
  • Manufacturer slide charts and websites
  • Mobile phone applications
    • Nebulous LLC’s VDrop iOS (FREE) application used for examples: iOS Apps
  • Bussmann method

Disclaimer

All information provided is for reference only (as-is and as-available), without guarantee of accuracy or otherwise.  In no way is this technical information able to consider your specific design considerations. Perform due diligence and consult a licensed professional as required.

References

  1. Hickey, R., 2002. Electrical Construction Databook. 1st ed. New York: McGraw-Hill, pp.Section 11.
  2. Hauck, J., 2011. Electrical Design Of Commercial And Industrial Buildings. Sudbury, Mass.: Jones and Bartlett Publishers, p.31.
  3. Miller, C., 2005. Illustrated Guide To The National Electrical Code. Australia: Thomson/Delmar Learning, pp.202-206.
  4. Clark, W., 1998. Electrical Design Guide For Commercial Buildings. New York [etc.]: McGraw-Hill, p.7.
  5. 1976. IEEE Recommended Practice For Electric Power Distribution For Industrial Plants. [New York]: Institute of Electrical and Electronics Engineers, pp.63-70.
  6. 1974. IEEE Recommended Practice For Electric Power Systems In Commercial Buildings. (New York): Institute of Electrical and Electronics Engineers, pp.68-79.
  7. National Fire Protection Association. (2016). NFPA 70: National Electrical Code (2017th ed.). Quincy, Ma.: Chapter 9, Tables 8 and 9

Revision: 5-19-2020