Voltage Drop Examples: Sizing Equipment Grounding Conductors for Voltage Drop

Dive into the intricate process of sizing Equipment Grounding Conductors. This comprehensive guide helps you grasp the basics of EGCs, their installation protocols, and the significance of Overcurrent Protection Device (OCPD) in determining their size. We also discuss the calculation of EGC size considering voltage drop, all while adhering to NEC specifications.

Equipment Grounding Conductor Basics

The Equipment Grounding Conductor, or EGC, is an important part of the electrical system. The goal of the EGC is to provide a ground to any metal parts of equipment that do not carry a current that could become energized due to a fault condition. The EGC provides a safe path for ground fault current to flow. Comparatively, NEC 250.110 and 250.112 both cover the specifics. Let’s break into some voltage drop examples to show how we deal with extremely long feeders.

Installation Requirements of the Equipment Grounding Conductor 

The equipment grounding conductor can be either an approved metallic conduit or a dedicated conductor run with the circuit or feeder conductors in the conduit. During this article, we will be discussing the wiring method of grounding conductors. NEC 250.118 discusses all the requirements when using the conduit as an EGC.

Sizing of Equipment Grounding Conductor

The sizing of the equipment grounding conductor depends on the Overcurrent Protection Device (OCPD) that protects the conductors of the electrical circuit. Essentially, the circuit breaker or fuse size ahead of the circuit or feeder determines the size of the EGC to be used.

NEC Table 250.122 serves as the reference table for sizing the EGC, providing essential guidelines for proper EGC selection. Match the fuse size or circuit breaker size used to determine the appropriate size of the copper or aluminum EGC.

NEC 250.122 (C): Multiple Circuits

Where a single equipment grounding conductor is run with multiple circuits in the same racewaycable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the racewaycable, or cable tray. Equipment grounding conductors installed in cable trays shall meet the minimum requirements of 392.10(B)(1)(c).

In a single raceway with multiple circuits, size the EGC based on the largest OCPD size.

Voltage Drop Example 1:We have a two-inch raceway with a 30 amp circuit, a 60 amp circuit, and a 100 amp circuit. What size EGC would we need?

Step 1: Find the largest OCPD in the raceway: 100 amps.

Step 2: Use table 250.122 to find the EGC based on 100 amps: 8-AWG copper or 6-AWG aluminum. 

Sizing Equipment Grounding Conductors With Parallel Feeds

NEC 250.122(F) covers the requirements for conductors in parallel and equipment grounding conductor requirements. 

  • NEC 250.122(F)(1)(a) Conductors in parallel but run in the same raceway: a single wire type equipment grounding conductor, sized based on Table 250.122
  • NEC 250.122(F)(1)(b) Conductors in parallel but run multiple raceways: a single wire type equipment grounding conductor shall be installed in each raceway sized based on Table 250.122.
This is from NFPA Handbook: https://link.nfpa.org/publications/70/2023/chapters/2/articles/250#ID000700001381

Sizing Equipment Grounding Conductors Based on Increase of Conductor Size Due to Voltage Drop

Increasing the conductor length leads to an increase in voltage drop along the conductor. To mitigate the overall voltage drop, one can increase the size of the conductors.

  • Per NEC 250.122(B), wire-type equipment grounding conductors shall increase in size proportionately to the increase in circular mil area of each ungrounded conductor.

Voltage Drop Example 2: A 250 amp load is supplied by a 250 amp breaker 500 feet away. The conductors are 250kcmil with a 4-AWG copper EGC. With a conductor size of 500kcmil, what is the appropriate size for the EGC?

METHOD 1 using cross-sectional area:

  1. Step 1: find the ratio of existing conductors. 500/250 = 2
  2. Step 2: find the cross-sectional area of 4 AWG EGC from Table 8 in Chapter 9 of NEC: 41,740 cmil
  3. Step 3: find the cross-sectional area of the new EGC = 41,740 cmil (4-AWG) x 2 = 83,480 cmil
  4. Step 4: use table 8 in chapter 9 of NEC to find the new GEC size based on 83,480 cmil: 1-AWG

Curious to learn more about voltage drop? Checkout Electrician U’s YouTube video to learn more:

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