Sizing Equipment Grounding Conductors for Voltage Drop
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 which could become energized due to a fault-condition. The EGC provides a safe path for ground fault current to flow. NEC 250.110 and 250.112 both cover the specifics.
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. We will discuss the wiring method of grounding conductors in this article. NEC 250.118 discusses all the requirements when using conduit as an EGC.
Sizing of Equipment Grounding Conductor
The equipment grounding conductor is sized based on the Overcurrent Protection Device (OCPD) protecting 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: This is the table used to size the EGC. Match the fuse size or circuit breaker size being used to find either the copper or aluminum EGC size.
When running a single equipment grounding conductor in the same raceway, cable, or cable tray, it shall be sized by the largest overcurrent device protecting conductors in the raceway, cable, or cable tray (NEC 250.122(C)). When running multiple circuits in a single raceway, the EGC is sized based on the largest size OCPD. Let’s take a look at an example.
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?
Step1: Find the largest OCPD in the raceway: 100 amps.
Step2: 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.
- Conductors in parallel but run in the same raceway: a single wire type equipment grounding conductor, sized based on Table 250.122.
- 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.
Sizing Equipment Grounding Conductors Based on Increase of Conductor Size Due to Voltage Drop
As conductor length is increased, the voltage drop along the conductor will also increase. To account for this, conductors can be increased in size to lower the overall voltage drop. When doing this, the size of the equipment grounding conductor must be considered.
- 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.
Example: A 250 amp load is supplied by a 250 amp breaker 500 feet away. The conductors are 250kcmil with a 4-AWG copper EGC. What size EGC would be needed if the conductor size is increased to 500kcmil?
METHOD 1 using cross sectional area:
- Step1: find ratio of existing conductors. 500/250 = 2
- Step2: find cross sectional area of 4 AWG EGC from table 8 in chapter 9 of NEC: 41,740 cmil
- Step3: find the cross-sec area of the new EGC = 41,740 cmil (4-AWG) x 2 = 83,480 cmil
- Step4: use table 8 in chapter 9 of NEC to find new GEC size based on 83,480 cmil: 1-AWG
