Section 3: Circuit Design

Introduction

Once the service type is determined, the circuits can be designed. This section covers the major considerations in the design of roadway lighting circuits. Designers should refer to the
National Electrical Code (NEC) Handbook
 for additional information.

Voltage Drop

The primary limitation on the length of circuits is the voltage drop. “Calculating Voltage Drop,” Section 4 of this chapter, explains how to calculate voltage drop.

Circuit Length Limits

At 480 volts, a 4,000 foot circuit of twin-arm poles can be served without unduly large conductors. At 240 volts, the circuit is limited to about 2,000 feet. (Conductor size and voltage drop are discussed in "Circuit Design" and "Calculating Voltage Drop" of this chapter.).

Conductors and Conduit Size

Another consideration is the number of conductors that can be installed in the various sizes of conduits. Annex C of the
National Electrical Code
 shows the maximum number of conductors allowed in each conduit based on the capability of the wire to dissipate heat. However, with conduit runs of any substantial length, it is usually not practical to install the maximum number of conductors allowed in the conduit. A good rule of thumb is to limit the number of conductors to approximately one-half of the number shown in Annex C.
For underground lighting conduit, TxDOT's standard practice is to use 2 inch PVC. Although this is usually larger than required by the NEC for typical TxDOT lighting circuits, the larger size conduit is easier to clean and to pull conductors through. Since the largest cost of trenched conduit is the trench, 2-inch trenched conduit has a similar cost to smaller sizes of trenched conduit.
For underground lighting conductors, TxDOT's standard practice has previously been to use a minimum size of 8 AWG copper. However, it is now recommended to use the smallest size conductor that the NEC and voltage drop will allow, with a minimum size of 12 AWG. Using the minimum size conductor gives three advantages:
  • Lower cost of installation
  • Reduced consequences of copper theft. Smaller conductors reduce the scrap value of the copper and can help to reduce the incentive to steal it. If the conductors are stolen, the smaller conductors are less expensive to replace.
  • Increased safety due to the possibility of reduced arc flash. Smaller conductors have higher resistance, which can reduce arcing if they are accidentally shorted or grounded during maintenance.

Conduit Run Length

As conduit run length and number of bends increases, the effort required to pull conductors also increases. Generally continuous conduit runs in excess of 700 feet are not recommended. Continuous conduit runs in excess 500 feet are not recommended when bends total 180 degrees. The NEC limits total conduit bends between pulling points to 360 degrees (no matter what the length of the run).

Overcurrent Protection

The ampacity of conductors is shown in Table 310.15(B)(16) and accompanying notes of the
National Electrical Code
. This ampacity should be observed in sizing overload protection for the circuit.
Lighting is considered a continuous load. NEC 210.20 requires that for continuous loads, the rating of the overcurrent device shall be at least 125% of the actual lighting load. For example, if the actual lighting load is 18 amps, then the minimum breaker size would need to be at least 1.25 x 18 amps = 22.5 amps. Since that is not a standard size, the next higher breaker size of 30 amps would be used.
When sizing overcurrent protection, the device closest to the load should be the smallest size. Devices should increase in size toward the service disconnect.
For example, a 10 amp fuse might be used in the pole base where the branch circuit breaker is 20 amps, and the main breaker is a 60 amp circuit breaker.

Grounding Conductor Size

An equipment grounding conductor (EGC) should be installed in every conduit. The EGC may be sized according to NEC table 250.122, or it may be the same size as the largest current-carrying conductor in the conduit. For traffic signal installations, the minimum size of the EGC in every conduit is 8 AWG.

Copper Theft

Theft of copper conductors from roadway electrical installations is an ongoing problem for TxDOT and many other public entities. Replacing stolen conductors can have a high cost in materials and manpower. Copper theft also leaves the installation inoperable, which decreases safety for drivers.
Although there is no universal solution to prevent all copper theft, three strategies can be used to help deter it. Law enforcement can reduce theft by taking thieves off the street and punishing those involved. Engineering methods can be used to make it more difficult for thieves to access the wiring. Reducing the value of conductors by using smaller sizes or alternate materials can help reduce the incentive for theft. Details on the three strategies follow.
Law Enforcement:
  • Texas Penal Code, Section 31.03, makes it a state jail felony to steal aluminum, bronze, copper, and brass up to a value of $20,000. This includes aluminum and copper conductors.
  • Texas Occupations Code, Chapter 1956, requires scrap and recycling yards to collect information on sellers of scrap wire, including driver's license or official ID, contractor license, thumbprints, vehicle information, and photos or videos of the person selling the scrap.
Engineering Methods:
  • Locking ground box covers, handhole covers, and T-base covers are available from several manufacturers.
  • Eliminate junction boxes during design to reduce the number of access points to conductors.
  • Ground boxes can be hidden a few inches underground. If this is done, then a method to locate them for maintenance is needed.
  • Tack weld bolts to metal junction box covers. The tack weld needs to be removed for maintenance inside the box.
Reduce Value of Conductors:
  • Use the smallest size conductor that will work according to the design and the NEC. The smaller wire is less expensive to replace and has less scrap value.
  • Aluminum conductors have lower scrap value than copper, but the NEC has restrictions on the use of aluminum conductors that makes it mostly unsuitable for underground wiring. NEC 250.120(B) does not allow bare aluminum equipment grounding conductors to come in contact with the earth or to be used in corrosive conditions. Also NEC 250.120(B) does not allow aluminum equipment grounding conductors to be terminated within 18 inches of the earth. For these reasons, aluminum conductors are usually not recommended for underground wiring of illumination.