Applying National Elelctric Code Class 2 Power for Remote DAS and Small Cells
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In today’s urban environment the ubiquitous macro cell with its highly visible tower is no longer a welcome sight, nor is it able to support the explosive growth in demand for wireless data and capacity. In many markets small cells and distributed antenna systems (DAS) are being deployed to add wireless data capacity and fill these coverage gaps.
In today’s urban environment the ubiquitous macro cell with its highly visible tower is no longer a welcome sight, nor is it able to support the explosive growth in demand for wireless data and capacity. In many markets small cells and distributed antenna systems (DAS) are being deployed to add wireless data capacity and fill these coverage gaps.
Locations that require improved capacity and coverage include densely populated office buildings, college campuses and stadiums, where users are rapidly forgoing traditional information technology (IT) equipment in favor of their wireless devices.
Power Challenges
Users expect wireless systems to be available – always! Loss of utility power does not excuse loss of service, so backup power is essential. Providing backup power, via uninterruptible power supply (UPS) modules to large numbers of small cells and DAS remotes, can be expensive both from infrastructure and maintenance standpoints. Being able to supply uninterruptible power from a central location significantly reduces maintenance costs, with a nominal increase in infrastructure cost to get power to the required locations.
Transmitting DC power to small cells or DAS remote locations from a central location requires the use copper wires over long distances, which incur resistance losses. Use of a higher voltage enables longer reach for modest amounts of power. Fortunately most small cells do not require large amounts of power.
Low Voltage Approaches
Power delivery to remote equipment at low voltage DC, typically 48 volt, can be achieved using National Electric Code (NEC) safety practices designated “Class 1 or 2.”
Class 1 circuits are not limited in the amount of power allowed in each circuit, but circuits must be installed in a protective conduit for fire and safety reasons. Class 1 circuits can carry higher currents and reach longer distances, but are more expensive, both in material costs and installation.
Class 2 circuits are limited by NEC specifications to 100 watts of DC power per circuit for fire and safety purposes, but can be installed without a protective conduit, significantly reducing material and installation costs. Each circuit requires a protective device, typically an active current limiter, which can limit total power to 100 watts per circuit.
For loads requiring less than 70 W, the 100 W power limited circuit may have sufficient reach. The power limitation can become restrictive when longer reaches are required or higher power loads are used. Multiple circuits cannot be directly paralleled to provide additional power or reach, as this would negate the Class 2 safety features by allowing larger potential fault currents. Multiple circuits may be run in parallel, however, if a combiner circuit is used to isolate fault currents from parallel circuits. This can significantly increase the load power that can be supplied or increase the reach of the combined circuits.
Reach is also limited by the need to make allowances for the discharged voltage of the batteries used to provide backup power. Reach must be calculated based on the “end of discharge” voltage of the battery. Typically this would be 42 volts for a nominally 48-volt battery. Use of a “boost converter” that maintains the line voltage at a constant 58 volts can more than double the effective reach of a Class 2 circuit (see Figure One).
Figure One – Voltage booster and current limiter used to implement Class 2 power delivery circuits
Figure Two – Reach of a Class 2 circuit, with and without voltage booster
For Class 2 circuits, it can be seen from Figure Two, that a load of 50 W can be located at about 1,300 feet from the source without the use of the voltage booster, whereas the same load could be located at a distance of 4,400 feet with the voltage booster. Not having to install the cable in a protective conduit is estimated to save around $8 per foot, totaling over ten thousand dollars for a 1,300 foot cable run.
For larger loads, Class 2 circuits can still be used, but to maintain the integrity of the Class 2 safety requirements circuits cannot be paralleled without the use of a combiner (see Figure Three).
Figure Three – Class 2 circuits cannot be directly paralleled
When an appropriate combiner is used calculations are straightforward; a 100 W load would require two circuits at 1,300 feet. The combiner used to parallel circuits can be used with or without the boost converter that extends reach.
Conclusion
Providing uninterruptible power to a proliferation of small loads requires many UPS units or the ability to transmit DC power over significant distances from a centralized location. The use of NEC Class 2 circuits is a cost effective way to transmit power over long cables while minimizing the installation cost by avoiding conduit.
The architecture typically used by DAS and small cell systems, with a centrally-located data processing site, lends itself very well to this power architecture. In most cases, the remote equipment requires a fiber optic cable connection for data communications. This means that installation can be further simplified by the use of a hybrid fiber cable, which contains multiple fibers along with copper conductors used to carry the power. Installation of a single, hybrid cable, without the need for conduit, is all that is required for these cases. This type of installation offers low cost, high reliability and low maintenance.
Power and reach limitations can be overcome by using multiple circuits and appropriate combiner circuitry to power larger loads at extended distances, without jeopardizing safety or reliability.
