Determining the ampacity of conductors is never easy. There are a lot of factors to consider, including derating and overcurrent protection. This article shows how to design a circuit for a practical medium voltage example.
Example: A feeder circuit serves 100A of continuous load and 50A of noncontinuous load at 15kV. The circuit utilizes single conductor aluminum cable pulled in a triplexed configuration. Routing near the source and load termination points is via aboveground conduit in a 40 Celsius ambient. Routing in the middle of the circuit is via underground conduit in a 25 Celsius ambient. The conduit is rated for a contact temperature of 105 Celsius. Determine the required conductor size and overcurrent protection device rating.
Solution: The first step is to identify the required load current for sizing when terminating, I'. This current is based on NEC Article 215's requirement to size to 125% of the continuous load and 100% of the noncontinuous load before adjustment factors.
I' = 1.25 * 100 + 50 =175A
Next, we have to determine the relevant ampacity table from NEC Article 311 (in the 2020 edition of the Code). Conductors are routed in aboveground conduit near their termination points, which means that 311.60(C)(74) is applicable. Before any adjustment factors, the 90 Celsius ampacity column requires a minimum of 2/0 AWG Aluminum.
We must also evaluate the ampacity for conditions-of-use throughout the circuit. Since the conduit is rated for 105 Celsius, we will assume that industry-standard MV105 cable is used and we can begin our derating from the 105 Celsius ampacity value. Feeder circuits are required to carry 100% of the continuous and noncontinuous load after derating. In this case, that is 150A. Since medium voltage systems aren't required to be protected at their ampacity, no additional multiplication factors on the sizing current are required.
When routed aboveground in a 40 Celsius ambient, there is no additional derating required. Referencing the 105 Celsius column, the minimum conductor size required per 311.60(C)(74) is 1 AWG Aluminum.
311.60(C)(78) is the ampacity table for underground conduit routing with (3) single insulated aluminum conductors. When routed in underground conduit in a 25 Celsius ambient, we must derate for ambient temperature. The applicable derating factor is .97 using the standard equation. A 1/0 AWG Aluminum conductor can carry 160A at 105 Celsius after derating.
Comparing all of the conditions-of-use and the termination requirements, the terminations represent the worst-case scenario. 2/0 AWG Aluminum is required.
The overcurrent protection device needs to be rated for a minimum of 175A per the continuous and noncontinuous load. If the device is rated for operating at 100% of it s ratings, this minimum becomes only 150A. The actual device size can be much larger, as long as engineering calculations show that withstand and overload are properly accounted for in the protection system.
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