Electrical Equipment Design Notes and Cable Sizing

Electrical equipment and cables shall be sized in accordance with the requirements of NFPA 70. In addition, the following shall be applied when determining equipment ratings and cable sizes. Main keywords for this article are Electrical Equipment Design Notes and Cable Sizing. Transformer Capacity. How to Select Switchgear. Motor Control Center.

Definitions

Eight Hour Maximum Demand

The eight hour maximum demand of loads is defined as the greatest rootmean-square value of the load during any eight hour period. It is the equivalent thermal aging load.

Fifteen Minute Maximum Demand

The fifteen minute maximum demand of loads is defined as the greatest average load which can occur for a fifteen minute period.

Firm Load Data

Firm load data is the load data derived from actual equipment performance characteristics and duty cycles.

Adjusted Maximum Demand Based on Firm Load Data

The adjusted maximum demand based on firm load data is equal to 1.0 times maximum demand.

Load Factor

The load factor is to the ratio of the average load over a designated period of time to the peak load occurring in that period.

Demand Factor

The demand factor is the ratio of the maximum demand of a system, or part of a system, to the total connected load of the system, or part of the system, under consideration.

Electrical Equipment Design Notes

What should be Transformer Capacity in Process Industry

An eight hour maximum demand shall be the basis for selection of transformer capacity.
When synchronous motors with a leading power factor are connected to a bus, the resulting leading kVAR shall be considered when calculating the transformer kVA capacity.
The initial design load connected to a transformer shall not exceed the following values:
a. The 55°C ONAN rating (ambient temperature 50°C) of transformer (except in case of secondary selective system with forced cooled rated transformers).
b. The 55°C forced cooling rating (ambient temperature 50°C) of transformer in case of secondary selective system.
c. Ninety percent of the ONAN or forced cooled rating on transformers with a single temperature rise.
Power transformers shall be specified with winding temperature rise requirements as per SES E04-S01.
Radial System
Liquid-filled transformers installed as individual units supplying a radial type load, shall have self-cooled ratings sufficient to supply the following:
a. In radial and primary selective substations, transformer self-cooled capacity shall be equal to or greater than 1.15 times the adjusted maximum demand
b. Peak demand of one-hour duration or less shall not exceed 110% of the transformer 55 C self-cooled rating c. The minimum transformer size for non-essential service shall be 750kVA for units with a 480 V secondary and 3750 kVA for units with 4160 Vand above secondary
d. The minimum transformer size for essential service (MCC) shall be 300kVA for units with 480 V secondaries
Secondary-Selective Systems
a. For a secondary selective system, if one transformer is out of service, the remaining transformer shall have sufficient capacity at the highest temperature rise forced-cooled FA rating to serve the maximum designed load of both the buses with the bus-tie breaker closed.
b. The minimum transformer size shall be 1000 kVA for units with a low voltage secondary and 5000 kVA for units with a medium voltage secondary
Captive transformers
Captive transformers for motors shall be thermally rated and mechanically braced for three repetitive motor starting duty cycles. kVA and impedance shall be selected for starting the motor and for limiting the voltage dip on the bus to which the transformer primary is connected.
Transformer impedance shall be in accordance with ANSI C52.12.10, Table 10 except for the following:
a. To meet voltage drop limitations
b. To realize economics by the use of switchgear with lower interrupting ratings

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How to Select Switchgear Rating

A 15 minute maximum demand shall be the basis for selecting switchgear continuous ratings.
The main breaker and the main bus of the switchgear shall have a current rating equal to or greater than the highest current rating of the transformer that feeds the switchgear.
Bus tie circuit breakers in secondary selective and spot network substations shall be interchangeable with the incoming breakers.
Each feeder breaker shall have a continuous current rating equal to the higher of the 15 minute maximum of the feeder load or 100% of the full load currents plus 25% of largest motor current. Loads shall include any provisions for future loads.
Circuit breakers used to control a single motor shall have a continuous rating at least equal to 1.25 times the motor full load current.
Generator breakers shall have a continuous rating at least equal to 1.15 times the maximum continuous generator rating.
Circuit breaker interrupting rating shall be adequate for the maximum short circuit level.
Switchgear momentary rating shall be at least adequate for the maximum short circuit level, with estimated motor contribution, which would be expected when the transformer is loaded to its self cooled rated capacity.

https://electrical-engineering-portal.com/design-installation-medium-voltage-switchgear

Motor Control Center (MCC) Bus Rating

480V MCCs shall have a main horizontal common power bus rated for continuous current rating of 600, 800, 1000, 1200 amperes or higher as required for a particular application. Vertical bus installed in a section shall have a minimum continuous current rating of 300 amperes.
4160V MCCs shall have a main horizontal bus rated for minimum 1200 amperes continuous. The maximum load shall equal the sum all the full loads of normally operating motors and process heaters.

Battery and Charger Capacities

The 125 VDC systems supplies control power to switchgear, Excitation power backup for synchronous motors, MCC protective relays, annunciator, etc. The system is composed of dual, 100% redundant battery chargers and 125 VDC battery bank.
The batteries are sized to provide power to trip the maximum number of breakers that might trip once, after supplying power to indicating lights, relays, etc. for four hours with both battery chargers off. An example would be a bus differential tripping the main breaker, the tie breaker, and all feeder breakers on one bus.
Each battery is sized to recharge the fully discharged battery bank in 8 hours.

Uninterruptable Power Supply Capacities

The uninterruptable power supply (UPS) supplies power to distributive control system, instruments, I/O loop power supplies, etc. The UPS is 100% redundant systems. The UPS has 480 V, 3 phase input, 120 V, single phase output.
The batteries for UPS shall be sized to deliver full rated output for 30 minutes operation in case of a single UPS system and 60 minutes (if there are two batteries each 30 minutes capacity) operation in case of parallel or dual redundant UPS system.
The battery charger is sized to recharge the batteries in six hours while the UPS is carrying 100% load.

Cable Sizing in Plants Industry

Cable sizing shall comply with the NEC. Conductor sizing shall take into account ambient temperature, continuous operating load, non-continuous loads, spared loads, spared loads, future loads, conduit fill, tray fill, tray covers, and other factors that may influence the allowable current rating.
For all cables other than transformer feeders, the eight hour maximum demand shall be the basis for cable capacity calculation.
Transformer feeders shall have a capacity not less than the transformer fancooled rating. When a feeder supplies more than one transformer, its rating shall be at least equal to the summation of the fan-cooled ratings of all secondary selective and spot network substations plus the self-cooled rating of all radial substation transformers supplied by the feeder.
Lighting feeders feeding lighting panels shall have a capacity not less than the maximum demand of the load.
Lighting and power wire and cable shall be derated under any of the following conditions:
a. Outdoor ambient temperature of 50 °C
b. More than three conductors in a raceway or cable
c. More than one cable in an underground duct bank or trench
d. More than one cable in a cable tray
e. Cables shall be sized to limit voltage drop
Motor branch circuit conductors shall be sized in accordance with Article 430 of the NEC.
Cable capacity for tray installation shall comply with the NEC including the effects of ambient temperature and solar radiation.
Feeders rated above 600 volts shall be sized to withstand short circuit thermal stress without damage to the feeders. The maximum short circuit level of the supply, times a 1.25 safety factor, and the clearing time of the feeder protective device shall be used to determine this condition.
Power cables in underground conduit banks shall be derated in accordance with the Neher-McGrath method in order not to exceed the conductor insulation temperature specified by the cable manufacturer. Cable capacity calculation shall take into account the applicable de-rating factors, such as for ambient temperature, soil thermal resistivity, depth of laying and grouped
installation of cables.
a. For sizing of underground cables, use the following criteria:
Earth temperature = 35 °C
Earth thermal resistivity = 120 °C cm/W
Selected backfill thermal resistivity = 120 °C cm/W
Load factor = 1.0
Cable sizing calculations shall include the 20% future cables installed in the top positions of the duct bank or trench. The rating shall be for the worst case location along the route, i.e., exiting the substation, approaching another duct bank, road crossing, etc.
Medium and high-voltage cables shall be selected to coordinate with the maximum 1 second short circuit rating of the connected equipment.
Medium-voltage feeders to load-center substations shall be sized for the maximum transformer rating. This size shall include the transformer rating obtainable by forced-air cooling.
Services, feeders and branch circuits for power and lighting installations other than specified above shall be sized in accordance with the requirements of NFPA 70.