Evaluation of Software’s for Ampacity Calculations Thermal Method (Neher-McGrath)

Evaluation of Software’s for Ampacity Calculations Thermal Method (Neher-McGrath) Edwin W. Marquez Engineering Manager in Southwire Company Electrical Engineer National Polytechnic Institute (IPN) Higher School of Mechanical and Electrical Engineering (ESIME - Zacatenco) Email: edwin.marquez@southwire.com

Abstract - This document presents the bases of the thermal method “Neher-McGrath� for ampacity calculations on cables. Also shows ampacity results of the evaluation of three different software’s used for ampacity calculations such as: AmpCalc, Etap and CYMCAP based on the NEC table 310.60(C)(77).

The current passing through a conductor produces losses in the form of heat, which elevate the temperature in the conductor. This heat must pass thru the cable insulation, the air in the raceway, and the raceway itself to the surrounding elements, where it is dissipated into the air by radiation or convection.

Figure 1 – Elements involved in the heat transfer

The Neher-McGrath Formula The abbreviated formula is shown in the NEC (National Electrical Code) in 310.15(C). It is a heat transfer method to determinate cable ampacity or cable current cable capacity. The full formula is a series of heat transfer calculations taking in consideration all heat sources and the thermal resistances between the heat sources and the free air.

The most common use of the Neher-McGrath formula is for calculation of cable ampacity in underground electrical systems such as: direct burial cables, cables in direct burial ducts, duct banks or other underground raceways.

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The conductor’s ampacity is the ability to dissipate heat through the thermal resistances of the surroundings materials of the conductor. Hence the thermal resistances of all components that shall dissipate heat must be known, for example: Insulation of the conductor: The thermal resistance value depends of the thickness of the insulation and the type of materials like thermoplastic PVC, CPE, TPE or thermoset XLPE or EPR. The insulation material also depends of the cable application and the thickness change depending the cable size and the voltage level.

Figure 2 – Cable Insulation

Where: Tc = Conductor temperature in degrees Celsius (°C) Ta = ambient temperature in degrees Celsius (°C) Rdc = dc resistance of 305 mm (1 ft) of conductor in microohms at temperature Tc Yc = component of the ac resistance resulting from skin effect and proximity effect Rca = thermal resistance between conductor and surrounding ambient

Duct Wall: The thermal resistance of the duct wall is based on the thermal resistivity of the type of material like PVC or Polyethylene (HDPE, MDPE, LDPE, etc.) and the thickness as for example schedule 40� or 80�.

Airspace: The thermal resistance in the airspace is based on the number of conductors in the duct, the temperature of the air in the duct. For NEC applications the number of cables in the duct must meet the NEC conduit filling guidelines.

90°C and 105°C. The ampacity of a copper 500 kcmil for 5001-35000 Volts at 90°C is 465 Amperes.

Figure 4 – NEC Table 310.60(C)77

Earth backfill: This resistance includes not only the thermal resistivity and ambient temperature of the earth but also the number of conductors inside the duct, the burial depth, the loss factor, and the mutual heating factor from other nearby systems.

Figure 3 – Heat transfer effect

Below are the installation conditions taken from the NEC to model ampacity calculations on AmpCalc, Etap and CYMCAP.

There are other factors that affect the heat dissipation with effects in the final conductor ampacity, for example: ▪ ▪ ▪ ▪

Conductor loss Skin effect heating for AC systems Hysteresis loss if the duct is steel or other metallic material Heating from other conductors, depending of the number of conductors, location, and proximity to other systems.

▪ Cable Type: Copper 500 kcmil 15 kV 133% EPR insulation with copper tape and PVC jacket ▪ Maximum conductor temperature 90°C ▪ Earth temperature 20°C ▪ Earth thermal resistivity 90 ▪ Load factor 100% ▪ Duct Bank thermal resistivity 85 ▪ Metallic shield bonded in two points or both ends ▪ PVC duct 6 inches ▪ Cable Arrangement: Triplex ▪ Frequency: 60 Hz Figure 5 – Medium Voltage Power Cable Type MV-105

Ampacity Evaluation based in the NEC using AmpCalc, Etap and CYMCAP Based in the NEC (National Electrical Code) Table 310.60(C)(77) Ampacities of Three Single-Insulated Copper Conductors in Underground Electrical Duct (Three Conductors per Electrical Duct) Based on Ambient Earth Temperature of 20°C, Electrical Duct Arrangement in Accordance with Figure 310.60(C)(3), 100% of Load Factor, Thermal Resistance (RHO) of 90, Conductor Temperatures of

AmpCalc Version 4.0 AmpCalc for Windows is a computer program from CalcWare which calculates power cable ampacity ratings and/or cable operating temperatures for user defined cable installations.

Ampacities are calculated for single or multiple underground systems including: ▪ ▪ ▪

Etap also offer Cable Thermal Analysis based in the below methods: ▪ ▪

Duct bank Direct buried cable Direct Buried conduit

Neher-McGrath Method IEC 60287 Method

The Cable Thermal Analysis features on Etap are: Calculations are in accordance with the industry accepted Neher-McGrath calculation procedures.

AmpCalc is suitable for cable installations utilized in industrial, commercial, and utility electrical power distribution systems. The software is suitable for use with Windows 7, 8.1 or 10.

After set the NEC conditions from Table 310.60(C)(77) and knowing that AmpCalc use the Neher-McGrath method to perform the calculations the result is 472.7 Amps @ 90°C.

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Figure 6 – Ampacity result using AmpCalc

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Graphical user interface Graphical manipulation of raceways, cables, conduits, etc.

Drag and drop cables from one-line diagrams Cables of different sizes in the same raceway Separate phases into different conduits or locations Unsymmetrical positioning of raceways Transient calculations use a dynamic thermal circuit model Grounded/ungrounded shielding Calculate thermal resistances, dielectric losses, and Yc, Ys, factors, etc.

User-defined armor cables Unbalanced load factors Multiple duct banks and direct-buried conduits Place raceways in multiple cross-sections

Setting the NEC conditions from Table 310.60(C)(77) and using the Neher-McGrath method the result from Etap is 477.5 Amps @ 90°C. Figure 7 – Ampacity result using Etap

Etap 19.5.0 Etap is a powerful tool to calculate cable ampacity or current carrying capacity, cable sizing, shock protection for LV and MV Cables. ETAP electrical engineering software offers a comprehensive set of cable current carrying capacity / ampacity and sizing modules based on the following standards and guidelines: IEC 60502, IEC 60092, ICEA P-54440, BS 7671, IEC 60364, NEC NFPA 70, and IEEE 399.

CYMCAP 7.2 rev 1 Figure 8 – Cross sectional cable view in CYMCAP

The CYMCAP software is dedicated to performing ampacity and temperature rise calculations for power cable installations. Determining the maximum current power cables can sustain without deterioration of any of their electrical properties is important for the design of electrical installations.

It addresses steady-state and transient thermal cable rating as per the analytical techniques described by Neher-McGrath and the International Standards IEC 60287 and IEC 60853.

This software was developed by Ontario Hydro (Hydro One), McMaster University and CYME International, under the auspices of the Canadian Electricity Association.

Using CYMCAP software with the NEC Table 310.60(C)(77) conditions for thermal analysis, the result is 477 Amps @ 90°C.

The Analytical Capabilities of CYMCAP are: Figure 9 – Ampacity result using CYMCAP

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Iterative techniques based on Neher-McGrath and IEC-60287 methods Detailed graphical representation of virtually any type of power cable. This facility can be used to modify existing cables data and enrich the cable library with new ones. This includes single-core, three-core, belted, pipe-type, submarine, sheathed, and armored cables Different cable installation conditions such as directly buried, thermal backfill, underground ducts or duct banks.

Pipe-type cables directly buried or in a thermal backfill Independent libraries and databases for cables, duct-banks, load curves, heat sources and installations Modeling of cables in air on riser poles, groups of cables in air, moisture migration, nearby heat sources and heat sinks, etc.

Cyclic loading patterns as per IEC-60853 Multiple cables per phase with modeling of the sheath mutual inductances which greatly influence circulating current losses and thus derates the ampacity of cables All types of sheath bonding arrangements for flat and triangular formations are supported with explicit modeling of minor section lengths, unequal cable spacing, etc.

CONCLUSION: The three Software’s evaluated on this document provide closing results using the same thermal method, the results are 2%-3% over the ampacity data from the NEC Table 310.60(C)(77).

AmpCalc offer a very simple interface and is focused only on ampacity calculations under thermal analysis for underground and aerial installations.

Etap is an integrated model-driven tool for design, simulation, analysis, optimization, monitoring, operation, and automation of electrical power systems, with different modules like data collection, network modeling, power flow analysis, short

circuit, protection and coordination, arc flash, ground grid systems, etc.

CYMCAP is the more used software to perform thermal analysis calculations because offer multiple analytical capabilities for installations such as: multiple duct banks, cables in tunnels, cables in troughs, multiple casings and the analysis can be performed for duct bank optimizer, magnetic fields, cable impedance and electrical parameters for cables, short circuit and circuit crossings.

The obtained results of the assessment of these three Software’s are show in the Table 1.

References: 1) NFPA 70, National Electric Code, 2017 2) IEEE Std 835 Power Cable Ampacity Tables 3) http://neher-mcgrath.com/ 4) https://www.calcware.com/ 5) https://etap.com/product/cable-thermalsoftware 6) http://www.cyme.com/software/cymcap/ 7) https://www.skm.com/cable-ampacity.html 8) https://www.cableizer.com/

Table 1 - Results NEC Table 310.60(C)(77) 465 Amps AmpCalc 472.7 Amps @ 90°C Etap 477.5 Amps @ 90°C CYMCAP 477 Amps @ 90°C These Software’s offer you the ability to perform any ampacity calculation with installation conditions or configuration does not included in the NEC. There are other two software’s available in the market to perform cable ampacity calculation based on thermal methods one is SKM from Systems Analysis, Inc and Cableizer that is an online cable application for thermal analysis.