Neher Mcgrath Study

The Neher-McGrath analysis is a technique for calculating the temperature increase brought on by electrical current flow in conductors. It is highly regarded and utilized in the field of electrical power systems and is based on the laws of thermodynamics and electrical engineering concepts. By aiding in the prevention of overheating and probable conductor failure, the Neher-McGrath calculation is a crucial instrument for assuring the safe and effective operation of electrical systems.

Why perform Neher McGrath Study?

The cables that network a power system together form the backbone of the system. It is only logical, therefore, that any complete analysis of a power system should include an analysis of its cable ampacities. This analysis is complicated since the ampacity of a conductor varies with the actual conditions of use.

As stated in NFPA 70 NEC Article No. 100, Ampacity is “the current in amperes a conductor can carry continuously under the conditions of use (conditions of the surrounding medium in which the cables are installed) without exceeding its temperature rating.”

Therefore, a cable ampacity study is the calculation of the temperature rise of the conductors in a cable system under steady-state conditions.

Prominent factors on which ampacity of conductors depends are:

1. Ambient temperature: May vary along the length and with time.
2. Thermal characteristics of surrounding medium: Affects the rate of heat dissipation.
3. Heat generated by conductor due to its own losses: Load current flow including fundamental and harmonic currents.
4. Heat generated by adjacent conductors: Raising the ambient temperature and impeding heat dissipation.

To account for the various items that affect ampacities of cables, NEC 1975th edition accepted for the first time the Neher-McGrath method.

The general calculation method is very broad and can be applied to a variety of cable constructions and installation conditions as compared to the NEC tables that are limited to certain conditions.

For most installations, the ampacity tables in the NEC are adequate. Engineers, installers, and inspectors may need to use one of the many software programs available to make actual NM (Neher–McGrath) calculations in some situations where the use of the ampacity tables, including the safety margins, is insufficient.

For instance, in NEC tables, a soil temperature of 20° C at a depth of 36 inches and thermal resistivity of 90 K cm/W is considered. But this narrow range of conditions is not always applicable. What happens if there are several inches of polyurethane foam around a conduit? Yet, the addition of excessive thermal insulation will affect the ampacity of a conductor. And the southern US states often reach a soil temperature of 25° C at a depth of 36 inches reducing the ampacity by 5% or more.

Through the utilization of the Neher McGrath method, power systems engineers can construct a model of irregular setups, leading to a decreased discrepancy between the determined and actual ampacity.

Producing Neher McGrath results and Identifying Issues:

To perform the Neher-McGrath study, the following inputs are required:

Cable System Layout: Provide the layout of the cable system, including the arrangement and configuration of cables, cable lengths, and connections between cables.

Electrical Load Data: Specify the electrical load data for the cable system, such as the current magnitude and duration, power factor, and any variations in load over time. This information is crucial for determining the thermal behavior of the cables.

Cable Specifications: Provide detailed information about the cables in the system, including the type of cable (e.g., XLPE, PILC), conductor material, insulation type, conductor size, and any additional protective layers or features. The cable specifications will impact the thermal and electrical characteristics of the cables.

Ambient Conditions: Specify the ambient conditions surrounding the cable system, including the ambient temperatures, wind speed, and soil or air temperature gradients. These conditions affect the heat dissipation and temperature rise of the cables.

Grounding Configuration: Describe the grounding configuration used in the cable system, such as the type of grounding (e.g., solidly grounded, ungrounded), grounding resistors or reactors, and any additional grounding measures implemented.

Cable Installation Details: Provide information about the cable installation method, such as direct burial, duct installation, or overhead installation. This information helps assess the cable's heat dissipation and heat transfer characteristics.

System Fault Data: If available, provide information about fault currents, fault durations, and fault clearing times. This data is essential for evaluating the cable's short-circuit performance and determining its ability to withstand fault conditions.

The Neher-McGrath calculation is typically performed using computer software, such as ETAP and SKM which can quickly and accurately process the required inputs and provide the resulting temperature rise. However, manual calculations can also be performed using specialized tables and equations.

Qualified and Certified Engineers:

Providing engineering recommendations about the causes of exceptions, the integrity of the system, or recommendations for corrective actions requires knowledge and skills beyond those of power systems engineering. Our team comprises certified professional engineers specialized in analyzing Neher McGrath results, particularly in critical areas related to life safety and essential components within facilities.

At AllumiaX, we understand the significance of the Neher McGrath study and its implications for the success of your projects. As a leading service provider in the field, we are dedicated to helping you harness the power of this study and apply it effectively to your electrical system designs.

By partnering with us, you can benefit from our comprehensive range of services, which include conducting Neher McGrath calculations, providing detailed reports and recommendations, and offering practical solutions to optimize your electrical systems. Whether you are working on a small-scale project or a large-scale industrial installation, we have the knowledge and resources to support you every step of the way.

Don't compromise on the safety and efficiency of your electrical systems. Trust the experts at AllumiaX to guide you through the intricacies of the Neher McGrath study and ensure the optimal performance of your projects.

What is the customer/client responsible for?

At AllumiaX, we evaluate the power system and flag potential problems and high-risk situations while also recommending various options for resolutions and best-in-the-market practices to our customers for a safe, regulated, reliable, and well-designed power system. Clients who choose not to perform the study miss out on a huge value proposition.

Clients should help develop a list of equipment to be evaluated via onsite data collection for the existing facility or by providing updated one-lines, BOMs and cutsheets for the equipment.

What to Expect?

Neher McGrath studies will be 100% in compliance with the latest NEC, OSHA & IEEE standards. Study outcomes and recommendations are expected to improve your facility in the following areas:

• Ensuring that electrical conductors are operated within safe temperature limits: The Neher-McGrath calculation is used to determine the maximum allowable temperature rise for electrical conductors, based on the conductor material and the type of insulation. This helps to prevent overheating and potential failure of the conductors.
• Optimizing the design of electrical systems: The Neher-McGrath calculation can be used to select conductor sizes and materials that will provide the desired level of performance while minimizing temperature rise and energy loss. This is important for maximizing the efficiency and reliability of electrical systems.
• Evaluating the impact of changes to electrical systems: The Neher-McGrath calculation can be used to select conductor sizes and materials that will provide the desired level of performance while minimizing temperature rise and energy loss. This is important for maximizing the efficiency and reliability of electrical systems.
• Troubleshooting electrical issues: The Neher-McGrath calculation can be used to help diagnose and resolve problems with electrical systems, such as overheating or failures. By analyzing the temperature rise of conductors, it may be possible to identify the cause of the issue and develop a solution.

AllumiaX is a professional electrical engineering company that offers specialty engineering studies, field services, and infrared thermography.

Contact us today to learn more and get started.  

Talha Qamar

Sales and Business Development

Office: (206) 687-4009 Ext.102

[email protected]

1187 N Willow Ave #103-40
Clovis, CA 93611