Neher–McGrath
Encyclopedia
The Neher–McGrath calculations calculate underground cable temperatures or maximum current ratings. From the basic principles that electric current
leads to thermal heating and thermal power transfer to the ambient environment requires some temperature difference, it follows that the current leads to a temperature rise in the conductors. The ampacity
, or maximum allowable current, of an electric power cable
depends on the allowable temperatures of the cable and any adjacent materials such as insulation or termination equipment. For insulated cables, the insulation maximum temperature is normally the limiting material property that constrains ampacity. For uninsulated cables (typically used in overhead installation), the tensile strength of the cable (as affected by temperature) is normally the limiting material property. The Neher–McGrath method is the electrical industry standard for calculating cable ampacity, most often employed via lookup in tables of precomputed results for common configurations.
Neher and McGrath are the surnames of two electricians who wrote a famous article about how to calculate the capacity of current (ampacity) of cables. Their article is so useful that it is used as reference for the ampacity in most of the standard tables.
The equation in section 310-15(C) of the National Electrical Code, called the Neher–McGrath equation (given below), may be used to estimate the effective ampacity I of a cable. In the Neher–McGrath (NM) equation, TC is normally the limiting conductor temperature derived from the insulation or tensile strength limitations.
is a term added to the ambient temperature, TA, to compensate for heat generated in the jacket and insulation for higher voltages. 
is called the dielectric loss temperature rise and is generally regarded as insignificant for voltages below 2000 V. Another term in the Neher–McGrath equation, (1+YC), is a multiplier used to convert direct current
resistance (RDC) to the effective alternating current resistance (which typically includes conductor skin effects
and eddy current
losses). For wire sizes smaller than AWG
No. 2, this term is generally regarded as insignificant. RCA is the effective thermal resistance between the conductor and the ambient conditions, which can require significant empirical and/or theoretical effort to estimate. With respect to the AC-sensitive terms, tabular presentation of the NM equation results in the National Electrical Code was developed assuming the standard North American power frequency of 60 hertz and sinusoidal wave forms for current and voltage.
The challenges posed by the complexity of estimating RCA and of estimating the local increase in ambient temperature obtained by co-locating many cables (in a duct bank) create a market niche
in the electric power industry for software dedicated to ampacity estimation.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
leads to thermal heating and thermal power transfer to the ambient environment requires some temperature difference, it follows that the current leads to a temperature rise in the conductors. The ampacity
Ampacity
Ampacity is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration.Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device can continuously carry while remaining...
, or maximum allowable current, of an electric power cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
depends on the allowable temperatures of the cable and any adjacent materials such as insulation or termination equipment. For insulated cables, the insulation maximum temperature is normally the limiting material property that constrains ampacity. For uninsulated cables (typically used in overhead installation), the tensile strength of the cable (as affected by temperature) is normally the limiting material property. The Neher–McGrath method is the electrical industry standard for calculating cable ampacity, most often employed via lookup in tables of precomputed results for common configurations.
Neher and McGrath are the surnames of two electricians who wrote a famous article about how to calculate the capacity of current (ampacity) of cables. Their article is so useful that it is used as reference for the ampacity in most of the standard tables.
The equation in section 310-15(C) of the National Electrical Code, called the Neher–McGrath equation (given below), may be used to estimate the effective ampacity I of a cable. In the Neher–McGrath (NM) equation, TC is normally the limiting conductor temperature derived from the insulation or tensile strength limitations.
is a term added to the ambient temperature, TA, to compensate for heat generated in the jacket and insulation for higher voltages. is called the dielectric loss temperature rise and is generally regarded as insignificant for voltages below 2000 V. Another term in the Neher–McGrath equation, (1+YC), is a multiplier used to convert direct currentDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
resistance (RDC) to the effective alternating current resistance (which typically includes conductor skin effects
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
and eddy current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
losses). For wire sizes smaller than AWG
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
No. 2, this term is generally regarded as insignificant. RCA is the effective thermal resistance between the conductor and the ambient conditions, which can require significant empirical and/or theoretical effort to estimate. With respect to the AC-sensitive terms, tabular presentation of the NM equation results in the National Electrical Code was developed assuming the standard North American power frequency of 60 hertz and sinusoidal wave forms for current and voltage.
The challenges posed by the complexity of estimating RCA and of estimating the local increase in ambient temperature obtained by co-locating many cables (in a duct bank) create a market niche
Niche market
A niche market is the subset of the market on which a specific product is focusing; therefore the market niche defines the specific product features aimed at satisfying specific market needs, as well as the price range, production quality and the demographics that is intended to impact...
in the electric power industry for software dedicated to ampacity estimation.
External links
- http://neher-McGrath.com
- http://neher-mcgrath.com/Heating_Calculations.pdf
- http://www.neher-mcgrath.org/
- http://www.neher-mcgrath.com/41/neher-mcgrath_FAQ.html
- http://www.neher-mcgrath.com/41/neher-mcgrath_calculation_example.html