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Sizing feeders in an electrical installation

In our world, there are many electrical systems, and they differ, both in their electrical infrastructure as well as in their technical and safety regulations. Costa Rica, for example, developed its infrastructure based on the North American electrical system. Therefore, electrical installations in buildings are regulated by the NFPA70 National Electrical Code (NEC). Its purpose is to protect people and property against the risks arising from electricity use. Its application produces a virtually hazard-free installation, though it may not necessarily be efficient or suitable for future expansion. The designer of the installation is the person responsible for achieving those characteristics. These are some normative aspects to be applied for the sizing of feeder conductors in a cable duct:

1. According to Section 110.14(C) of the NEC 2017 (1) there are two circuit ranges for electrical installations:

1.1. Circuits rated 100 A or less, with a conductor with an ampacity for an equipment terminal temperature of 60°C (or higher if the equipment is listed or certified and approved for those temperatures).

1.2. Circuits rated higher than 100 A, with a conductor with an ampacity for an equipment terminal temperature of 75°C (or higher if the equipment is listed and approved for those temperatures).

The rating of these circuits is defined by the size of the overcurrent protection device (circuit breaker [CB] or fuse).

The allowable conductor ampacity used to determine equipment terminal arrangements should be based on Table 310.15(B)(16) and the appropriate amendments to Section 310.15(B)(7).

2. Section 240.6(A) contains the List of Standardized Values for Overcurrent Protection Devices (CB or fuse).

3. Ampacity is defined in Article 100 of the NEC as the maximum current, in amperes, that a conductor can carry continuously under conditions of use without exceeding its temperature rating.

4. Continuous load is defined in Article 100 of the NEC as an electrical load where the maximum current is expected to continue for three hours or more.

5. Section 240.4 indicates that the conductor must be protected with an overcurrent protection device, using the ampacities in Section 310.15.

6. Normal working (or operating) conditions are considered to be no more than 3 current-carrying conductors in a cable duct, cable, or directly buried, 0 to 2000 V, 60 Hz, with an ambient temperature of 30°C, as outlined in Table 310.15(B)(16).


a) The required ampacity of a circuit conductor is calculated as the non-continuous load plus 125% of the continuous load following Section 215.2.

b) Choose the Overcurrent Protection Device (CB or fuse) defined in Section 240.6(A), with a rating no smaller than the required ampacity, which is calculated beforehand according to Section 215.3.

c) The allowable ampacity of the conductor is determined as a value no smaller than the rating of the protection device defined above in Section 240.4, Table 310.15(B)(16), in the copper or aluminum column, according to the terminal temperature of the equipment.

d) The circuit conductor's size is calculated using the allowable ampacity determined above in the Gauge column of Table 310.15(B)(16).

The conductor gauge determined by this procedure is the minimum allowed in a circuit for its maximum current capacity. EXAMPLE 1: The size of a copper EcoPlus THHN/THWN-2 conductor and the overcurrent protection (CB) of an electrical circuit are required to supply a continuous load of 120 A under normal working conditions. The voltage drop is not taken into consideration since the length to be considered is small.

Applying the procedure indicated above:

Ampacity required = 1.25 x 120 = 150 A according to Section 215.2

Protection Device = CB = 150 A from Sections 240.6(A) and 215.3

Allowable Ampacity of the Conductor = 150 A Table 310.15(B)(16), 75°C copper column

Conductor gauge = 1/0 AWG from Table 310.15(B)(16)

The circuit rating in this example is 150 A, and the maximum continuous load that can flow through the conductor is 120 A; or a 150 A non-continuous load.


From a value engineering perspective, the designer could choose a gauge the next larger size compared to the one obtained with this procedure. This provides a more operationally efficient installation with a lower voltage drop, which is profitable economically, with a return on investment in only a few years, a longer service life (by operating at lower temperatures), and a better response to overload and short circuits due to its bigger diameter. This option also makes it possible to increase the installation's future load; it has a smaller ecological footprint with less impact and fewer CO2 emissions.

Ecological option for Example 1:

Recommended gauge = 2/0 AWG 1 gauge larger than 1/0 AWG In this option, the operating temperature of the EcoPlus THHN/THWN-2 2/0 AWG conductor is less than 60°C, which increases its service life (60°C would produce a current of 145 A, and only 120 A circulate; see Table 310.15(B)(16)). It has 21% fewer Joule losses than the 1/0 AWG conductor, so the additional investment in the installation has a one- year return, approximately; and its carbon footprint is lower due to the reduction in energy consumption. (1) The current version, which is mandatory in Costa Rica, is the NEC 2017.


  • Zúñiga Javier 1 March 2023 at 06:10 Reply

    Excelente muchas gracias, muy claro el procedimiento, soloun detalle, el NEC que en este momento está vigente en CR es el 2014 Muchas gracias

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