There are many ampacity and cable sizing charts and calculators on the internet, but none that Ive seen correctly take into account all requirements imposed by the National Electrical Code on feeder conductors and their overcurrent protection, or encompass the use of 100%-rated or adjustable electronic-trip breakers.
In sizing cables and breakers, it’s important to appreciate the distinction between NEC rules protecting equipment versus NEC rules protecting conductors. These are the four requirements in a nutshell:
1. In order to protect terminations and connected equipment from excess heat, feeder conductor ampacity at the rated temperature of the terminations or equipment (whichever is lower) must be at least 125% of the continuous load plus 100% of the non-continuous load (110.14(C), Example D3(a)). This well-known but not always fully understood requirement is not for conductor protection, but for thermal protection of the terminations and OCPD. The cables are sized larger for continuous loads in order to better conduct heat away from the terminations and equipment. Specifying cable ampacity at the rated temperature of the terminations or equipment is a roundabout way of ensuring that the temperature of the terminations will not exceed their rated value.
2. Feeder conductor ampacity after derating for ambient temperature and raceway fill must be at least 100% of the total load (215.2, 310.15, Example D3(a)). This rule protects the conductor's insulation. While there is derating for ambient temperature and raceway fill, no extra 25% factor is imposed for continuous loads, because cables do just fine with continuous loads at their full rated ampacity.
3. Overcurrent protection must be provided for the conductor, with its (long-time) trip point no higher than one standard size up from the conductor's derated ampacity (240.4, 240.6, Example D3(a)). This rule ensures that the OCPD will open before conductor insulation damage occurs.
4. Circuit breakers, unless 100%-rated, must be rated for at least 125% of the continuous load plus 100% of the non-continuous load (215.3, Example D3(a)). As with #1 above, this rule is for thermal protection of the breaker, and to ensure that nuisance trips of thermal-magnetic trip units does not occur as a result of high temperatures. Electronic trip units are rated for accurate operation at least up to their termination temperature, so nuisance tripping due to heat is rarely a concern when they are used.
This calculator, based on the 2017 NEC, determines the required conductor size, breaker size, and breaker trip setting for a given load per code requirements, and indicates which criterion is the determing factor. It is primarily intended for use in industrial applications, and is valid for feeder conductors 600V or less at currents of 100A - 800A. Hover your cursor over the various fields for more information and details.
A report of the entered and generated data may be exported via the clipboard for pasting into a spreadsheet, or downloaded as a csv file for import to a spreadsheet.
DISCLAIMER: This calculator is provided as an informational tool and convenience only. The National Electrical Code is in some instances ambiguous and open to interpretation; the calculator is based on our interpretation and research into interpretations of others. Neither the author nor publisher warrant its accuracy or assume any liabiity for its application.
Electrical installation and design should be performed only by licensed electricians and Professional Engineers.
