NEC Basic Conductor Sizing Requirements

 Introduction:

• It is important that an Electrical Engineer should have a knowledge on the basic sizing requirements in any electrical installations. That is why the NEC set a basic requirement to protect the public against wrong design details that may result on the loss of life and property. 
• The problem may arise several years after the installation have been completed therefore it is necessary that the design guide must be properly followed and duly assessed by the local building officials or any other regulating body.

The following are the basic standard requirement in sizing conductors and overload protection:

1. The ampacity of the branch circuit must not be less than the maximum load served.
2. The maximum load to be served by the branch circuit must not be more than 80% of the ampacity of the conductors.
3. The ampacity of the branch circuit conductors must not be less than the rating of the branch circuit.
4. The rating of a branch circuit is established or defined by the rating or setting of its protective device.
5. The total load on any over-current device in a panel board must not exceed 80% of the rating of the over-current device.
6. The normal ampacities of conductors in cables or raceways are given in NEC Tables 310-16 (copper) and Table 310-8 (aluminum) based on a 30 degrees celcius ambient temperature. For ambient temperatures over or under 30 degrees celcius, correction factors must be applied.
7. The current permitted to be carried by the branch circuit conductors may have to be reduced if the load is contibous. This does not mean a change in ampacities of the conductors but the rule refers to a limit of the load to be carried by the conductors.
8. Continuous load refers to a load that operates for three hours or more, such as store lighting, office lighting and the like. Continuous loads does not change the current carrying capacity of the conductor but it is used for safety considerations.
9. Over-current protection for any single non-motor operated equipment or appliance with ratings of 10 ampere or more must not be more than 150% of its ampere rating.
10. General purpose receptacle outlet other than dwelling units shall be considered as a load of 1.5 amperes (180 VA).
11. Circuit conductors shall be protected against over-current in accordance to their ampacities, but where the ampacities of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes.
12. However rule no. 11 contains twelve rules that modify the general requirement and permit the conductors not to be protected in accordance with their ampacities, they include:

• Power Loss Hazard
• Devices Rated 800 Amperes or Less
• Tap Conductors
• Motor-Operated Appliance Circuit Conductors
• Motor and Motor-Control Circuit Conductors
• Phase Converter Supply Conductors
• Air-Conditioning and Refrigeration Equipment Circuit Conductors
• Transformer Secondary Conductors
• Capacitor Circuit Conductors
• Electric Welder Circuit Conductors
• Remote-Control, Signaling, and Power-Limited Circuit Conductors
• Fire Alarm System Circuit Conductors

Summary:

1.  Never use a conductor more than its rated current carrying capacity.
2. Apply the derating factor set by the National Electrical Code in case the installation is more than its ambient temperature;
3. Apply the modification of the general requirement as set stated on the rule no. 12 above.

 Resources: 

• National Electrical Code
• CESEEPS Red Book, Low Voltage Systems and Applications in Industries