Conductor Sizing, based on 2017 NEC^® - Part 1

Figure 01

For EC&M Magazine
By Mike Holt, NEC^® Consultant

Do you understand the basic concepts behind conductor sizing?

The NEC requires a given conductor to be a minimum size for a given load under specific conditions of use. The reason for this is to protect the conductor insulation from overheating. So to understand conductor sizing, you need to understand a little about conductor insulation.

Table 310.104(A) provides information on conductor insulation properties such as letter type, operation temperature, application, insulation, and outer cover properties. Only conductors in Tables 310.104(A) though 310.104(E) can be installed, and a given conductor can be used for only the application(s) the tables identify it for.

In general, you can use only conductors contained in Tables 310.104(A), except where otherwise permitted in the NEC (an example being PV wire [690.31(C)]).

It’s common to see conductors with a dual insulation rating, such as THHN/THWN. You can use this type of conductor in a dry location at the THHN 90ºC ampacity. If you use it in a wet location, you must adhere to the THWN ampacity rating of the 75ºC column of Table 310.15(B)(16) for THWN insulation types.

When the insulation type has the suffix “-2”, you can use the conductor in a wet or dry location at the 90ºC ampacity rating. THWN-2 is an example.

Conductor sizes
Conductor sizes are expressed in American Wire Gage (AWG), typically from 18 AWG up to 4/0 AWG. Conductor sizes larger than 4/0 AWG are expressed in kcmil (thousand circular mils) [110.6].

There’s a misconception that 12 AWG copper is the smallest conductor permitted for commercial or industrial facilities. Although this isn’t true based on NEC rules, it may be a local code requirement or otherwise specified for the project.

The smallest conductor permitted for branch circuits for residential, commercial, and industrial locations is 14 AWG copper, except as permitted elsewhere in the NEC [310.106].

You can use conductors smaller than 14 AWG for:

• Class 1 remote-control circuits [725.43].
• Fixture wire [402.6].
• Motor control circuits [Table 430.72(B)].
  

Conductors 8 AWG and larger must be stranded when installed in a raceway unless specifically permitted elsewhere [310.106(C)]. Solid conductors are often used for the grounding electrode conductor [250.62] and for bonding pools, spas, and outdoor hot tubs [680.26(C)].

You must size conductors using their ampacity from the insulation temperature rating column of Table 310.15(B)(16) that corresponds to the lowest temperature rating of any terminal, device, or conductor of the circuit.

For motors marked with design letters B, C, or D, you can use conductors having an insulation rating of 75ºC or higher if the ampacity of such conductors doesn’t exceed the 75ºC ampacity listed in Table 310.15(B)(16) [110.14(C)(1)(a)(4)].

For equipment rated 100A or less, you must size the conductors using the 60ºC temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(2)].

Where you have terminals for equipment rated over 100A and/or pressure connector terminals for conductors larger than 1 AWG, you must size the conductor per the 75ºC temperature rating listed in Table 310.15(B)(16) [110.14(C)(1)(b)(1)].

Conductors rated 90ºC and that terminate on separately installed pressure connectors can be sized to the 90ºC column of Table 310.15(B)(16) if the conductors and pressure connectors are rated at least 90ºC [110.14(C)(2)]. Figure 01

Overcurrent protection
Conductor sizing and overcurrent protection sizing go hand in hand, so let’s look briefly at overcurrent protection.

Article 240 covers the general requirements for overcurrent protection and the installation requirements of overcurrent protection devices (OCPDs). Overcurrent is a condition where the current exceeds the rating of equipment or ampacity of a conductor due to overload, short circuit, or ground fault [Article 100].

An OCPD protects the circuit by opening when the current reaches a value that will cause an excessive or dangerous temperature rise (overheating) in the protected conductor(s). OCPDs must have an interrupting rating sufficient for the maximum possible fault current available on the line-side terminals of the equipment [110.9]. Electrical equipment must have a short-circuit current rating that permits the circuit’s OCPD to clear short circuits or ground faults without extensive damage to the circuit’s electrical components [110.10 Note].

There are many different rules for protecting conductors and equipment. It’s not simply using 12 AWG wire and a 20A breaker. The general rule is that conductors must be protected at the point where they receive their supply per their ampacities, as listed in Table 310.15(B)(16) [240.21].

Except as permitted by 240.4(A) through (G), conductors must be protected against overcurrent per their ampacity after ampacity correction and adjustment required in 310.15. 

For OCPDs rated 800A or less [240.4(B)], the next higher standard rating of OCPD listed in 240.6(A) (above the ampacity of the ungrounded conductors being protected) is permitted. But only if all the following conditions are met:

(1) The conductors aren’t part of a branch circuit supplying more than one receptacle for cord-and-plug-connected loads.
(2) The ampacity of a conductor, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, doesn’t correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
(3) The next higher standard OCPD rating from 240.6(A) doesn’t exceed 800A.

If the circuit’s OCPD exceeds 800A, the conductor ampacity must have a rating of not less than the rating of the OCPD [240.4(C)]. But that’s only after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both.

Small conductors
Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed the following [240.4(D)]:
(1) 18 AWG Copper, 7A
(2) 16 AWG Copper,10A
(3) 14 AWG Copper, 15A
(4) 12 AWG Aluminum/Copper-Clad Aluminum, 15A
(5) 12 AWG Copper, 20A
(6) 10 AWG Aluminum/Copper-Clad Aluminum, 25A
(7) 10 AWG Copper, 30A

What about transformer secondary conductors? The primary OCPD sized per 450.3(B) is considered suitable to protect the secondary conductors of a 2-wire (single-voltage) system. But only if the primary OCPD doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio [240.4(F)].

Overcurrent protection for specific applications
Overcurrent protection for specific equipment and conductors must comply with the requirements referenced in Table 240.3.

Air-conditioning and refrigeration equipment, and their circuit conductors, must be protected against overcurrent per 440.22. Typically, the branch-circuit ampacity and OCPD size are marked on the equipment nameplate [440.4(A)].

Motors and their circuit conductors must be protected against overcurrent per Article 430 [240.3].

Getting it right
Factors that affect conductor sizing include conductor ampacity, equipment terminal temperature rating, continuous load factors, overcurrent current protection requirements, conductor insulation temperature rating, ambient temperature conductor ampacity correction, and conductor bundle conductor ampacity adjustment.

You must account for all of these to arrive at the minimum safe size for a given conductor. You may want to exceed that size after considering factors such as voltage drop, to optimize performance. But you can never go below that size.

If you’re sizing the conductors for a given system, you are probably also sizing the overcurrent protection, so you need to understand both to do the job right.