Underlined text denotes changes to the Code for the 2017 NEC.

Q1. What are the NEC rules for sizing transformer secondary conductors?

A1. Transformer Secondary Conductors. A set of conductors supplying single or separate loads is permitted to be connected to a transformer secondary without overcurrent protection in accordance with (1) through (6) [240.21(C)].

The permission of the “next size up” protection rule when the conductor ampacity doesn’t correspond with the standard size overcurrent protection device of 240.4(B) doesn’t apply to transformer secondary conductors.

(1) Overcurrent Protection by Primary Overcurrent Protection Device. The primary overcurrent protection device sized in accordance with 450.3(B) is considered suitable to protect the secondary conductors of a 2-wire (single-voltage) system, provided the primary overcurrent protection device doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary- to-primary transformer voltage ratio.

Example: What’s the minimum size secondary conductor required for a 2-wire, 480V to 120V transformer rated 1.50 kVA with 60ºC terminals?
Solution:
Primary Current = VA/E
VA = 1,500 VA
E = 480V
Primary Current = 1,500 VA/480V
Primary Current = 3.13A
Primary Overcurrent Protection [450.3(B)] = 3.13A x 1.67
Primary Overcurrent Protection [450.3(B)] = 5.22A or 5A Fuse
Secondary Current = 1,500 VA/120V
Secondary Current = 12.50A
Secondary Conductor = 12 AWG, rated 20A at 60ºC,
[Table 310.15(B)(16)]
The 5A primary overcurrent protection device can be used to protect 12 AWG secondary conductors because it doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Overcurrent Protection Device = 20A x (120V/480V)
Overcurrent Protection Device = 20A x ¼ (25%)
Overcurrent Protection Device = 5A fuse
Answer: 12 AWG

(2) 10-Foot Secondary Conductors. Secondary conductors can be run up to 10 ft without overcurrent protection if installed as follows:
(1) The secondary conductor must have an ampacity not less than:
a. The calculated load in accordance with Article 220, and
b. The rating of the overcurrent protection device termination or equipment containing overcurrent protection devices supplied by the tap conductors.

(4) Outside Secondary Conductors of Unlimited Length. Secondary conductors located outside a building or structure can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:
(1) The conductors are protected from physical damage within a raceway or manner approved by the authority having jurisdiction.
(2) The conductors terminate at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors.
(3) The terminating overcurrent protection device for the outside secondary conductors is part of the building feeder disconnect.
(4) The building feeder disconnecting means is readily accessible, either outside the building, or nearest the point of entrance of the outside secondary conductors.

(5) Secondary Conductors from a Feeder Tapped Transformer. Transformer secondary conductors must be installed in accordance with 240.21(B)(3).

(6) 25-Foot Secondary Conductor. Secondary conductors can be run up to 25 ft without overcurrent protection if they comply with all of the following:
(1) The secondary conductors must have an ampacity that’s not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent protection device protecting the primary of the transformer.
(2) Secondary conductors terminate in a single overcurrent protection device that limits the load to the secondary conductor ampacity.

(D) Service Conductors. Service conductors must be protected against overload in accordance with 230.90 and 91.

(H) Battery Conductors. Overcurrent protection is installed as close as practicable to the storage battery terminals.

Q2. What are the working space rules around the electrical panel according to the NEC?

A2. [110.26] For the purpose of safe operation and maintenance of equipment, access and working space must be provided about all electrical equipment [110.26].
(A) Working Space. Equipment that may need examination, adjustment, servicing, or maintenance while energized must have working space provided in accordance with 110.26(1), (2), (3), and (4):

Author’s Comment:
•The phrase “while energized” is the root of many debates. As always, check with the AHJ to see what equipment he or she believes needs a clear working space.

Note: NFPA 70E, Standard for Electrical Safety in the Workplace, provides guidance in determining the severity of potential exposure, planning safe work practices, arc‑flash labeling, and selecting personal protective equipment.

(1) Depth of Working Space. The working space, which is measured from the enclosure front, isn’t permitted to be less than the distances contained in Table 110.26(A)(1).

(a) Rear and Sides. Working space isn’t required for the back or sides of assemblies where all connections and all renewable or adjustable parts are accessible from the front.

(b) Low Voltage. If special permission is granted in accordance with 90.4, working space for equipment that operates at not more than 30V ac or 60V dc can be less than the distance in Table 110.26(A)(1).

Author’s Comment:
•See the definition of “Special Permission” in Article 100.

(c) Existing Buildings. If electrical equipment is being replaced, Condition 2 working space is permitted between dead‑front switchboards, switchgear, panelboards, or motor control centers located across the aisle from each other where conditions of maintenance and supervision ensure that written procedures have been adopted to prohibit equipment on both sides of the aisle from being open at the same time, and only authorized, qualified persons will service the installation.

Author’s Comment:
•The working space requirements of 110.26 don’t apply to equipment included in Chapter 8—Communications Circuits [90.3].

(2) Width of Working Space. The width of the working space must be a minimum of 30 in., but in no case less than the width of the equipment.

Author’s Comment:
•The width of the working space can be measured from left‑
to‑right, from right‑to‑left, or simply centered on the equipment, and can overlap the working space for other electrical equipment.

The working space must be of sufficient width, depth, and height to permit all equipment doors to open 90 degrees.

(3) Height of Working Space (Headroom). The height of the working space in front of equipment isn’t permitted to be less than 6½ ft, measured from the grade, floor, platform, or the equipment height, whichever is greater.

Equipment such as raceways, cables, wireways, cabinets, panels, and so on, can be located above or below electrical equipment, but must not extend more than 6 in. into the equipment’s working space.

Ex 1: The minimum headroom requirement doesn’t apply to service equipment or panelboards rated 200A or less located in an existing dwelling unit.

Author’s Comment:
•See the definition of “Dwelling Unit” in Article 100.

Ex 2: Meters are permitted to extend beyond the other equipment.
Ex 3: For battery systems, see 480.10(D) for top clearance requirements.

(4) Limited Access. Where equipment is likely to require examination, adjustment, servicing, or maintenance while energized is located in a space with limited access, all of the following conditions apply:
(a)(1)
Above Suspended Ceiling. Equipment installed above a suspended ceiling must have an access opening not smaller than 22 in. x 22 in.
(a)(2)
Crawl Space. Equipment installed in a crawl space must have an accessible opening not smaller than 22 in. x 30 in.
(b) The width of the working space must be a minimum of 30 in., but in no case less than the width of the equipment.
(c) The working space must permit equipment doors to open 90 degrees.
(d) The working space in front of the equipment must comply with the depth requirements of Table 110.26(A)(1), and horizontal ceiling structural members are permitted in this space.