Fuses must be installed at the point where the conductor receives its supply, i.e., at the beginning or lineside of a branch circuit or feeder (240.21).
(B)(1) Fuses are not required at the conductor supply if a feeder tap conductor is not over ten feet long; is enclosed in raceway; does not extend beyond the switch board, panelboard or control device which it supplies; and has an ampacity not less than the combined computed loads supplied, and not less than the rating of the equipment containing an overcurrent device(s) supplied, unless the tap conductors are terminated in a fuse not exceeding the tap conductor ampacity. For field installed taps, the ampacity of the tap conductor must be at least 10% of the overcurrent device protecting the feeder conductors [240.21(B)(1)].
(B)(2) Fuses are not required at the conductor supply if a feeder tap conductor is not over 25 feet long; is suitably protected from physical damage by being enclosed in an approved raceway or other approved means; has an ampacity not less than 1⁄3that of the device protecting the feeder conductors and terminate in a single set of fuses sized not more than the tap conductor ampacity [240.21(B)(2)].
(B)(3) Fuses are not required at the conductor supply if a transformer feeder tap has primary conductors at least 1⁄3the ampacity of the overcurrent device protecting the feeder, and secondary conductors are at least 1⁄3the ampacity of the overcurrent device protecting the feeder, when multiplied by the transformer turns ratio. The total length of one primary plus one secondary conductor (excluding any portion of the primary conductor that is protected at its ampacity) is not over 25 feet in length; the secondary conductors terminate in a set of fuses rated at the ampacity of the tap conductors; and if the primary and secondary conductors are suitably protected from physical damage [240.21(B)(3)].
(B)(4) Fuses are not required at the conductor supply if a feeder tap is not over 25 feet long horizontally and not over 100 feet long total length in high bay manufacturing buildings where only qualified persons will service such a system. Also, the ampacity of the tap conductors is not less than 1⁄3of the fuse rating from which they are supplied. The size of the tap conductors must be at least 6 AWG copper or 4 AWG aluminum. They may not penetrate walls, floors, or ceilings, and the taps are made no less than 30 feet from the floor. The tap conductors terminate in a single set of fuses that limit the load to the ampacity of the tap conductors. They are physically protected by being enclosed in an approved raceway or other approved means and contain no splices.[240.21(B)(4)].
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©2014 Eaton
Conductor Protection
5. For field installations where the secondary conductors leave the enclosure or vault where they receive their supply, the secondary conductor ampacity is not less than 1⁄10of the rating of the over-current device protecting the primary of the transformer multiplied by the turns ratio. [240.21(C)(2)]
(C)(3) Transformer secondary conductors do not require fuses at the transformer terminals when all of the following conditions are met.
1. Must be an industrial location.
2. The conditions of maintenance and supervision in a given industrial location ensure that only qualified personnel service the system 3. Secondary conductors must not be more than 25 feet long. 4. Secondary conductor ampacity must be at least equal to the
secondary full-load current of transformer and sum of terminating, grouped, overcurrent devices. Selecting the next higher standard size overcurrent protective device is NOT allowed.
5. Secondary conductors must be protected from physical damage in an approved raceway or other approved means. [240.21(C)(3)] Note: Switchboard and panelboard protection (408.36) and transformer protection (450.3) must still be observed.
(C)(4) Outside conductors that are tapped to a feeder or connected to the secondary terminals of a transformer do not require fuse protection when all of the following are met:
1. The conductors are protected from physical damage in an approved means.
2. The conductors terminate in a single set of fuses, no larger than the ampacity of the conductors.
3. The conductors are outside, except for point of load termination. 4. The overcurrent device is near or a part of the disconnecting
means.
5. The disconnecting means is readily accessible outdoors or, if indoors, nearest the point of the entrance of the conductors or where installed inside per 230.6 nearest the point of conductor entrance [240.21(C)(4)].
Tap Conductor Exception for Listed Surge Protective Devices Exceptions to 240.21(B)(1)(1)b. and 240.21(C)(2)(1)b. permits sizing of tap conductors for listed surge protective devices and other listed non-energy consuming devices to be based on the manufacturer’s instructions.
This surge protective device is prewired with specific conductors that are shown in the device’s instructions. Surge protective devices are non-energy consuming devices that do not have a calculated load as referenced by 240.21(B)(1)(1)b and 240.21(C)(2)(1)b. For surge protective devices see
"http://www.cooperbussmann.com/surge" www.cooperbussmann.com/surge. Note: Smaller conductors tapped to larger conductors can be a serious
hazard. If not adequately protected against short-circuit conditions (as required in NEC®110.10 and 240.1(FPN)), these unprotected conductors can vaporize
or incur severe insulation damage. Molten metal and ionized gas created by a vaporized conductor can envelop other conductors (such as bare bus), causing equipment burndown. Adequate short-circuit protection is
recommended for all conductors. When a tap is made to a switchboard bus for an adjacent panel, such as an emergency panel, the use of Bussmann cable limiters is recommended for protection of the tapped conductor. These current-limiting cable limiters are available in sizes designed for short-circuit protection of conductors from 12 AWG to 1000 kcmil. Bussmann cable limiters are available in a variety of terminations to make adaption to bus structures or conductors relatively simple.
(B)(5) Fuses are not required at the supply for an outside tap of unlimited length where all of the following are met:
1. The conductors are outdoors except at the point of load termination. 2. The conductors are protected from physical damage in an approved
manner.
3. The conductors terminate in a single set of fuses that limit the load to the ampacity of the conductors.
4. The fuses are a part of or immediately adjacent to the disconnecting means.
5. The disconnecting means is readily accessible and is installed outside or inside nearest the point of entrance or where installed inside per 230.6 nearest the point of conductor entrance [240.21(B)(5)]. See the following Figure.
(C)(1) Fuses are not required on the secondary of a single-phase two-wire or three-phase, three-wire, delta-delta transformer to provide conductor protection where all of the following are met:
1. The transformer is protected in accordance with 450.3.
2. The overcurrent protective device on the primary of the transformer does not exceed the ampacity of the secondary conductor multiplied by the secondary to primary voltage ratio. [240.21(C)(1)]. Selecting the next higher standard size overcurrent protective device is NOT allowed.
(C)(2) Fuses are not required on the secondary of a transformer to provide conductor protection where all of the following are met:
1. The secondary conductors are not over 10 feet long.
2. The secondary conductors’ ampacity is not less than the combined computed loads.
3. The secondary conductor ampacity is not less than the rating of the device they supply or the rating of the overcurrent device at their termination. Selecting the next higher standard size overcurrent protective device is NOT allowed.
4. The secondary conductors do not extend beyond the enclosure(s) of the equipment they supply and they are enclosed in a raceway.
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