Question

What are the rules as per the NEC providing overcurrent protection when utilizing autotransformers?

Answer 1

Over Current protection for autotransformer is similar to that of transformer.

Protection at the primary side of the transformer

Transformer primary protection — Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. 450. If a transformer serves a fire pump (Art. 695), its primary OCPD shall be sized per the locked-rotor current of the fire pump, and transformer secondary protection shall not be permitted [695.5(B)].

Transformer primary feeder protection — The NEC does not list any specific Exception for transformer primary feeder protection. An OCPD is required for the transformer primary conductor. The only Exception is found in Sec. 240.4(A). If overload protection of the OCPD would create a power loss hazard, such as fire pump application and some processes at refinery facilities, the overload protection shall not be required. However, short circuit protection is still required. As a reference, Sec. 695.4(B)(2)(a)(1) and (2) requires the overcurrent device for a fire pump(s) to be sized per locked-rotor current of the largest fire pump motor and shall not open within 2 minutes at six times the full-load current of the fire pump motor(s).

Protection at the secondary side of the transformer

Transformer secondary protection — Although OCPDs are required as primary protection for transformers with no Exception, there are a few scenarios where overcurrent protection is not required at the transformer secondary, as per Tables 450.3(A) and 450.3(B).

Transformer secondary protection is not required for the following conditions:

For transformers with primary voltage greater than 1,000V at supervised locations, with the following maximum primary protection [Table 450.3(A)]:
a. Circuit breaker size up to 300% of or fuse rating up to 250% of transformer-rated current, for transformers with secondary voltage greater than 1,000V.
b. Circuit breaker size or fuse rating up to 250% of transformer-rated current, for transformers with secondary voltage 1,000V or less.
For transformers with primary voltage 1,000V or less and current of 9A or more, with primary protection not more than 125% of transformer-rated current, the “next-size-up” rule can be applied [Table 450.3(B)]. Please note that requirements for transformers with current less than 9A are not listed here.
As per Note 5 in Table 450.3(A), “A transformer equipped with a coordinated thermal overload protection device by a manufacturer shall be permitted to have separate secondary protection omitted.”
As per Note 3 of Table 450.3(A), “A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation.”

Transformer secondary feeder protection — Typically, transformer secondary feeder protection is required except for a few conditions listed in Sec. 240.21(C)(1) through (6). For those Exceptions, the next-size-up rule shall not be permitted. Section 240.4(B) notes that the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted, provided the conditions listed on 240.4(B)(1) through (3) are met.

a. Primary side protection is allowed as secondary feeder protection when the following requirement is met [240.21(C)(1)]. Per 240.21(C)(1), for a single-phase transformer having a 2-wire (single-voltage) secondary or a 3-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, secondary feeder shall be permitted to be protected by an OCPD on the primary side of the transformer, “provided this protection is in accordance with Sec. 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.” A note in Sec. 450.3 mentions that it is not necessary to duplicate overcurrent protection at the transformer secondary side, as long as the OCPD on the primary side protects the transformer (meeting 450.3 requirements) and protects the feeder (meeting 240.21 requirements). A possible reason to have Sec. 240.21(C)(1) is that the primary side OCPD sees the same per unit fault current at the transformer primary side as that at the secondary side.

For any other type of transformer, the secondary conductor is not required to be protected by an OCPD at the transformer primary [240.4(F)]. For example, if an OCPD is only applied to the high-voltage, delta side of a delta-wye grounded transformer, the device can have a problem providing sensitive fault protection for the transformer. For a low-voltage (wye side) line-to-ground fault, the high-side line current is only 58% of the low-voltage, per-unit fault current [Sec. 11.9.2.2.3.1 of IEEE standard 242-2001].

b. A secondary conductor OCPD is not required when the conductor is short and meets the following requirements — providing other requirements in the NEC are met, such as minimum cable ampacity [240.21(C)(2),(3),(5) and (6)].

Secondary conductor is not greater than 10 ft in length [240.21(C)(2)];
Industrial installation secondary conductor is not greater than 25 ft in length [240.21(C)(3)];
Secondary conductor from a feeder tapped transformer, primary plus secondary, is not greater than 25 ft in length [240.21 (C)(5)];
Secondary conductor is not greater than 25 ft in length [240.21(C)(6)].

c. Overcurrent protection is not required for outside secondary conductors when the following requirements are met [240.21(C)(4)].

It is common to use an exterior transformer to feed a single building or multiple buildings through outside secondary conductors. As long as they meet ALL of the following requirements listed in Sec. 240.21(C)(4), an OCPD is not required at the secondary side of the transformer.

The conductors are protected from physical damage in an approved manner.
The conductors terminate at a single circuit breaker or a single set of fuses.
The overcurrent device for the conductor is an integral part of a disconnect means or shall be located immediately adjacent thereto.
The disconnect means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure.
b. Inside, nearest the point of entrance of the conductors.
c. Where installed in accordance with Sec. 230.6, nearest the point of entrance of the conductors.

A possible reason to have Sec. 240.21(C)(4) is there is a very low incidence rate of faults occurring on the exterior protected secondary conductor in the “approved manner.” Therefore, the risk factor for fire hazard or property damage may be low. The NEC does not specify what “approved manner” is for the conductor protection. It might require AHJ’s interpretation and approval. Using an underground concrete-encased duct bank with required concrete strength, encasement thickness and bury depth can be an “approved manner” for this application.