Published: June 2026
Technical Level: Advanced
Category: Electrical Design
The sizing and protection of a transformer in a building electrical system must satisfy both the engineering requirement to serve the load and the National Electrical Code's requirements for overcurrent protection, conductor sizing, and grounding that ensure the installation is safe. This paper develops the procedure: the determination of the load and the selection of a standard transformer rating to serve it, the sizing of the primary and secondary conductors to the transformer, and the application of the Code's rules for the overcurrent protection of the transformer and its conductors. The objective is to give the engineer a clear path from the load requirement to a code-compliant transformer installation, with the relationships among the transformer rating, the currents, the conductor sizes, and the protective-device ratings made explicit.
The sizing of a transformer begins with the determination of the load it must serve, computed from the connected load through the demand and diversity factors that reflect the load's actual coincident demand rather than the sum of its nameplate ratings. The transformer must be rated to supply this demand load continuously, with an allowance for any anticipated growth, and a transformer is selected from the standard kVA ratings at or above the computed demand. The selection avoids both gross oversizing, which carries unnecessary no-load loss for the life of the transformer, and undersizing, which leads to overheating and shortened life, while providing a reasonable margin for growth.
The rated currents of the transformer follow from its kVA rating and its primary and secondary voltages. The full-load current on each side is the kVA rating divided by the product of the voltage and, for a three-phase transformer, the square root of three:
Where:
is the full-load line current in amperes.
is the transformer rating in kVA.
is the line-to-line voltage on the side considered in volts.
These full-load currents, computed for both the primary and the secondary, are the basis for sizing the conductors and the overcurrent protection on each side, and they are the reference against which the Code's percentage limits for protective-device ratings are applied.
The conductors connecting to the transformer on its primary and secondary sides must be sized to carry the respective full-load currents continuously, with the Code's continuous-load provisions applied where the load is continuous. The conductor on each side is selected for an ampacity, after the applicable adjustment and correction factors for ambient temperature and grouping, equal to or exceeding the full-load current of that side increased by the continuous-load factor where applicable. The secondary conductors, carrying the larger current of the lower-voltage side, are generally larger than the primary conductors carrying the smaller current of the higher-voltage side, in inverse proportion to the voltages.
The conductors must also be checked for voltage drop where the runs are long, by the methods applicable to feeder conductors generally, and enlarged where necessary to keep the drop within acceptable limits. The selection of each conductor is therefore the larger of the ampacity requirement and the voltage-drop requirement, as for any feeder. The conductor sizing and the overcurrent protection are interrelated, because the Code's rules for transformer protection govern the relationship between the protective-device rating and the conductor ampacity, and the two must be selected together to satisfy the Code.
The National Electrical Code requires that a transformer be protected against overcurrent, and its rules establish the maximum rating of the protective device in terms of a percentage of the transformer's rated current, with the permissible percentage depending on whether protection is provided on the primary side alone or on both the primary and secondary sides, and on the transformer's impedance and voltage class. The rules reflect the need to protect the transformer against damaging overcurrent while permitting the magnetizing inrush and the legitimate overload capacity of the transformer without nuisance operation, and they distinguish the protection of the transformer itself from the protection of the conductors connected to it.
The protective device must be coordinated with the transformer's inrush and damage characteristics, lying above the inrush point so that energizing the transformer does not trip the device and below the transformer's damage curve so that the device operates before a sustained fault damages the transformer. Where the Code permits a protective device rated above the transformer's full-load current to accommodate inrush and overload, the conductors must still be protected, either by the same device within the conductor's ampacity or by the Code's specific provisions for transformer secondary conductors. The engineer applies the applicable percentage limits to determine the maximum protective-device rating, selects a standard device at or below that maximum, and confirms that the device protects both the transformer and its conductors and coordinates with the inrush and damage characteristics. This reconciliation of the transformer protection rules, the conductor protection rules, and the coordination requirement produces a protective arrangement that is both code-compliant and properly coordinated.
The most consequential point for the practicing engineer is that transformer sizing, conductor selection, and overcurrent protection are not a checklist but a system of interdependent constraints: the rating sets the full-load currents, the currents set the conductor ampacity, the conductors set the protection limits, and the NEC percentage rules tie the protection back to the rating. A change to any one ripples through the others, and the design is complete only when all four are reconciled rather than when each is individually satisfied.
The most common implementation failure is selecting primary and secondary overcurrent protection from the transformer percentage tables without confirming that the same devices also protect the conductors they feed, because the NEC's transformer-protection allowances and its conductor-protection rules are separate requirements that must both be met, and a device sized correctly for the transformer can leave the secondary conductors unprotected.
The engineer should next carry a specific case end-to-end (for example a 75 kVA, 480 to 208/120 V dry-type unit), from the demand load through the standard rating, the primary and secondary conductors, and the coordinated overcurrent protection, because the worked sequence exposes the interdependencies that the rule-by-rule reading leaves implicit.
[1] NFPA 70-2023, National Electrical Code, Article 450, National Fire Protection Association, 2023.
[2] IEEE Standard C57.12.00-2015, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEEE, 2015.
[3] IEEE Standard 242-2001, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book), IEEE, 2001.
[4] IEEE Standard 141-1993, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book), IEEE, 1993.
[5] IEEE Standard C57.91-2011, IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators, IEEE, 2011.