Transformers and the 2008 NEC
By D. Jerry Flaherty, Chief Electrical Inspector, Electrical Inspection Service Inc.
(631) 286- 6642 - (516) 466-6486
WWW.EISLongIsland.com
A transformer installation can be a mind boggling experience with code requirements in many different
NEC articles. In this article I hope to summarize the NEC requirements for typical dry-type transformer.
Sizing Transformers: A transformer must be sized to satisfy 100% of the load connected to the
transformer. Article 220 should be used to calculate the transformer load. The load should be evenly
distributed across the phase. To calculate the minimum transformer kilo-volt-amps (kVA) use the
following;
1Ø Amps x volts / 1,000 = kVA
Ex: 42 amps x 240 volts / 1,000 = 10.08 kVA
3Ø Amps x volts x 1.732 / 1,000 = kVA Ex: 56 amps x 208 volts x 1.732 / 1,000 = 27.16 kVA
If the transformer load is a motor, use Table 430.248 for single phase motors or 430.250 for three phase
motors to determine the full-load current then multiply by the motor service factor then multiply by the
duty-cycle (Table 430.33(E) if required.
Ex: The transformer supplies power for one 10 HP, 3Ø, 208 volt motor with a service factor of 1.15 running
continuously. Referring to Table 430.250 a 10 HP, 3Ø, 208 volt motor has a full load current of 30.8 amps.
30.8 amps x 208 volts x 1.732 x 1.15 / 1,000 = 12.76 kVA
If the calculation does not equal a standard size transformer, the next larger standard size transformers
would be selected.
Accessibility, Working Space and Dedicated Space: Section 450.13 requires that transformers “be
readily accessible to qualified personnel for inspection and maintenance”. This means that a transformer
should be in an enclosed area not accessible to non-qualified persons and the requirements of section
110.26 for working space and dedicated space apply to the transformer installations.
Section 450.13(A) does allow dry-type transformers under 600 volts to be installed “in the open on walls,
columns or structures” and 450.13(B) allows 50 kVA or smaller transformers to be installed in hollow
spaces meaning that a transformer can be installed above any area, in a dropped ceiling, etc. It also
negates the requirements for being readily accessible.
Transformer Disconnect: The 2008 NEC does not require a means of disconnect within sight of the
transformer however the 2011 NEC will require a means of disconnect within sight of a transformer or a
sign indicating location of the transformer disconnect .
NOTE: I will be use a 45kVA, 480 volt delta primary with a 208 volt wye secondary enclosed transformer in an
office environment as an example. I will also be using copper conductors.
1. Calculate the Primary Current: To calculate the current of the transformer primary divide the
transformer kVA by the primary voltage. For 3 phase, multiply by the square root of 3.
Ex: 45kVA / (480 volt x 1.732) = 54 amps
2. Calculate the Primary Over-current Protection: 450.3 allows the primary overcurrent protection to
be a maximum of 250% of the transformer rated current if the transformer secondary has overcurrent
protection. Section 240.4(F) requires three phase wye transformers to have overcurrent protection on
the secondary to protect the secondary conductors. In this example there is no in rush current so I will
use 125% of the transformer current.
Ex: 125% x 54 amps = 68 amps The next standard size overcurrent device is 70 amp. (240.6)
3. Calculate the Primary Conductor Size: The primary conductor should be sized using 100% of the
non-continuous load and 125% of the continuous load. Since our example is an office (full load on for
over 3 hours) I’ll use a continuous load 125%. If required, conditions of use and voltage drop should
be applied conductors. Assuming the circuit breaker and the transformer terminals are rated at 75°C.
Ex: 54 amps @ 125% = 68 amps = 4 AWG @ 75°C (Table 310.16)
4. Calculate the Equipment Grounding Conductor: The equipment grounding conductor is sized
using Table 250.122 based on the overcurrent protection.
Ex: 70 amp = #8 AWG EGC or a metal conduit system (250.118)
5. Calculate the Secondary Current: Use the same formula use for calculating the primary current to
calculate the secondary current.
Ex: 45kVA / (208 volts x 1.732) = 125 amps
6. Calculate the Secondary Overcurrent Protection: 450.3 require the maximum secondary
overcurrent protection be 125% of the transformer current.
Ex: 125 amps x 125% = 156 amps
Requiring a 150 amp overcurrent device (240.6)
Note 1 allows the next standard size circuit breaker or fuse. The next standard size overcurrent
device is 175 amp. (240.6)
7. Calculate the Secondary Conductors Size: The transformer secondary conductors are considered
continuous, requiring the conductor amperage be at least 125% of the transformer amperage.
Ex: 125 amps x 125% = 156 amps
However conductor will have to be adjusted for conditions of use. 310.15(B)(4)(c) requires that the
neutral be considered a current carrying conductor if the major portion of the load is nonlinear. In an
office environment, lighting electronic ballasts and computers are nonlinear loads requiring the
neutral has to be considered as a current carrying conductor. The four current carrying conductors
requiring an adjustment factor of 80% per 310.15(B)(2)(a).
Ex: 156 amps / 80% = 195 amps = 3/0 AWG @ 75°C (Table 310.16)
8. Calculate the Secondary Equipment Grounding Conductor: The equipment grounding conductor
is sized using Table 250.122 based on the overcurrent protection.
Ex: 175 amp = #6 AWG EGC or a metal conduit system (250.118)
9. Grounding and Bonding: Just like a service, the neutral conductor (transformer X0), non-current
carrying metal parts, the grounding electrode conductor and the equipment grounding conductors
must be connected together by a low-impedance connection. This connection can be in the
transformer housing or in a panel or disconnect located within 10 feet of the transformer.
10. Bonding Jumper: 250.4(A)(2)&(3) and 250.30(A)(1) require non-current carrying conductive
material to be connected together and be connected to the supply. 250.102(C) requires a bonding
jumper between X0 and the transformer case. The bonding jumper is sized using Table 250.66 based
on the largest ungrounded conductor.
Ex: 3/0 AWG = #4 AWG bonding jumper
11. Grounding Electrode Conductor: 250.4(A)(1) and 250.30(A)(3) requires that the electrical system
be connected to earth by a grounding electrode/grounding electrode conductor. 250.30(A)(3)
requires the grounding electrode conductor between the transformer X0 and the grounding
electrode be also sized by table 250.66 based on the largest ungrounded conductor.
Ex: 3/0 AWG = #4 AWG bonding jumper
The grounding electrode must conform to Part III of Article 250, the same requirements as for a
service. If building steel is used as the grounding electrode it must conform to 250.52(A)(2) requiring
10 feet of contact with the earth directly or via “J” bolts.
(250.30(A)(4) has the requirements for multiple separately derived systems/ transformers.)
I hope you find this article informative. Please send any comments, questions or suggestions to
JerryFL@optonline.net .