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26. Buck-boost transformers are almost always installed as auto-transformers. Does the N.E.C. (National Electrical
Code) permit the use of autotransformers?
Yes. Please refer to N.E.C. Article 450-4, “Autotransformers 600 Volts, Nominal, or Less.” Item (a) explains how to overcurrent
protect an autotransformer; item (b) explains that an insulating transformer such as a buck-boost transformer may be field
connected as an autotransformer.
27. When a buck-boost transformer is connected as an autotransformer such as boosting 208V to 230V, the kVA is
greatly increased. What is the procedure for determining the size (ampere rating) of the overcurrent protective device
such as a fuse or circuit breaker?
The National Electrical Code Article 450-4 addresses overcurrent protection of autotransformers. A copy is reproduced below
for easy reference.
450-4. Autotransformers 600 Volts, Nominal, or Less.
(a) Overcurrent Protection. Each autotransformer 600 volts, nominal, or less shall be protected by an individual overcurrent
device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more
than 125 percent of the rated full-load input current of the autotransformer. An overcurrent device shall not be installed in
series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between
Points A and B as shown in Diagram 450-4.
Exception: Where the rated input current of an autotransformer is 9 amperes or more and 125 percent of this current does
not correspond to a standard rating of a fuse or non-adjustable circuit breaker, the next higher standard rating described in
Section 240-6 shall be permitted. When the rated input current is less than 9 amperes, an overcurrent device rated or set at
not more than 167 percent of the input current shall be permitted.
(b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as an autotransformer shall be
identified for use at elevated voltage.
28. I have noted the reprint of the N.E.C. (National Electrical Code), Article 450-4 shown in the previous question
covering autotransformer overcurrent protection. Could you explain this article in detail by citing an example?
An example of an everyday application is always a good way to explain the intent of the “Code.” Example: A 1 kVA transform-
er Catalog No. T111683 has a primary of 120 x 240V and a secondary of 12 x 24V. It is to be connected as an autotransform-
er at the time of installation to raise 208V to 230V single phase.
When this 1 kVA unit is connected as an autotransformer for this voltage combination, its kVA rating is increased to 9.58 kVA
(may also be expressed as 9,580 VA). This is the rating to be used for determining the full load input amps and the sizing of
the overcurrent protect device (fuse or breaker) on the input.
Full Load Input Amps =
9,580 Volt Amps
= 46 Amps
208 Volts
When the full load current is greater than 9 amps, the overcurrent protective device (usually a fuse or non-adjustable breaker)
amp rating can be up to 125 percent of the full load rating of the autotransformer input amps.
Max. amp rating of the overcurrent device = 46 amps x 125% = 57.5 amps
The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the
overcurrent device. This is shown in Article 240-6 of the N.E.C.
Max. size of the fuse or circuit breaker = 60 amps