Question: I have been unable
to find in the NEC that prohibits the stacking of transformers. For example, I have a
300kVA 480/208 transformer and a 75kVA 480/208 transformer that I need to fit in one electric
room. There is not sufficient floor space to accommodate both. Can the transformers be
stacked on top of each other with the smaller supported by a steel rack?
Mike Holt's Answer: As long
as the working space requirements of 110.26 are maintained, the ventilation is adequate
to dispose of the transformer full-load losses without creating a temperature rise that
is in excess of the transformer rating [450.9], and the manufactures instructions are
complied with [110.3(B), this should be fine.
Mike, This type of transformer installation
has always concerned me, especially when located in close quarters. I don't understand
why NEC does not address it formally. I understand that NEC is not a substitute for good
engineering judgment, however many installers believe that if NEC doesn't expressly prohibit
it, it is OK to do.
In doing due-diligence facility analysis and performing mission critical renovations,
it is not un-common to find stacked dry transformers, mostly 30 to 150 kVA, supporting
heavy harmonic laden loads, with windings carrying nearly full-load amperes, crammed into
tight spaces and ventilated with natural convection. The transformers just cook.
Your assistance in promoting sound dry transformer placement in accordance with UL listing
installation requirements would be helpful to others, I'm sure.
Although the NEC may not address a stacked configuration for transformers specifically,
the insulation system design and UL listed temperature rise are premised on an ambient
temperature, usually 40 Degree C, for general purpose 150 deg C rise transformers.
The transformer kVA capacity rating is a function of the rating of the winding insulation
system and the transformer losses. For example: A 220 deg C insulation = (30 Deg C winding
hot spot + 150 deg C rise + 40 Deg C ambient temperature).
The life of the insulation is premised on a design number of hours at operating temperature.
By subjecting the insulation to an increase of 10 degrees C above its rating effectively
reduces the life of the insulation by about one-half.
By stacking transformers, the transformer's ambient temperature will be likely be greater
than if floor mounted, due to the heat source of the lower transformer and also compounded
by heat stratification, if there is a low ceiling height. The end result will be a transformer
that runs at over-temperature when operated at capacity or may need to be loaded at a
significantly reduced capacity. Additionally, there is likely to be improper winding overcurrent
protection, which was determined based on a 40 deg C ambient location.
The result: premature failure of the transformer, and possibly fire.
The code requires that electrical equipment be installed in accordance with listing and
labeling. NEC 110.3 (B): "Installation and Use: Listed or labeled equipment shall
be installed and used in accordance with any instructions included in the listing or labeling."
This includes maximum ambient temperature conditions, upon which transformer capacities
For more background on transformer temperature ratings visit:
Your dedication to the promotion of workmanship quality and sound electrical engineering
judgment in the trade is second to none.
CSI Engineering, PC
Mission Critical Facilities Integrators
12240 Indian Creek Court, Suite 140
Beltsville, MD 20705