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Transformer Stacking

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 are based.

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.

Mike Balog
Principal Engineer
CSI Engineering, PC
Mission Critical Facilities Integrators
12240 Indian Creek Court, Suite 140
Beltsville, MD 20705

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