Distribution Transformers: Low-Voltage Dry-Type


Low-voltage dry-type transformers (LVDT) are generally used inside buildings to reduce voltage to the values necessary to power appliances, lighting and other products. Unlike liquid-immersed transformers, these are generally owned by the building owner. The customer purchases electricity at a voltage level that must be stepped down for use. "Dry-type" is a reference to the type of insulation medium, which means that the core and coil is cooled and insulated by air, as opposed to "liquid immersed" transformers that use oil as the coolant/insulant.  “Low-voltage” means that it has an input voltage of 600 volts or less.


Initial standards for low-voltage dry type transformers were set in EPAct 2005 and became effective in 2007. In March 2010, DOE began a new rulemaking to update standards for the three main categories of distribution transformers: liquid-immersed, low-voltage dry-type and medium-voltage dry-type. In February 2012, DOE proposed a very modest increase in efficiency for LVDT, below the level agreed to by manufacturers in 2011 transformer negotiations facilitated by DOE. However, in the final rule published in April 2013, DOE improved the standard, reducing electrical losses by about 18 percent compared to products meeting the current standard. The standard for dry-type transformers is based on a voluntary industry level called “NEMA Premium”, developed by the National Electrical Manufacturers Association.The standards went into effect on January 1, 2016. 

In September 2021, DOE published a final rule for the distribution transformer test procedure, effective mar 14, 2022. 


Transformers are generally very efficient — electricity losses are usually below 1 or 2%. However, since all power generated goes through one or more transformers, even small improvements can yield very large national savings benefits. In general, transformers can be made more efficient by using better quality windings (which can be aluminum or copper) and through improved core designs, material (electrical grade steel), and construction. Amorphous core material offers the biggest step up in efficiency.


Savings through what year?: 2045
Energy saved (quads): 2.43
CO2 savings (million metric tons): 161.6
Net present value savings ($billion) 3% discount rate: 9.04
Net present value savings ($billion) 7% discount rate: 2.67


Federal Date States
Potential Effective Date of Updated Standard 2024
Updated DOE Standard Due 2021
2nd Federal Standard Effective 2016
2nd Federal Standard Adopted (DOE) 2013
2008 AZ Standard Effective *
2007 NJ Standard Effective *
2007 WA Standard Effective *
2007 RI Standard Effective *
1st Federal Standard Effective 2007
2006 CT Standard Effective
Test Procedure - Last Revised - Active Mode 2006
2005 MD Standard Effective
EPACT Initial Federal Legislation Enacted 2005
2005 NJ Standard Adopted
2005 WA Standard Adopted
2005 AZ Standard Adopted
2005 RI Standard Adopted
1st Federal Standard Adopted (Congress) 2005
2004 CT Standard Adopted
2004 MD Standard Adopted
2003 CA Standard Effective
2002 CA Standard Adopted
1998 MA Standard Adopted
1998 MA Standard Effective

* State standard never went into effect due to preemption by federal standard.

Timeline reflects state standards from 2001 to present; federal standards from inception to present.