Key:voltage-low

This feature has been labeled as deprecated. The recommended replacement is: guidelines below.
The reason is documented in Deprecated features. You are still free to continue to use or interpret this tag as you see fit since OpenStreetMap does not have “banned features”.
Under no circumstances should you (semi-)automatically change “deprecated” tags to something else in the database on a large scale without conforming to the automated edits code of conduct. Any such change will be reverted.

Replacement guidelines

Please see dedicated documentation to get more information about convention reviewed by OSM contributors in 2017.

The convention OSM reviewed makes primary on the original producer side, like power generator or distant power plant whereas other sides including secondary sees consumers. Tertiary, quaternary and further sides are intended for lower voltages auxiliary services inside power plants or substations.
Most producers usually step up their voltages for transmission (generator) toward consumers which makes by convention primary > secondary anywhere else in the infrastructure even if energy can actually flow in all directions on many intermediate nodes.

The most important for OSM is the consistency between transformer=* and voltages, particularly for facilities that both produce and consume power at different times. No choice is wrong when values are consistent.

voltage-low=* is in use in many places. Some mappers may be more confident by using high and low voltages. However these high and low collide with high and low voltages (like high voltage for transmission and low voltage for domestic usage) and transmission transformers deal with two different high voltages for instance. Not to mention there are often more than two voltages in transformers used in transmission networks. Such information may help match both ways as following:

• voltage-low=* = voltage:secondary=* in case of down transforming, anywhere outside a power generating facility. The power normally flows from higher voltage grid to lower voltage grid towards consumers.
• voltage-low=* = voltage:primary=* in up transforming places like power plants, for generator only. Generators produce pretty low voltage power whereas transmission power grids operate at very high voltage. According to convention, the power generator side is definitely the primary interface and here at lower voltage than the secondary connected to power grid.