What is one requirement to check before paralleling two transformers?

One crucial requirement to check before paralleling two transformers is that the voltage ratings are identical. This ensures that both transformers can operate harmoniously together without causing overloading or damage to either unit. When transformers are connected in parallel, they need to operate at the same voltage level to maintain system stability. If their voltage ratings differ, it can lead to circulating currents between the transformers, resulting in inefficiencies and potential overheating.

The other options, while they may be relevant in specific scenarios, do not have the same critical impact as voltage compatibility. Color matching doesn't guarantee electrical compatibility; it's more related to identification rather than function. The size of the transformers is important for understanding their capacity and load handling but does not directly address their operational compatibility in terms of voltage. Similarly, the age of the transformers can provide insights into their condition but does not directly influence their ability to be paralleled safely.