Electrical lines breaking under low power

In Oxygen Not Included I am experiencing breaks in electrical lines even though they are not carrying over 1000 W of power. The entire system has more than 1000 W, but the branches where the breaks are occurring do not.

Is this just a bug in the design of the game?

Most likely the total wattage of equipment connected to your power source exceeds 1000W. This will cause the wires to degrade over time. Even if they are not using the power, when they do, the spike will be over 1000W and will cause damage.

The solution is to use transformers and heavy wires. Using this approach you can create a single high wattage line, and step the wattage down using a transformer, so that each sub-circuit has a maximum draw of 1000W. That is to say that the total wattage of equipment attached to the circuit is less than 1000W.

You might have multiple sub-circuits in order to meet the 1000W restriction, at least until you reach the upgraded wires technology.

If you'd like a more in-depth explanation this guide video is a good start.