Why 3% and 5%?

Wire has resistance, so some voltage is lost as heat before it reaches your device. Lose too much and motors run hot, LEDs dim or flicker, chargers underperform, and breakers can trip for no obvious reason.

The widely used rule of thumb (based on a recommendation in the U.S. National Electrical Code): keep the drop on a branch circuit at or under 3%, and the total from your panel to the device at or under 5%. These are guidelines for good performance, not laws — low-voltage systems like 12 V solar are often even stricter, because 3% of 12 V hurts a lot more than 3% of 240 V.

Why don't other calculators agree with each other?

Almost always one of two things: some tools want round-trip length where others want one-way (we always use one-way and double it for you), and some use slightly different resistance values for the wire. Open "How we calculated this" above and you can check our numbers against anything.