I've seen several Nuclear Reactor video tutorials, and they all seem to use a HV Transformer to allow their reactor to safely output its power to an MFSU. However my reactor has no transformer and safely outputs to a MFSU. What exactly determines the reactor's EU packet output so that I know when I must use a transformer?
The size of output packets is determined entirely by how much EU/t the reactor is putting out, which in turn determines whether and which transformers are needed.
An HV transformer is typically used because MV equipment maxes out at 128 EU/packet (regardless of EU/t), and a reactor design can easily scale up beyond that. The HV transformer is unusual in that it can accept any size of packet, which allows it to handle the typical High Voltage or Extreme Voltage that a reactor can put out.
Technically, a piece of equipment or wire can exceed its rated voltage if there are multiple sources of packets – two 32 EU/t sources will put 64 EU/t into the wire, but since the packets are only 32 EU each it still counts as LV. This makes using an EU meter to figure out wiring tolerance not actually very accurate, and you simply have to know what everything is outputting instead. So, a reactor being a single source, you can know its EU/packet will always match its EU/t, and you can figure out how big the packets it outputs are by measuring its EU/t.