How to Wire a 12/3 Romex Cable | eHow www››Home Repair & Maintenance›Electrical RepairsHow to Wire a 12/3 Romex Cable. Romex 12/3 wire is used in buildings for circuits that use higher amperage than the standard 20 amps. Most people are not familiar with this type of wire because it is used to hook up heavy-draw appliances such as water heaters and central air conditioning. In most cases, there is no plug outlet because it is.. Two 20amp Circuits On 12/3 Wire - Electrical - DIY wwwhatroom›Home Improvement›ElectricalAug 14, 2009Is this to code, to put 2 circuits on a 12/3 wire, or should the wire guage match the total amperage since it's sharing the neutral? (ex: 40) I am designing a PDU for a custom server rack and what I I believe stubbie has another wonderful diagram to illustrate the broken neutral on a MWBC and the effects it has on the connected loads.