At the currents required for the heat bed on the 12-inch printer, wire resistance can be significant and affect the heater current draw substantially. To minimize power losses in the cabling and reduce bed heat-up time, it is usually a good idea to go with some beefy wire from the power supply to the heat bed relay and possibly even replace the lighter gauge wire that comes preinstalled on the heat bed.

I only have an 8-inch printer, but from what I've read others doing, N5QM is probably correct. Those that have really optimized their wiring to minimize cable losses have been pushing the limit on their power supplies.