It is usually quite simple to calculate the electrical load on a circuit. If there is only one electrical device connected to the circuit, the load is simply the amount of current the device consumes when it is running. If a circuit only powers an electric bilge pump, and the pump draws 8 amps, then this is the load. The wiring and fuse or breaker must be adequate to handle 8 amps.
If there are several devices connected to the circuit and all are often running simultaneously, then the total load can be calculated by adding up the current requirements of each device. For example, if we have an autopilot that requires 6 amps, a GPS that draws 2 amps, and a chartplotter that draws 3 amps, then the total load on the circuit would be 11 amps.
But what if we have a number of devices connected to a circuit, some of which may be on all the time and some of which are on only part of the time? Do we tally up the loads for every device and design the circuit to handle this worst case load? Do we take the average load? Maybe the average load plus a fudge factor? If we guess wrong in one direction, we will be popping breakers or blowing fuses on a regular basis. If we design the circuit for the worst case scenario, we will be spending a lot more on wiring and breakers than we probably need to.
Fortunately, the National Fire Protection Association (NFPA) as well as the American Boat and Yacht Council (ABYC) provide some guidance. The ABYC provides a worksheet developed for boats, and it is quite helpful in determining the likely total load on a circuit that has multiple devices. I've modified it slightly and show it below.
The two columns on the left side are for devices connected to the circuit that are on either continuously or for long periods of time. There are several blank lines provided, so if your boat has other devices on the circuit, add them in 'Column A'. Next enter the current required for each device in the 'Amps' column, and add these all up and enter the total in the cell labeled 'Total of Column A'.
The columns on the right are for devices that are only on occasionally. Again, add any additional devices in 'Column B' and then enter the current required for each device in the 'Amps' column. Add these all up and enter this total in the cell labeled 'Total of Column B'. Now multiply this by 0.1 and enter it in the following cell, '10% of Total of Column B (1)'.
Scan the 'Amps' column on the right side and find the largest single load. Enter this in the cell labeled 'Largest Single Item of Column B (2)'. (Not too difficult to follow so far, eh?). Enter the larger of the two values, either (1) or (2), in the applicable cell on the left side, and add it to the 'Total of Column A' cell. The resulting sum is the total load likely to be seen at any given time.
I find the process to be fairly straightforward, and much easier than, say, filling out a 1040 Tax Form.
Next week, I'll talk about circuit breakers and fuses. Stay tuned.