Circuit Wizard-Case Study

August, 20 2007

Using Blue Sea Systems Circuit Wizard to Size Main and Branch Circuits

Blue Sea Systems' DC Circuit Wizard simplifies the calculations that are required to determine appropriate DC circuit protection and wire size for DC feeder wire and Branch power distribution panel circuits. This case study provides a practical example of how to use the Circuit Wizard to perform these calculations.

Follow this four-step process*:

1. Compile an inventory of loads, and wire length to and from each load
Inventory all electrical loads and the length of the wire between the distribution panel and each load, and back. Be sure to measure the length of wire from panel to load and return.

2. Calculate total load for panel feed wire
For this example, the total load for the panel is calculated to be 108 Amps (refer to ABYC E11 and the books referenced in the footnote for a detailed description of this process). The feeder wire must be suitable to carry this amount of current. The feeder wire is 20 feet in length (from battery to panel, and from the panel back to the battery).

3. Determine appropriate voltage drop for the panel feed wire, and Branch circuits
Panel feed wires should be sized to limit voltage drop—3%. For the Branch circuit wires that feed critical loads like communication radios and navigation lights, use a 3% voltage drop. For less critical loads like microwave and cabin lights, 10% voltage drop is acceptable. These values will be entered in the Circuit Wizard.

4. Use DC Circuit Wizard to calculate DC circuit protection rating and wire size
a) Calculate panel feed wire size and Main circuit protection
Enter the values for the panel feeder and Main circuit protection.

Click Calculate and the DC Circuit Wizard displays suitable wire size, and fuse and circuit breaker options. In this example, the recommended wire for the panel feeder is AWG 2. Examples of acceptable circuit protection options are 175A ANL fuse (PN 5128) or C-Series toggle circuit breaker (PN 7268).

b) For each Branch circuit, calculate wire size and circuit protection
Calculate wire size and circuit protection rating for each Branch circuit. For example, the bilge pump draws 4.2A, the wire length from panel to pump and return is 20 feet, it is a variable load, duration is 20 minutes.

The DC Circuit Wizard displays wire size, and fuse and circuit breaker options. In this example, the recommended wire for the branch circuit wire is AWG 16. The results indicate circuit protection devices with 12A and 15A ratings. Some acceptable options for this application are 12A Push Button Reset-Only circuit breaker (PN 7055), or A-Series toggle or rocker circuit breaker (PN 7208, 7303). Repeat this step for each branch circuit and load.  Circuit protection is normally selected to protect the wire, but in the case of pumps, a smaller circuit protector may be recommended by the bilge pump manufacturer to prevent overheating and a fire hazard if the pump is blocked.  The recommended protection for this pump is a 5A fuse.

The DC Circuit Wizard simplifies the complex process of wire size and circuit protection selection. However, it can’t take into consideration all of the unique characteristics of your boat, and can’t replace the experience of a qualified marine electrician. It is available at

*Steps 1 through 3 of this process are presented in ABYC E11 and described in several books on boat wiring, for example, Boatowner’s Illustrated Electrical Handbook by Charlie Wing, Boatowner’s Mechanical and Electrical Manual by Nigel Calder, and The Marine Electrical and Electronics Bible by John Payne.
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