If you have ever installed or replaced a commercial circuit breaker or fuse panel then you need to ask yourself… Have I ever heard of the”Total Cumulative Current” specification? Most people probably haven’t and this will include people who do this type of work. The more familiar specifications such as number of positions or fuse/breaker positions, the current ratings of the individual breakers/fuses, the size description are pretty much self explanatory. But a more obscure specification is the “Total Cumulative Current” specification sometimes called ” total current draw” or “max amperage” on the product data sheet. This specification tells you how much current you can draw from ALL of the circuit positions at one time. If you exceed this spec you may be on your way to a panel failure or worse a fire. Here is an example, If you have an 8 position panel with eight 15 amp circuit breakers or fuses installed, you might think that you can draw about 15 amps from each circuit simultaneously. This means drawing 120 amps total out of your panel. But wait, the total cumulative current rating of this panel might be 100 amps. This means you would be drawing 20 amps more than the panel is rated for. This will overheat the main panel feed wire and could potentially cause a fire. You must keep the total current under 100 amps.
Most installations probably will not exceed the total cumulative rating. However if you are installing a panel and are trying to keep costs down you may want to get the most out of a smaller panel by heavily loading each breaker/fuse position. In this case you may find you are overloading your panel. To avoid this, plan your installation carefully and do not try to shortchange yourself on breaker positions. If you have a lot of electrical equipment and need many breaker positions, you may need to divide them up around multiple panels. You may come away from this with the idea that designing your system will be harder but keep in mind that how much power you need is somewhat dependent on how big your boat is. A larger boat could potentially hold a lot of equipment and probably require a couple of panels minimum to do the job, but a small boat may easily get along with a single 8 position panel. The number of panels needed and the total current draw from the panel(s) may simply work itself out based on boat size and complexity but check it anyway! Also, the number of panel breaker/fuse positions do not determine the “Cumulative current” specification. In other words a 16 position panel may not have a larger cumulative current draw than an 8 position panel. So always CHECK THE DATA SHEET! Panels from different manufacturers do not have the same specifications just because they have the same number of breaker/fuse positions.
With this new knowledge take a look at your existing panel(s) or your new installation and figure out your total current draw to see if you are exceeding your panels limit. You will need the panel’s data sheet to figure this out. When you are doing this calculation, carefully assess whether or not all of your equipment would ever realistically be “on” simultaneously in normal or abnormal operating conditions. Under these conditions everything may be okay BUT if there is a chance that every load on your boat can be turned on simultaneously (such as kids playing) and that in turn could exceed your panels cumulative rating, then either change your system design or make sure someone is ALWAYS monitoring what is going on on your boat. You are the captain and ultimately responsible for everything that goes on on your boat.
The Calculations:
Calculation 1
Calculate the total current draw from each breaker/fuse position on your panel and add these current values together. This normally will not be the actual breaker/fuse rating installed.This total quantity must be less than the panels cumulative rating. You need to know the current draw of all of your equipment to do this calculation.
Calculation 2
Sometimes you can multiply the total number of positions by the breaker/fuse value (if the values of fuses/breakers are all of the same) and if that value is less than the cumulative current then you are good to go without looking up the current draw of all of your equipment. This works for the smaller panels (less than 8 positions) but for the bigger panels you need to know all of your current requirements and add the amp values all together like calculation 1. This second calculation is also dependent on the panel’s manufacturer.
The best way to do the calculation is by using Calculation 1. You need the current draw of all of your equipment to do it, but you should know what all of your equipment draws anyway, particularly if you are doing a new installation.

Here is Part 3 of my 4 Part Marine Electrical article. This article is about Fuses and Circuit Breakers.  Circuit protections devices are as important as determining the proper wire sizes when designing your electrical system. Use this article with the other two articles on Marine Wire listed in these posts

Click on the following link:  Fuses and Circuit Breakers

If you read the article mentioned in the post on “Marine Wire” you are ready to read the next article. This article is on sizing the wire for your boat project. Determining the right wire size is CRITICAL to your boats safety in terms of fire safety. Overloaded wire will almost certainly cause a fire on your boat, a situation none of us want. In addition to fire safety, the proper wire size  does affect the operation of most of your electrical equipment on board your boat. Read the article to find out more  by clicking this link.

Sizing Marine Wire

There is more to wire than meets the eye! This is the first of four articles appearing In “The Breeze”, the magazine put out by The Traditional Small Craft Association written by me. Click the link to read part one of the article.  http://www.baysidemarinedesign.com/BLOG pdfs/The Scoop on Marine Wire.pdf