SV Pilgrim - 1979 Morgan 382 - Homeport: Beaufort, NC

Sunday, October 18, 2015

Let's Talk Marine Wire

Within days of Pilgrim arriving in Beaufort, Feb. 2014, we removed all of the existing wiring.

Creative and dangerous wiring in Pilgrim's bilge.

34 years of modifications and repairs to the electrical system by many hands left a hodgepodge of wiring.  Almost all of the wiring was sub-standard.  Some was outright dangerous - note unfused bilge pump wire run joined with wire nuts in image above – YIKES!

Unfortunately Morgan Yachts did not use marine grade wire in the original construction.

Factory installed shore power circuit breaker removed from Pilgrim.

So all the original wiring was also removed.

Gutting the original electrical panel.

All the wire going back into Pilgrim is marine grade.  So what makes marine grade wire different from home or automotive wiring?

  • Marine grade wire is composed of tiny copper strands.  The wiring in a land based structure is typically a single thick copper wire. Stranded wire has more flex.  This allows the wire more easily fit into the tight spaces on a small vessel while placing less stress on connections.
  • All the tiny strands of copper in a marine grade wire are tinned (plated with tin.)  The tin coating protects the copper from corrosion due to moisture and heat.
  • The plastic insulation covering the exterior of marine grade wire is designed to resist degradation if exposed to fuel or oil.  This is very important for wiring in bilges and engine compartments.

What about the size of the wire (yes it does matter – haha.)  Size of electrical wiring here in the USA is designated by the American Wire Gauge (AWG). Wiring on marine vessels can vary from tiny 22 to 26 AWG in electronics and NMEA backbones to 00 (2/0) or 0000 (4/0) AWG wire in high amperage wiring for starter motors, windlasses, electric winches, etc.  Typically wires equal to or less than zero are referred to as “aught” (e.g. 2/0 is “two aught” and 4/0 is “four aught”).  

Wire must be properly sized based on type of current (AC or DC), voltage (12, 24, 120, 220), load current required (Amps), and total length of circuit (Feet).  If you are looking for more information, I recommend Blue Sea Systems: Choosing the Correct Wire Size for a DC Circuit.

The article above is the source for my preferred 12V DC Wire Sizing Table.

The sizing chart does not tell the entire story.  Each wire size also has a maximum ampacity. 

Ampacity is defined as the maximum amount of electric current a conductor or device can carry before sustaining immediate or progressive deterioration.

Thanks to Wikipedia for the speedy definition.  Factoring a wire’s ampacity and fusing wire runs accordingly is critical in primary wiring (i.e. feeds to battery selector switches, to electrical panels, to ground bus, etc.) and wiring for high amp components (i.e. starter motors, windlass, electric winches, etc.)  Here is a handy chart for maximum ampacity.

Choosing the correct size wire and fusing it below its rated ampacity is critical for the proper functioning of on-board systems and for the safety of all aboard.  I’ve printed both of the tables above and keep them handy in a plastic sheet protector when working on electrical systems.

Hope this information is helpful.  If anyone reading this post feels the information is stated incorrectly or it is misleading please let me know via a comment. 

I’m in full support of boat owners getting dirty and tackling their own projects, but if uncertain when assessing, altering, or installing electrical systems in your vessel please seek professional assistance.


  1. I think everything you have said is correct Jeff. BUT. I use wire of size which matches either the marine charts you show, OR the standards for house wiring on land... whichever calls for the LARGER wire size. For example, the 20 amp outlet circuits in your house are wired with 12 gauge wire (and hopefully fused for 20 amps). I would never use 18 or 14 gauge wire for a 20 amp circuit on a boat, even for short runs...


  2. Bob, Thanks for sharing your thoughts. I'm going to go find a chart for house wiring and compare.