Why Tin-Coated Marine-Grade Wire?
CAST Lighting abandonned the use of all-copper wire for Landscape Lighting - here is why.
Author: Steve Parrott
The corrosive effects of the outdoor environment present unique challenges to Landscape Lighting. Every component of the lighting system is subject to heat, moisture and physical abuse. Landscape Lighting wire is especially critical since its corrosion leads to degradation of lamp performance and (eventually) to complete system failure.
The following discussion addresses these key factors that determine long-term integrity of landscape lighting wire:
- Inhibition of wire corrosion
- Preservation of wire conductivity
- Optimization of wire solderability – during both initial installation and future additions of fixtures to the system
What Happens to Landscape Lighting Wire after Installation
- All outdoor wire is subject to 'wicking'. This is the migration of water and electrolytes under wire insulation. This moisture travels from wherever the wire has been cut including splice points, socket connections and transformer terminals. Wicking causes a progressive oxidative corrosion of the wire.
The wicking problem has long been recognized by marine engineers since moisture is constantly present in the marine environment. These engineers, working in concert with regulatory and advisory agencies, developed marine-grade standards for boat wiring. These standards require tin-coated copper wire. (UL 1309 and 1426)
The presence of moisture in the landscape makes outdoor lighting wiring more akin to marine applications than to wiring used in the low-moisture indoor environment. For this reason, CAST has adopted the Marine-Grade standard by introducing No-Ox tin-coated wire.
- Oxidative corrosion of copper and its effect on conductivity. Electrical conductivity is a measure of a materials ability to carry an electrical current. Copper is an excellent conductor making it ideal in all electrical applications. However, in the presence of air and moisture its surface oxidizes forming a layer of copper oxides that conduct electricity very poorly. This layer is not initially a problem since the layer is very thin and actually serves to protect the underlying copper.
In an outdoor corrosive environment, however, the oxide layer progressively extends deeper into the copper strand and eventually oxidizes the entire thickness. The resultant decrease in conductivity severely compromises the lighting system. Landscape lighting wire is especially prone to this severe corrosion because it is a stranded wire with very thin strands. Lighting installers often see the devastation of this effect when they pull old wire from the ground and see the strands completely blackened and brittle.
Tin-coating the wire protects from this type of progressive corrosion and loss of conductivity in two ways.
- Sacrificial. The tin coating differs in electrical potential from copper in a way that causes the tin to be oxidized in preference to the copper. In other words, instead of a progressive deepening of the corrosion into the strand, the tin must completely oxidize before the copper interior is subject to corrosion.
- Greater Conductivity of Tin Oxides. While copper oxides are very poor electrical conductors, tin oxides maintain good conductivity. This benefits the long-term conductivity of the entire wire bundle. Note: the initial conductivity of the tin coating is lower than copper, leading to a slightly higher voltage loss at time of installation. This difference is offset over time since the tin coating largely maintains its conductivity while the oxidized copper conductivity significantly decreases over time.
Solderability – Differences between Tin and Copper.
Landscape lighting installers use a variety of methods for making wire connections in the field. Of these methods, soldering provides the most secure, corrosion-resistant splice points. CAST teaches this soldering method at all its seminars and workshops.
Since tin solder is the preferred type, it stands to reason that tin-coated wire is the best recipient of this solder. The resultant connection provides a seamless surface of tin that extends from the solder point along the entire length of the wire.
This is critical for corrosion prevention since corrosion occurs most aggressively when two dissimilar metals are in contact with each other and exposed to the environment. A tin solder connection on an all-copper wire is an invitation for corrosion to occur at the copper-tin boundary.
Soldering can be especially problematic when old wire is soldered. This situation occurs frequently since installers are often called upon to add fixtures to an existing system. Tin-coated wire maintains good solderabilty as it oxidizes over time. Copper oxides, on the other hand, are extremely resistant to solder.
An installer trying to add new fixtures to a system with all-copper wire, may be forced to replace entire wire runs while a system with tin-coated wire may be cut and re-soldered to make the new connections.
More Reasons to use Tin-Coated Wire
- UL specified. In addition to the Marine-Grade applications mentioned above, tin-coated copper wire is required (various UL listings) for the internal wiring of any appliance subject to heat and humidity. CAST also uses tin-coated No-Ox wire for all internal wiring of its fixtures and transformers.
- Homeowner Perception. Homeowners readily see the value in using Marine-Grade wire. If you, the professional, assure them that the use of this wire will add longevity and reliability to their lighting system, they will be willing to absorb the extra cost. (Keep in mind that No-Ox wire is just a few dollars more than all-copper wire.)
- Reputation. No-Ox wire is unquestionably the highest quality wire available. Lighting Designers who maintain an uncompromising attitude toward quality will earn the best reputations and get the best jobs.
Sources for this article include various documents from UL, NEMA and IEC, as well as a thorough research of journals on corrosion and metallurgy. In addition, several experts in these fields were interviewed and overwhelmingly corroborated our conclusions.