Again!?

There I was sitting up in the flybridge enjoying the view and a morning cup of coffee while reading about some of the new things discovered by the JWST (James Web Space Telescope). Cool stuff that, enough to keep one's attention. So the first whiff of something getting way too hot didn't really register. The second snootful however, clearly hot rubber smoke mixed with electrical fire stink, set off every emergency warning bell in my brain. Within seconds I was down the ladder and in the salon, startling Deb with the worst words one can hear on a boat, “What's on fire?” But my brain, informing my feet without bothering to inform my head, already knew what was wrong and had them (my feet that is) heading out the side door and onto the dock. Sure enough our brand new shore power cord was smoking, turning black and, even as I moved to throw the dock-side breaker, bursting into flames. It wasn't a big fire. Grandson Eldest, having been rousted out of his berth when the AC went down, was right behind me, fire extinguisher in hand. A quick squirt was all it took to go from “EMERGENCY” mode to “What the hell was that all about?” curiosity. 

Two fires in less than a month? Something is clearly amiss. The first thought was that the “Y” splitter, 50A > dual 30s, was the culprit. That side of the connector to the 50 foot shore power chord was clearly the most heavily damaged and the original source of the way-too-hot. But that particular connector was brand new, one I had replaced after the first fire. I'm not perfect and may not be any good at a lot of things. But I am pretty good around electrical work and putting a connector on the end of a wire is not exactly brain surgery. But discretion is better than arrogance. Something was seriously wonky and I didn't know what is was. But, whatever it was, I was at least in some way clearly responsible for it.

We went out and bought a fancy new Marinco “Y” adapter complete with EEL connectors. No more splicing a new connecter onto an old chord. And though the damage was limited to the last six inches of the brand new (Marinco again) 50' 30 amp shore power cord, it also went in the trash and was replaced with new. (Yep, Marinco.) But I went poking around some more because something in the back of my my mind was telling my brain that I was being stupid and missing something obvious. Prodded by that feeling, we powered up the Air Con side of the AC power panel and brought the units on line one at a time. With just the forward unit running the amp gauge showed 12 amps. With both units on line I was looking at 28 amps, maybe a touch higher. More musing and I felt like was was looking right at the problem and not seeing it. Then...

The something floating around was a vague memory that the amp rating is for intermittent loads, not continuous loads. Thirty amps intermittent would be roughly 20% lower; 24 amps in this case for a continuous load. Both times we have smoked a shore power cable was with the Air Con running full blast for days on end. But it was (is) only a vague memory at best. So I went looking, starting the search by pulling up the Marinco Operations manual for the ELL shore power cords: ZX449 MAR_TL_032_0112. I read through it several times and didn't see a word about the 30 amp rating being for intermittent loads only. Not even under the “overheating” troubleshooting guide does it suggest that the cord is not rated for 30 amps continuously. Other Marinco literature also suggests that overheating is almost always a corrosion problem, not a word about amp draw over a long period of time. In addition a West Marine DYI shore power post also suggests that almost all cord failures are due to corrosion, not pulling too many amps. So running a 30 amp system at or near 30 amps continuously, at least for all of the official documentation that we could find, should not pose a problem.

And yet, now that I am deep in the rabbit hole of this whole ordeal, I am sure I remember picking up somewhere along the years that the amp carry ratings are always intermittent. After several hours of both Deb and I swimming through the murky waters of the internet we did find a post on the Sailboat Owners dot com forum that talks about the difference between continuous and intermittent.  (We also found and interesting read with USCG Safety Circular 81. It says nothing about continuous vs intermittent ratings, but it is still a good read.)

So, I'm going with I was stupid (or uninformed, or forgetful, depending on who is reading this). I smoked two cables and nearly burned the marina down by running the shore power at its max intermittent rating rather than at max continuous for long periods of time. For now we will run the salon (aft) AC in the day and the berth (forward) AC at night. The inside of the boat is about 15 degrees warmer both night and day, but I've had enough fires for one summer.

My bad. But being stupid (or uninformed, or forgetful, depending on who is reading this) is not the same as staying stupid (or uninformed, or forgetful, depending on who is reading this). And now you know what (I think) I know.  

Ps: I'm still going to ask if the Marina owner will check the output voltage at the dock. We are at the end of that circuit. Though this is a bit of a reach, low input voltage could cause or exacerbate an overheating problem caused by amp draw. I'm feeling pretty confident that running just one AC at a time will keep us out of trouble. But I would really like to know that I have scraped every side of this rabbit hole, and actually grasp how it is I nearly burned down the marina...twice.