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Old 03-07-2009, 17:54   #1
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Shore Power - 30amp Max?

I'm looking into buying a boat.

It appears that the standard hook up for everyone is a 30 amp shore power cord.

30 amps ain't much. Down in the South, an a/c can easily eat 30 amps by itself. Then there is a water heater, likly a decent 13 amps. Then all the lights, fans, water pumps, rechargeing the battery bank, and 12v appliances.

I've even seen boats advertized with a/c systems that will use a maximum of 45 amps during start up. What the heck?

What do people do to mannage their power at the shore to keep from burning up the cord and/or tripping the shore breakers? Two cords? Or just be careful? Do people find a way to sink up the shore power with some sort of on board power source?

I've seen a bunch of yachts advertized with 8kW generators, and 30amp cords. 30 amps at 110, is only 3300 watts. That doesn't seem like a good match to me. Or are all the cords 220V? Thats still only 6600 watts or so.
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Old 03-07-2009, 20:28   #2
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A single 30A 120V shorepower inlet would be sufficient for a small-to-medium sized boat. Larger boats use either two 30A inlets or a single 50A 120/240V inlet. The big megayachts use one or more 100A inlets, or even larger 3-phase systems.

Standard shorepower cords are sized for the appropriate amperage - longer cords use heavier gauge wire, shorter ones use smaller.

Remember that the amperage ratings are continuous. Startup surges for AC induction motors can be two or three times the continuous draw, but typically don't last long enough to trip a breaker - unless there are other loads running.

What size air conditioner have you seen that draws 30A? I have two MarineAir units - one's 16k btu, the other's 12k btu. They draw a little less than 13 and 15 amps, respectively. The minimum recommended breakers are 24 and 26 amps. One's on one 30A shorepower circuit, the other's on a second. I do have to be careful about running other high-current draw appliances, like the battery chargers or the hot-water heater simultaneously. But hey - it's a boat, not a house! You have to be more conscious about power consumption, anyway - especially away from the dock!
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Old 03-07-2009, 20:48   #3
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I was seeing a marine air 16000 BTU unit. That's 11 amps when it's running, and maybe 35 or 40 amps on start up. The mannufactor calls for a 45 amp breaker. Then you figure a 1500 watt water heater, that's 13 amps, give or take. Just between those two peices, your looking at 24 amps. And thats when everything is running nice and stable. If you've got the water heater heating up, and then the a/c kicks in, I would expect it to go way over the 30 amp rating of a cord.

Then you figure you'll have a 12 volt billage pump cycling from time to time. The possibility of a 12V powered head, or any of the other 12V stuff. I guess the battery bank might stabalize out the loading on the 12V, but still, those are going to be pretty decent hits on the battery charger.

I guess 30A might be enough at stable, but I would have been expecting the a/c to be triping the shore breaker all the time.
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Old 03-07-2009, 21:28   #4
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Originally Posted by ViribusUnitis View Post
I was seeing a marine air 16000 BTU unit. That's 11 amps when it's running, and maybe 35 or 40 amps on start up. The mannufactor calls for a 45 amp breaker. Then you figure a 1500 watt water heater, that's 13 amps, give or take. Just between those two peices, your looking at 24 amps. And thats when everything is running nice and stable. If you've got the water heater heating up, and then the a/c kicks in, I would expect it to go way over the 30 amp rating of a cord.
But that surge is only short term - it usually won't trip the 30A shorepower breaker.

FYI - Per the MarineAir spec sheet on the Cabin Mate series (document L-2314.pdf at Dometic's website), the recommended maximum sized breaker for either the 16k or the 12k unit is 40A @ 115V; minimum breaker size I've already stated. You're right - you have to be careful and manage your usage appropriately - just as I said.

Does one really need a 1500W water heater? Isotherm's water heaters are 750W - based on their thinking that when on shorepower, you have lots of time for water to come up to temperature. They include an inlet for cooling water from your engine to power your water heater while under way - it's probably the equivalent of a 1500 or 1750W element. I'm in the process of ordering such a unit to replace our old 1500W 19-gallon water heater - way too large for two people on a 51-foot sailboat.

That being said - any boat is a compromise. There's limited space (and usually budget) for all the comforts of a home on land. And things tend to be more expensive because they have to stand up to the marine environment, and there's extra safety considerations because of the higher probability of electrical faults due to moisture, vibration, and movement - much more so than a home on land. Things like stranded wire versus solid wire (handles vibration much better), oil/water resistant insulation for wiring, etc.

If you do end up purchasing a boat with only a single 30A inlet, you could always add a second - but that would entail some re-wiring, and you'd inevitably find that the initial install (even if factory installed) is probably not up to current ABYC recommendations. Or you could replace it with a single 50A inlet...

And in case there's still some confusion - yes, with two 30A inlets you do need two cords hooked up to two shorepower circuits. Increasingly, though, it seems most marinas have shorepower pedestals with a single 30A and a single 50A - so you could run a single 50A cable to a 50A-to-2x30A adapter - that's what we normally do...
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Old 03-07-2009, 22:21   #5
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I've been looking at boats in the ~40 foot LOA range. I'm wanting to have it set up for a family of 3. My wife, our kid, and me.

Well, my major issue was with is the semingly very common 30 amp cord up to the task. Just a quick math told me that 1 microwave, 1 water heater, 1 a/c system and all the 12V stuff on some of the boats is too much. The 12V loads could be quite large between 12V refer system, heads, lights, and god knows what else. Expecally if start up inductive loads are added up. Microwaves, and a/cs being bad about that.

I was thinking that the boats really were not designed to use the a/c or the water heater. Or that it was a somewhat poor design to use a 30 amp cord, but have an 8k generator set onboard. It would seem to my mind that if you've got around 8000 watts of generator, you should have a shore cord capible of handling around 8k.

You seem to indicate with with proper power mannagement that it probably is. Apperently the start up loads arn't long enough to effect the plug, or the shore breaker. That would seem to me to be a poor design compromise. It would involve pulling some rather high amperages, on rather hot days. I would expect that a better compromise would be a 30am 110/220 plug, which isapperently uncommon. A 50 amp 110/240 plug would certanly be overkill on a ~40 foot boat. (50 amp at 240 = 120,000 watts!)
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Old 04-07-2009, 03:41   #6
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Quote:
Originally Posted by ViribusUnitis View Post
I... I guess 30A might be enough at stable, but I would have been expecting the a/c to be triping the shore breaker all the time.
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Originally Posted by Beausoleil View Post
But that surge is only short term - it usually won't trip the 30A shorepower breaker...
Molded case circuit breakers have an inverse time-delay characteristic (specified on a time-current curve*), and an instantaneous trip mechanism which responds to a predetermined value of overload without any purposely delayed action (generally a short circuit).
Only reference to the specific breaker’s time-current curve, and the load’s starting current specifications, will determine whether the breaker will delay tripping long enough to permit the equipment to start.

However, I wouldn’t expect most 30A 2P circuit breakers, used on boats, to "nuisance" trip upon typical (short time) 40A A/C inrush currents, even when combined with other loads to a total of 50A (momentary).

Selective coordination is the selection and application of circuit protective devices in series, such that under overload or fault current conditions, only the device just up stream from the overload or fault (branch cct. breaker) will open to clear the fault. The remainder of the circuit’s protective devices (main breaker) will remain closed passing power to their individual loads. Selectivity can be based upon time or current levels.

* Time current curves are plots of the amount of current (vertical scale) flowing in the circuit to the time (horizontal scale) required for the breaker to clear the fault current.
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Old 04-07-2009, 05:08   #7
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Based on 26 years experience in a liveaboard marina, and on working on the electrical systems of many boats from 30-130', here are a few observations which might be helpful for the OP:

1. A single 30A power cord is likely to be insufficient for your liveaboard requirements. This is because the 30A design is itself flawed and -- unless brand new and sporting tight, clean contacts -- it won't really support a continuous load of 30A without overheating/melting. I can't tell you how many we've seen with burned contacts, meltdowns, etc.

2. The usual way of getting more power aboard for many boats is to add a second 30A circuit. Sometimes, even a 3rd 30A circuit is added, as was the case on my 44' houseboat. The additional power is required for heaters, appliances, and even ice-eaters in winter, and A/C in summer. While three 30A circuits seem like a lot, remember that it's really not safe to load any one of them more than about 25A.

3. Many boats use "splitters" which can take a 120/240V circuit and "split" it into two 30A 120V circuits. These are serviceable but, IMHO, are potentially very dangerous because it's possible to attempt to draw as much as 50A through EACH 30A cord attached, as the CPD upstream on the dock is 50A. Again, this practice frequently spawns overheating and meltdowns.

3. A much more satisfactory way to get additional power aboard is to use a 50A 120/240 circuit. The inlet at the boat is the same size, and carries four AWG6 wires from the dock. This can provide up to 50A in each of two 120V circuits aboard, for a total of 100A. We have converted many boats from two 30A circuits to a single 50A system with great success and satisfaction from owners. A single thick shore cord makes a neat connection which is far more robust than the 30A alternative. The downside of this is that it's a bit expensive: 50A shore cords are quite expensive, but they are the standard among larger yachts. If you shop around, you can find some deep discounts, though, and these 50A cords will generally last a lot longer than the 30A cords for reasons cited above.

4. Re: generator sizing, remember that the power factor of attached loads is important: inverter/chargers, A/Cs, frigs, etc. can place very heavy loads on generators. There's another very informative thread on this subject here: Gen / Xantrex Charging Problem

Hope this helps a bit.

Bill
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Old 04-07-2009, 06:00   #8
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It seems to me that, if you find a boat with enough shore power input to suit your needs for power consumption.........then you will have found the boat that will suit your needs for living aboard!
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Old 04-07-2009, 06:03   #9
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I assume that was in jest :-)

Lotsa boats with adequate power which are unsuitable for living aboard!

However, existing power capability should not be a huge factor in deciding on a liveaboard, because it's very possible -- and not all that expensive -- to add power capability if needed.

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Old 04-07-2009, 07:21   #10
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1 & 2: Circuits are should only be loaded to 80% of their nominal nameplate rating (30A x 0.8 = 24A continuous)*.
For instance, NEC Table 210.21(B)(2) shows that the maximum load on a given circuit is 80% of the receptacle rating and circuit rating. Thus, if you are planning to supply 20A on one circuit, that circuit must be at least 30A.
Also, you must size conductors no less than 125% of the continuous loads, plus 100% of the noncontinuous loads [NEC 210.19].
* According the 2002 National Electric Code (NEC), in Section 210-20:
"Where a branch circuit supplies continuous, or any combination of continuous and non-continuous loads, the rating of the over-current device shall not be less than the non-continuous load plus 125% of the continuous load."

However, every failure I’ve seen, I would attribute to either disconnecting under load, incomplete insertion, or some other operator failure.


3: For the most part, “cheaters” (adapter splitters) are dangerous, and always non-compliant /w code.

Quote:
Originally Posted by btrayfors View Post
Based on 26 years experience in a liveaboard marina, and on working on the electrical systems of many boats from 30-130', here are a few observations which might be helpful for the OP:

1. A single 30A power cord is likely to be insufficient for your liveaboard requirements. This is because the 30A design is itself flawed and -- unless brand new and sporting tight, clean contacts -- it won't really support a continuous load of 30A without overheating/melting. I can't tell you how many we've seen with burned contacts, meltdowns, etc.

2. The usual way of getting more power aboard for many boats is to add a second 30A circuit. Sometimes, even a 3rd 30A circuit is added, as was the case on my 44' houseboat. The additional power is required for heaters, appliances, and even ice-eaters in winter, and A/C in summer. While three 30A circuits seem like a lot, remember that it's really not safe to load any one of them more than about 25A.

3. Many boats use "splitters" which can take a 120/240V circuit and "split" it into two 30A 120V circuits. These are serviceable but, IMHO, are potentially very dangerous because it's possible to attempt to draw as much as 50A through EACH 30A cord attached, as the CPD upstream on the dock is 50A. Again, this practice frequently spawns overheating and meltdowns.

3. A much more satisfactory way to get additional power aboard is to use a 50A 120/240 circuit. The inlet at the boat is the same size, and carries four AWG6 wires from the dock. This can provide up to 50A in each of two 120V circuits aboard, for a total of 100A. We have converted many boats from two 30A circuits to a single 50A system with great success and satisfaction from owners. A single thick shore cord makes a neat connection which is far more robust than the 30A alternative. The downside of this is that it's a bit expensive: 50A shore cords are quite expensive, but they are the standard among larger yachts. If you shop around, you can find some deep discounts, though, and these 50A cords will generally last a lot longer than the 30A cords for reasons cited above.

4. Re: generator sizing, remember that the power factor of attached loads is important: inverter/chargers, A/Cs, frigs, etc. can place very heavy loads on generators. There's another very informative thread on this subject here: Gen / Xantrex Charging Problem

Hope this helps a bit.

Bill
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Old 04-07-2009, 09:36   #11
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It seems to me that, if you find a boat with enough shore power input to suit your needs for power consumption.........then you will have found the boat that will suit your needs for living aboard!
I hope you kidding with that. Everything that I've looked at so far would suit the liveablrd requirements as I see them. But almost every one has inadquate shore power by the above stuff.

Which interstly enough, is just as I suspected.

Very very intersting.
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Old 04-07-2009, 10:24   #12
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Living aboard in the Pac northwest we ran a small electric heater most of the time. We would burn up a 30 amp cord about once a year. I investigate 50 amp and even with my wholesale account it's extrememly expensive. If I remember right, the cord alone was $250+. Most people go with two 30 amp services instead of 50 amp service due to the cost and the availability of 50 amp connections at marinas.
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Old 04-07-2009, 10:34   #13
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ViribusUnitis,

enter www.victronenergy.com and download "Energy Unlimited" book there;
it has many answers, including all you needed when starting this thread.
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Old 04-07-2009, 10:37   #14
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that 50 amp cord is a bear to roll up and store
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