Air Conditioning Circuit Breaker

[britinusa]

My new (nearly installed) Dometic AC unit requires a circuit breaker with max of 25amps.

I'm thinking of using the 'AC Aux' but it's only a 15amp breaker.

Anyone know if this is the correct 25amp breaker to fit?

http://amzn.com/B004XS6WN4

Thanks.

Paul

[Jim Hardesty]

Paul,
The breaker may be sized for the wire.  Before running the AC check that you have the right sized wire (#12 I think) all the way to the shore power plug.
Jim

[sailaway]

Shore power plug Is rated at 30 amp If you run back to the shore you need 10 gauge wire. 10 gauge wire is rated at 30 amps your shore power is on a 30 amp breaker. Most people will put in an extra shore power plug just for the a/c. If your a/c pulls 25 amps you only have 5 amps left to run your boat. hot water tanks uses 10 amps alone. My a/c only uses 15 amps but is only 12k btu so it works will at nite. Charlie

[britinusa]

The 25Amps max is as per the AC manual. I suspect that it'll draw much less more like 14amps.

The question is if the breaker in the link is appropriate to the panel.

Paul

[sailaway]

Paul the breaker is the right one I have changed them. Charlie

[britinusa]

Thanks Charlie.

Paul

[KWKloeber]

Paul, I would use the smallest breaker for the load.  If it's 14-a max amp at start up, then I wouldn't install a 25, even if the cable can handle it.  I'd first measure amps at start up and then next-up-size the breaker accordingly.  20-a?

kk

[Stu Jackson]

The Electrical Systems 101 Topic includes these:

Blue Sea Tables  https://www.bluesea.com/support/reference  Scroll down to find the PDF download:  Protect Your Boat with the Correct Size Wire & Fuse Chart.  Handy to keep around.

Battery Fuse Sizing with tables (from Maine Sail)

http://forums.sbo.sailboatowners.com/showthread.php?t=130775

[KWKloeber]

Another good article to download and live by...  Unfortunately the PDF is too large to post here.

http://assets.bluesea.com/files/resources/technical_briefs/ten_deadly_conditions_part_1.pdf
http://assets.bluesea.com/files/resources/technical_briefs/ten_deadly_conditions_part_2.pdf

Ken

[mainesail]

My new (nearly installed) Dometic AC unit requires a circuit breaker with max of 25amps.

I'm thinking of using the 'AC Aux' but it's only a 15amp breaker.

Anyone know if this is the correct 25amp breaker to fit?

http://amzn.com/B004XS6WN4

Thanks.

Paul

Paul,

#1 You can not just swap out a 15A breaker, which will be using 14GA wire, for a 25A breaker which SHOULD use 10GA. Standard practice for AC Branch Circuits; 15A=14GA, 20A=12GA and 30A=10GA

#2 You can technically get away with 12GA on a 25A circuit breaker with 105C wire but most good electricians would use the standard practice in #1 for breaker/wire sizing for AC loads.

#3 A competent electrician would not add an air conditioner to an existing 30A serviced vessel that also has a battery charger & water heater. Your shore power cord, even when new, should really not be loaded at any more than about 80% of its rating except for brief periods.. Course we are on boats in a corrosive environment with a fairly inadequate shore power standard which uses "twist-lock" shore power cords and, well, $hit happens. Loading these twist-lock services at anywhere close to 80% (24A) is simply foolish to do and leads to boat fires.. You are best to not allow your 30A shore circuit to exceed 20A. If you want to play it safe simply reduce the 30A breaker to 25A or 20A..

#4 A properly done AC install on a 30A service boat would include a new 30A service and AC panel all properly wired and dedicated to the AC system.

#5 The manufacturer has specified a beaker to work with that system and they also usually specify a "circuit ampacity" for system design. I know Dometic is usually about 15-16A for a 10k BTU unit and about 25A for a 16k BTU unit. This puts you squarely in the 12GA to 10GA wire range if using a 10K - 16K unit.. This means your minimum system design needs to be for the ampacity the maker asks for. Put yourself in a courtroom after your boat was found at fault for burning down the marina when you added a high current draw AC unit to an existing 30A service also including water heater and battery charger and ran the AC off the existing 14GA wire and simply swapped the breaker from 15A to 25A. Probably not a position anyone wants to be in. Follow the manufacturers suggestions for "circuit ampacity" and err on the side of larger wire if you are in-between sizes. Don't round down with wire size, round up.

#6 Don't forget that the locked rotor rating, often stated as LRA, will occur nearly every time the unit starts up. This is called the in-rush and the 25A breaker, if that is what they call for takes LRA into account. LRA on many units can exceed 60A. Also if the unit starts running poorly, improper pressures in the unit, the current draw can increase. The average current ratings are based on a properly working unit not one that has been over-filled with 134a, under-filled or has a problem.

When I am quoting a job like this it includes:

-New AC shore power service & dedicated AC panel, or converting the vessel to 50A service, if possible.
-Smart Plug shore power inlet and cord set.
-All new wire from the AC panel to the AC unit.
-Proper seacock installation for the pump
-Proper strainer for the pump
-Etc. etc...


On a 30A shore power vessel, a done correctly installation for a 10K BTU to 16K BTU unit should always include a second complete 30A AC shore power system installed & dedicated to the AC/reverse cycle system or to have the 30A 120V system re-wired for 50A 120V or 120V/240V... I CAN NOT stress this enough!! Yes, Done correctly is costly...

One should not run an AC & water heater on a single 30A shore power vessel yet installs like this are done every single day and shore power cord / boat fires occur at surprisingly high rates. In reverse cycle mode many of these draw upwards of 20-30% more than just AC mode, not including the 1A +/- for the water pump. Stack a water heater on that, & a battery charger, and now our crappy "Twist-Lock" shore power cord sockets are all of a sudden glowing red hot.

Please be careful!!!!

Don't become this..

[britinusa]

Thanks Mainesail. & Stu.

Your points are very much valued (as always)

The wiring from the breaker to the AC unit is 12awg, I will not be reusing the original wires on the old breaker (I am removing them - not used) (BTW, love your electrical marking process!)


How about if I installed a mechanical interlock to prevent the AC breaker being turned on unless both the battery charger, outlets and the water heater breakers were off?


One consequence of the dual shore power concept is that will mean either AC or the other stuff when running off our Honda EU2000i Companion Generator.



Paul

[britinusa]

Here's my concept of the Interlock

The default position is down which allows for the Outlets/Water Heater/Battery Charger to be turned on as required.

In the Up position, the Outlets/Water Heater/Battery Charger have all to be in the Off position then the AC Aux (AirConditioner) can be switched On

Paul

[KWKloeber]

Here's my concept of the Interlock

The default position is down which allows for the Outlets/Water Heater/Battery Charger to be turned on as required.

In the Up position, the Outlets/Water Heater/Battery Charger have all to be in the Off position then the AC Aux (AirConditioner) can be switched On

Paul

Paul, I think with everything, common sense can factor in. 
Is it detrimental to have the outlets powered?  No, if you're charging an iPad or running a fan.  Yes if you're running a hair dryer or whatever else high load. 
Do you have a Mwave?

Ken

[Ron Hill]

Guys : The simplest way to know when 110/115V AC switch is ON, is to wire in a light!!

Radio Shack has red, yellow or green peanut neon bulbs.  They are the size of a #2 pencil eraser.  They are easily wired in next to the switch - then you absolutely know (from a distance !) when a switch is ON. 

A thought

[sailaway]

Paul you are going to love your new ac mine was the best thing ever. I mounted it under the v-berth and used the useless drawer hole as cold air return. I put an air filter in place of the drawer hole an made it larger. then I took the ac air duct into the hanging locker star forward an put a tee with one outlet to the top an another in the main cabin lots of work but will worth it. Charlie