30 amp service to 15 amp system

jviss · Jun 14, 2023

Welcome to the asylum! Ha, ha.

What you say doesn't make a lot of sense to me. Maybe others know better. But, if you have a 30A service and 15A circuits, it's no different than having a 200A service at home with 15A circuits, no?

Interested in what others have to say.

Dave · Jun 14, 2023

jviss said:
Welcome to the asylum! Ha, ha.

What you say doesn't make a lot of sense to me. Maybe others know better. But, if you have a 30A service and 15A circuits, it's no different than having a 200A service at home with 15A circuits, no?

Interested in what others have to say.

Welcome to SBO.

There are likely more serious issues with this than just the number of amps being pulled. The biggest issue is not the current draw, the bigger issue is AC systems on boats are different from home wiring. To begin with there needs to be a main breaker that breaks both the hot (black) and neutral (lines). Unlike a house where the neutral and ground are tied together at the panel, boats should not be wired in that manner. These precautions are necessary to prevent electrocution and to limit galvanic corrosion on your boat.

If you are unfamiliar marine AC wiring, I'd suggest finding someone who is and has an ABYC certification.

The 200 amp service coming into a house goes through fuses on both legs in the meter box and then a 200 amp circuit breaker before the individual breakers. There is no main breaker on this boat making it possible to draw over 30amps if each circuit is drawn between 10 and 15 amps and we're hoping the power cord is properly sized (10ga) and the breaker on the pedestal is functioning and sized properly.

walt · Jun 14, 2023

Breakers protect the wire from getting too hot. Anytime you go to a smaller (higher number) gauge wire, you need a new breaker/ fuse.

The dock pedestal has 30 amp breaker and the cord you bought is likely 10 gauge wire. Inisde the boat, you are wired for 15 amp service and your picture shows 15 amp breakers. 120 wiring inside the boat is likely 12 or 14 gauge and is adequatly protected by the 15 amp breakers.

The reason that boats "require" breakers on both the hot and the nuetral is that there is a chance that the marina pedestal is wired with hot and neutral reversed. You have much less risk of the power coming into the house being screwed up polarity wise so homes only have breakers on the hot side.

Maybe the 15 amp breaker in your picture already has both hot and nuetral covered. It the breakers are only on the hot and you are sure the polarity at the pedistal is correct, you are fine as is. Easy to buy a polarity tester (at least for the 15 amp side).

Of course the totally correct answer is that if you dont have the double breaker on both hot and neutral, you probably are going to die a fiery death.

jviss · Jun 14, 2023

Dave said:
Welcome to SBO.

There are likely more serious issues with this than just the number of amps being pulled. The biggest issue is not the current draw, the bigger issue is AC systems on boats are different from home wiring. To begin with there needs to be a main breaker that breaks both the hot (black) and neutral (lines). Unlike a house where the neutral and ground are tied together at the panel, boats should not be wired in that manner. These precautions are necessary to prevent electrocution and to limit galvanic corrosion on your boat.

If you are unfamiliar marine AC wiring, I'd suggest finding someone who is and has an ABYC certification.

The 200 amp service coming into a house goes through fuses on both legs in the meter box and then a 200 amp circuit breaker before the individual breakers. There is no main breaker on this boat making it possible to draw over 30amps if each circuit is drawn between 10 and 15 amps and we're hoping the power cord is properly sized (10ga) and the breaker on the pedestal is functioning and sized properly.

1. I don't know why you're replying to and quoting my post;
2. I don't understand what you're saying in your last paragraph. How do you know there's no main breaker on the boat? And, what do you mean by "making it possible to draw over 30amps if each circuit is drawn between 10 and 15 amps?"

Properly sized cords and breakers don't necessarily prevent fires and overheating! Breakers don't detect heat, they only detect current.

jviss · Jun 14, 2023

walt said:
Breakers protect the wire from getting too hot.

That's not exactly correct. Breakers protect circuits from current in excess of the breaker rating - and that's it. It can't tell if a wire is getting hot, as it can from a poor connection, etc. It doesn't detect heat.

walt · Jun 14, 2023

I think it is "primarily" correct. Try google "are fuses sized for wire gauge"

A 30 amp breaker will always have 10 gauge or larger wire. Example, if I had a 30 amp breaker with a 30 amp outlet but the wiring was 20 gauge (ie, way too small of wire). Plug in a 30 amp load and that wire will at least get very hot possibly melting insulation. In this case, the 30 amp outlet is part of the safety as it doesn't allow you to plug in some device intended for 15/ 20 amp outlet which almost for sure uses smaller wire (like 12, 14 or 16 guage).

Device protection. If I have a 15 amp breaker, it will have wire rated for 15 amp and a specific type of socket that will only allow a device designed for that breaker and wire to be plugged in. If the device needs protect beyond that, it will have some sort of internal fuse.

FYI, one time I bought a 30 amp to 15 amp converter at a big box hardware store. One side plugged into a 30 amp outlet, the other side allowed a standard 15 amp plug. Simple wire to wire connection. You really should not be able to buy such a device. I checked for a UL logo on the adapter, of course there wasnt any.

Arcing sockets... that is why we have those wonderful AFCI breakers that I think are required on new home builds. I guess they trip fairly easily when something like a vacuum cleaner or table saw is turned on.

dlochner · Jun 14, 2023

jviss said:
1. I don't know why you're replying to and quoting my post;
2. I don't understand what you're saying in your last paragraph. How do you know there's no main breaker on the boat? And, what do you mean by "making it possible to draw over 30amps if each circuit is drawn between 10 and 15 amps?"

I was responding to both the OP and your post.

The photo of the panel shows 3 15 amp circuits and the OP said:

Dantheman said:
I think I need to install a main breaker that runs from a 30a inlet on the boat

Finally, it is simple arithmetic, 3 circuits each drawing 11 amps will have a total draw of 33 amps above the 30 a rating of the cable and breaker. Why is this a problem, shouldn't the breaker break? If the breaker on the pedestal is good, new, and accurate it should. But that is not always the case in all pedestals. You will note that I included these assumptions in my post.

jviss · Jun 14, 2023

dlochner said:
I was responding to both the OP and your post.

The photo of the panel shows 3 15 amp circuits and the OP said:

Finally, it is simple arithmetic, 3 circuits each drawing 11 amps will have a total draw of 33 amps above the 30 a rating of the cable and breaker. Why is this a problem, shouldn't the breaker break? If the breaker on the pedestal is good, new, and accurate it should. But that is not always the case in all pedestals. You will note that I included these assumptions in my post.

Ah, I see, David. I think you are laboring under a common misconception. The individual branch circuit breakers have no reflection on the service capacity; they only serve to limit the current to the branch they serve, based on the maximum current the branch circuit wire can support safely. Consider a typical household service, a 200 Amp service into a 40 circuit cabinet. If half the 40 circuits are 20A, and the rest 15A, that would be 700A total, far in excess of the 200A service. But that's not how it works. The circuit breakers are sized to the wire, not the available Amperage.

Maine Sail · Jun 14, 2023

Dantheman said:
Hi all,

Trying to set up shore power on my Irwin 27 for when I move aboard in September for refrigeration, air conditioning, and computer needs. My marina's pedestal has a standard 30 amp outlet, and I have purchased a 30A cable. The previous owners did not use the AC system, so it is not active, though it is partially wired with 120V outlets throughout the boat and a 15 amp breaker already installed (see attached). After some research, some say that running 15a from 30a shore power is unsafe, even using a GFCI with a 30/15 adapter, as a short could cause a firey overload before the breaker is tripped. I think I need to install a main breaker that runs from a 30a inlet on the boat, then run wires to the existing subpanel (attached) from the main breaker for the individual circuits, right? Any suggestions appreciated. Thanks!

-Dan

That photo is not a safe or correct AC installation. You need a double pole AC main breaker that is ELCI and opens hot and neutral, as well as a reverse polarity indicator. This is a bare minimum there's a lot more to it to keep it safe.

obxjeff · Jun 14, 2023

An analogy is a tree, the trunk is the 30a source, each branch connected to the trunk has 30a available but are smaller and so need to limit the amps flowing into them. This is what a fuse/breaker does. Even with only one branch, as long as that branch is correctly fused for its wire size/guage everything is fine. So a 30amp trunk with a 15amp fused branch is perfectly safe and typical.

Expanding on this, if you have five 15amp branches for a total of 75 amps, the trunk can handle this assuming only 30amps total is being used at any given time. If you exceed 30 amps total use, the shore pedestal breaker will trip killing all power.

dlochner · Jun 14, 2023

obxjeff said:
An analogy is a tree, the trunk is the 30a source, each branch connected to the trunk has 30a available but are smaller and so need to limit the amps flowing into them. This is what a fuse/breaker does. Even with only one branch, as long as that branch is correctly fused for its wire size/guage everything is fine. So a 30amp trunk with a 15amp fused branch is perfectly safe and typical.

Expanding on this, if you have five 15amp branches for a total of 75 amps, the trunk can handle this assuming only 30amps total is being used at any given time. If you exceed 30 amps total use, the shore pedestal breaker will trip killing all power.

This is correct, except in a boat you want the main breaker on the boat to trip and break both the hot and neutral. The breaker on the pedestal will only trip one lead which leaves the possibility of the other lead being energized. The most obvious situation in which this could occur is when the polarity is reversed.

FastOlson · Jun 15, 2023

FWIW, in on our boat (when doing projects) I learned, early on, that my boat's 15 amp outlet circuit would let me run a small AC heater in the winter OR the vacuum cleaner, but not at the same time.

Trip that breaker, every time. And for the right reason, too!
That said, when the vacuum is in use it outputs quite a bit of heated air, from cooling its motor, anyway....

We are all so used to having oodles of AC power in our houses, and adapting to limited amperage (per circuit) in our boats is sometimes a learning process.

JBP-PA · Jun 15, 2023

Dantheman said:
Thanks Maine Sail. So I should pick up something attune to this west marine breaker, linked below, as well as a galvanic isolator?
A-Series 120V AC Main Circuit Breaker, Main + 1 Positions | West Marine

Yes, you could use that panel but you can also get the main breaker and use it in addition to your existing panel, such as

Amazon.com

Which is a lot cheaper, though neither of those are ELCI protected (think GFCI). ELCI will cost you another $100-$200

Blue Sea 8100 Elci Gfci Panel

BLUE SEA 8100 ELCI GFCI PANELELCI Main 30A Double PoleOne 30A AC ELCI Main Residual Current Circuit Breaker, leakage trip amperate 30 mAELCI provides overcurrent and leakage protection per ABYC E11 for whole boat shore power protectionRed Revers...

www.hodgesmarine.com

Or

Amazon.com

FastOlson · Jun 15, 2023

Here is a picture of the Blue Seas breaker I installed about 20 years ago. Surveyors approved of it.
It is wired up to our 30 amp inlet, about a foot away.

dlochner · Jun 15, 2023

JBP-PA said:
Yes, you could use that panel but you can also get the main breaker and use it in addition to your existing panel, such as

Amazon.com
Which is a lot cheaper, though neither of those are ELCI protected (think GFCI). ELCI will cost you another $100-$200

Blue Sea 8100 Elci Gfci Panel

BLUE SEA 8100 ELCI GFCI PANELELCI Main 30A Double PoleOne 30A AC ELCI Main Residual Current Circuit Breaker, leakage trip amperate 30 mAELCI provides overcurrent and leakage protection per ABYC E11 for whole boat shore power protectionRed Revers...

www.hodgesmarine.com

Or

Amazon.com

Be careful on Amazon and eBay as there are a lot fakes and knock offs being sold.

heritage · Jun 15, 2023

You have gotten lots of expert advice here. Installing a 30 amp receptacle is pretty simple after you get the courage to cut a fairly large hole on the side of your cabin. Then run a properly sized marine grade ac cable to your new 30 amp panel. I would then use your existing ac panel as a sub panel.
I have great memories of my time at the Boca Chica marina. You got to love the sound of Navy jets though.

kappykaplan · Jun 16, 2023

heritage said:
You got to love the sound of Navy jets though.

The sound of freedom! :beer:

Maine Sail · Jun 16, 2023

JBP-PA said:
Blue Sea 8100 Elci Gfci Panel

BLUE SEA 8100 ELCI GFCI PANELELCI Main 30A Double PoleOne 30A AC ELCI Main Residual Current Circuit Breaker, leakage trip amperate 30 mAELCI provides overcurrent and leakage protection per ABYC E11 for whole boat shore power protectionRed Revers...

www.hodgesmarine.com

Or

Amazon.com

That is the panel you want. It would then feed the branch breaker panel. A fail-safe galvanic isolator is a bare minimum..
Galvanic Isolator Article - Marine How To

30 amp service to 15 amp system

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