Basic Branch Circuit Question

Hi All,

I have likely a pretty newbie question:

If my boat has a 30A shore power and it comes into my inverter/charger.
And if my inverter/charger has an output current of 100A.
And assuming that I stay under the wattage of the inverter (let's say 3000 W).

Can I run move items than 30A? That is if I have three appliances that each pull 15 amps is that going to be ok because my invertor has an output current of 100A? Or is that going to pop the shorepower circuit because that is only 30A?

Just trying to figure things out as I am looking to live aboard and need to organize my electrical loads and determine what I am able to draw while I am living at the dock.

Thanks,

Chris

The short answer is NO.

The 100 amp output refers to the 12 volt DC charging output.

If the inverter is rated at 3000 watts then you have around 25 amps of usable power on the AC output while inverting and 30 amp pass through while charging.

Agree with TG. When you are connected to shore power, you are not inverting, you are charging only. The shore power passes through a relay in the I/C straight to the AC load panel.

Actually it depends on the inverter. A standard Magnum inverter will only pass through shore current. So if you try to pull 40 amps on a 30 amp circuit a breaker some where will pop. Now if you have a Magnum Hybrid things are different. With the Magnum Hybrid, if you pull 40 amps you will get 40 amps, 30 amps from shore and 10 additional amps from your batteries.

Oh. I like that idea.

I am going to live aboard and my current inverter/charger (Freedom 20) is acting flaky. So I anticipate (plan) to replace it. The thing is, I am trying to understand what would be best to move to. As I will be mostly at the dock, and I have a diesel stove for heat, I am looking at the load that I will need and the option to improve from what I have.

The idea of having an inverter/charger that will "top up" the needs is appealing. Especially if I need to have an electric heater on and then do something like run the microwave or expresso machine!

Chris

You mention in another thread you have a CHB for a full time liveaboard. In this thread you mention the possibility of needing to run an electric heater and wanting to run a microwave or other intermittent load. You are considering a hybrid inverter/ charger to make up the excess draw. Remember you have to replace the excess draw which reduces available power while the batteries are recharged.

I want to welcome you to the 30 AMP dance. You will be actively managing your loads especially in winter.

I encourage you to keep some basics in mind.

Don't count on drawing the full 30 AMPS shore power continuously. Things will likely overheat possibility to the point of risk of fire. Consider 24 AMPS as your max continuous load. 30 x 80% = 24. This is especially important on older boats.

24 AMPS @ 120 volts is 2880 WATTS
A 1500 WATT space heater plus the 1500 WATT water heater element when it kicks on puts you in the red zone. There's nothing left for lights, refrigeration, entertainment etc.

Find ways to lower your electric demands.

Your Dickenson with water coils is OK once you get it burning correctly. But for a full time liveaboard in your area I'd want more. 30,000 to 40,000 BTU for comfortable living. I'd want a hydronic system that also heats the hot water tank taking that load off your electric system.

A hydronic system installation is mind bogglingly expensive. If it's more than you want to spend then consider a forced air system. And swap out your water heater to one with a 750 WATT element.

A way to ease winter sleeping discomfort is to use heated mattress pads. They don't draw much power but they increase comfort.

Cook with propane not electricity.

Good stuff ^

OP: @Portage_Bay #6 is spot on.

I will emphasize that a 10 amp AC load will require 100 amp DC (+/-). That is considered a big load on most moderate house lead acid (FLA, AGM, GEL) battery banks and is not sustainable for any length of time.

The idea of the inverter "helping" to limit shore power current is a good one, and several manufacturers support that feature.

My current inverter/charger setup is using Victron Multiplus units. When tied to shore power I set the shore power limit to 20 amps.

If the load passing through the inverter wants to exceed 20 amps then the inverter uses the batteries to support the load.

@ksanders - when you do that (use the inverter to pick up the load beyond the 20 amps), this obviously draws down the batteries. Is it correct to assume that the charger side of the inverter/charger will continue to charge the batteries so what you are pulling is being topped up continuously?

TheLake said:

@ksanders - when you do that (use the inverter to pick up the load beyond the 20 amps), this obviously draws down the batteries. Is it correct to assume that the charger side of the inverter/charger will continue to charge the batteries so what you are pulling is being topped up continuously?

Click to expand...

The Victron will limit your shore power draw based on the way you set it. If you set it to 20 amps, it will only draw 20 amps for all uses. If you use 30, you are drawing 10 (AC amps) from the batteries (100 amps DC). It will do this until you reduce load or your batteries hit the cut-off voltage. Once you reduce load, if there are amps available (out of the 20 you've selected) to charge, it will do so.

Lower end inverter/chargers that don't have shore power input limits will pass through the 30 amps from shore power to your AC system AND also attempt to charge the batteries on top of that, exceeding the 30 amp breaker at the shore power pedestal and/or the shore power breaker on the boat's panel or the inverter itself. You have to be aware that if your batteries are being charged, you are cutting into the 30 amps of AC you have available for other loads.

TheLake said:

@ksanders - when you do that (use the inverter to pick up the load beyond the 20 amps), this obviously draws down the batteries. Is it correct to assume that the charger side of the inverter/charger will continue to charge the batteries so what you are pulling is being topped up continuously?

Click to expand...

SBMAN explained it perfectly in the post above

I lived on a Uniflite 42. I had 30 amps of power and a Trace 2512sw inverter/charger which is very similar to the Magnum 3000 watt Hybrid. The inverter was programed to limit to 30 amps. Now the hot water heater alone used 13 amps, then two space heaters set at 750 watts each used used another 13 amps. This left 4 amps for lights and computers ect. So, as long as no one turned on a microwave or a hair dryer life was fine. The beauty of my system was that I had 1250 amp hour of battery power. This meant I could have everything going in the morning with no worry of tripping a circuit breaker. Fortunately you only exceed the 30 amps for short periods of time and there was plenty of battery to pick up the slack. Then after the hot water was replenished the Trace Unit would sense this and would crank up the charger to replenish the batteries. I went on with this system for years never worrying about not have enough amps.

I must caution everyone. I wired in a 50 amp socket into the boat and used a 50 amp cord with a 30 amp adapter. This moved all the danger of fire to the dock pedestal. This system spent way too much time at max capacity and I burned up several 30 amp adapters. I would not do this with just a 30 amp socket and main cord, they can not take the load for extended time.

Automatic Load shedding is fairly easy to set up.

A NC (normally closed, current passes thru) relay is required . When an item like a HW heater is hooked to a lead wire from the item you wish to power, like a reefer or microwave , the lead wire triggers the HW heater relay to open.

Living well on a 15A plug can be done with a handful of relays and some thought.

Grainger used to sell 30A relays that could be adjusted to NO or NC .

FF said:

Automatic Load shedding is fairly easy to set up.

A NC (normally closed, current passes thru) relay is required . When an item like a HW heater is hooked to a lead wire from the item you wish to power, like a reefer or microwave , the lead wire triggers the HW heater relay to open.

Living well on a 15A plug can be done with a handful of relays and some thought.

Grainger used to sell 30A relays that could be adjusted to NO or NC .

Click to expand...

this is a good idea, and can be done in a variety of ways. i never got around to it, but on my last boat i was going to use a programmable panel meter with an amp coil on it to drop the water heater relay out if the load is too high. a timer to hold it off for a reasonable time value so it wouldn't keep cycling. usually dropping out the water heater is enough to keep from tripping the shore breaker. my new boat has double 30a shore inlets, so that thought became a very low priority.

Great idea about the relay for the hot water heater. I will look further into that.

Thanks for all the insights and help everyone!

It’s a good idea to learn about the subject of load shedding. Also, your boat likely doesn’t have a decent breaker near the shore power boat receptacle. It’s a good idea to look at the cabling to the AC panel and rewrite that short distance. Also to understand 12 volts DC management, battery health and so forth. Nigel Calder is an excellent read.

Jim