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Old 10-20-2021, 11:28 AM   #1
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Watts versus Amps

When I did work on my house the factor that was most important to me was the amps draw on a single circuit to ensure that I didn't pop the breaker.

In the reading that I have done about the marine environment, it seems like Watts is a more important measurement.

Do I need to be concerned about both? That is, ensuring that the total Watts draw from all the circuits remains under what the inverter is rated for (and how does that relate to when shore power is just being passed through) - and - ensuring that each circuit is only pulling a load that is under the breaker for that circuit - and - that the total amp draw is less than what is able to be provided?

Sorry - just a bit confused as I am trying to learn how to do the 30-amp dance.
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Old 10-20-2021, 11:53 AM   #2
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At your home you really don't need to be worried about total amperage or wattage draw. Homes usually have a 200 amp at 240V supply and it would take running the A/C, the stove, the oven, the microwave, the electric hot water heater, etc all at the same time to come close to that limit.

Boats under 40' generally use a 30 amp 120V supply through the shore power cord. There should be a 30A breaker at the main panel and ABYC recommends another one near the shore power inlet.

A marine 16,000 btu/hr A/C will draw 15 amps, a microwave maybe 10 and almost anything else, a coffee maker, toaster, etc will blow the 30A circuit. So even without an inverter you have to manage your power draw. You can think in amps or watts, either work, but I prefer amps. Just convert each appliances wattage draw if that is what you have to amps and total the amperages up. Or do the same in watts but use 120*30= 3,600 watts as the maximum your shore power can supply.

If you have an inverter it depends on how it is wired. It sounds like yours may take power from the shore power cord and then has an internal transfer switch to either pass it through back to the main power panel or use inverter power to supply the AC. How much the inverter can supply depends on its rating usually in watts. Since I use amps to manage loads I would convert its 1,2,3,000 watts to amps and manage against that total.

Inverters generally do have the ability to exceed their rating for a few seconds such as the inrush current starting load for an A/C. So a 2,000watt inverter that could supply the running current of 15 amps of an A/C can sometimes handle the inrush current. But not always. Depends on the inverter.

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Old 10-20-2021, 12:10 PM   #3
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Ohms Law - Amps x Volts = Power (Watts) can get a bit complicated when DC/Inverter is inserted into the mix. If you plug-in a 1250 watt draw, it wants 1250-watts. Ignoring inverter loss, that means 100-amps at 12.5-volts. But.....there are a two places where there may be significant voltage drop: Undersized Cables; and Undersized Battery Bank. So while the Inverter may not exceed the wattage, it may have an under-voltage shut-down.

In my opinion, when dealing with an inverter tied to DC, amperage needs immense consideration. It's by far the biggest factor. CLICK HERE for the Blue Sea chart on cable sizing. I have not been able to find anything that accurately predicts battery-drop. I can tell you that LiFePO4 batteries are limited by a BMS and have very small voltage drop within those limits. Deep Cycle FLA batteries are not designed for large current pulls so can experience relatively high voltage drops in small battery banks (under 200-300 AH banks).

Good luck

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Old 10-20-2021, 02:11 PM   #4
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Amps are ALWAYS important. Mainly because, if you try to push too many amps through too small of a wire, it gets hot and can start a fire. So you ALWAYS have to be sure that your wiring is adequate to handle the amps you intend to push through it.

That said, if you have a 1,000 watt inverter, and you try to run 1,500 watts through it, you're going to have a problem. So, if you have an inverter, you have to be aware of how many watts it can handle.

As mvweebles pointed out, conversion from volts and amps to watts is pretty simple. As is conversion the other direction.


Good luck!
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Old 10-20-2021, 03:25 PM   #5
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OP If you haven't already got a decent meter I suggest something like this AC Digital Multi-Function Meter with Alarm. Put it on the input feed to your main AC panel and at a glance now how much power you are drawing. You can view AMPs, WATTs and Volts. I use the AMP display. Very easy to install. Not too expensive to hire a pro if you are unsure of your marine electrical skills.

I said elsewhere to you that you should not go above 24 AMPs continuous on your 30 AMP shore power service. That's 80% of 30 AMPs. I got that advice from someone far more knowledgeable about marine electrical systems than I. Your boat's 30 AMP system was designed to withstand 30 AMPs peak. Your shore power is essentially a extension cord. One that has been exposed to the marine environment. There will be resistance due to corrosion, looseness of the plugs due to wear and tear. Resistance will cause among other things heat, that's where the fire risk comes in. You have a CHB, likely 30 - 40 yrs old. Unless the electrical system has been re-wired there will be resistance aboard the boat as well.

Do be careful. I've lived on 30 - 40 yr old boats in your climate with 30 AMP service. It's far too easy to stress the system beyond safe levels especially in winter. I rewired one of them and as I pulled the out wire out I found places where it had gotten so hot the insulation was cooked. I found a lot of corrosion as well. Don't take chances.
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Old 10-20-2021, 03:39 PM   #6
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Another thing to consider, especially with an inverters AC power output is the type of power the load requires. Inverters are rated at resistive power output only. If you have a device that has capacitive or inductive elements, there is reactive power. So you inverter output is de-rated. A 1000W AC inverter may not be able to power even a 900W AC load that is reactive. And this is separate than inrush current, which is also a factor.
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Old 10-20-2021, 03:52 PM   #7
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Another thing to consider is that some inverters have "power assist" feature. When that feature is enabled, you can get a lot more AC amps out that is actually coming in the AC in. As an exemple, a victron milti plus with a 30 amp AC input in passthrough can actually power 50 amps of load on the AC out. It's configurable but you have to plan your wiring/breakers for that kind of amperage if you use the power assist function. Pretty sure other inverters out there have similar abilities.
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Old 10-20-2021, 06:17 PM   #8
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Ready?
2X30 cables 1 service for house, 1 service for the 2 reverse cycle A/C-heat
1x1200btu and 1x16000btu
3X200amp house batteries
1800 watt inverter.
40 amp charger
2x130 watt solar panels
130amp alternator on the Cummins
1X6kw generator

electric 4 burner stove and oven
electric water heater.
1x1200watt microwave
1 Splendide washer/dryer, which eats up the amps.
add in a 1200btu and 1600btu built in compartment electric heaters (use these at the same time and the house banks is almost totally 'full')

2 thrusters on the house batteries.

We can all see where this is going.

We cannot fit all the loads on the 30amp 'house' service, at the same time.

In the beginning, when making breakfast, I spent a lot of time going to the pilot house to reset the 30amp house breaker or reaching behind the saloon cushion to reset the breaker there.

1st thing I did was install an 50amp meter in the galley (a piece of red tape at the 30amp level.) This allows me to perfect 'my 30amp dance', almost.
The first load I shed is the 11 gallon electric water heater. There is more than enough VERY hot water to get at least one hot shower. Of course, if the Cummins is running, it heats the water.
Realize on the electric stove/oven, the coils do not cool down quickly so I can use the microwave by momentarily shutting off the coils as necessary. I know, I know, if I had a gas range and oven I would not need to worry about this load. The boat came with the electric stove/oven and I personally do not want LP on the boat.

Soooooo, this is the long, long story of how with installation of one 50amp meter in the galley/saloon area, life is easier and the '30amp dance' is easier.
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Old 10-20-2021, 06:59 PM   #9
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In my mind, the reason why you consider the 2 is the wattage of an appliance stays the same (I believe)....yet as voltage varies with supply and other issues, amps change to meet the wattage requirement.

So you want to calculate power use for some things in wattage, but size wiring and batteries on amperage.
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Old 10-20-2021, 08:34 PM   #10
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Originally Posted by Portage_Bay View Post
OP If you haven't already got a decent meter I suggest something like this AC Digital Multi-Function Meter with Alarm. Put it on the input feed to your main AC panel and at a glance now how much power you are drawing. You can view AMPs, WATTs and Volts. I use the AMP display. Very easy to install. Not too expensive to hire a pro if you are unsure of your marine electrical skills.

I said elsewhere to you that you should not go above 24 AMPs continuous on your 30 AMP shore power service. That's 80% of 30 AMPs. I got that advice from someone far more knowledgeable about marine electrical systems than I. Your boat's 30 AMP system was designed to withstand 30 AMPs peak. Your shore power is essentially a extension cord. One that has been exposed to the marine environment. There will be resistance due to corrosion, looseness of the plugs due to wear and tear. Resistance will cause among other things heat, that's where the fire risk comes in. You have a CHB, likely 30 - 40 yrs old. Unless the electrical system has been re-wired there will be resistance aboard the boat as well.

Do be careful. I've lived on 30 - 40 yr old boats in your climate with 30 AMP service. It's far too easy to stress the system beyond safe levels especially in winter. I rewired one of them and as I pulled the out wire out I found places where it had gotten so hot the insulation was cooked. I found a lot of corrosion as well. Don't take chances.
Portage Bay, that link to the meter got me to thinking that I might benefit from having it. Then I realized that I already have two Blue Sea analog ammeters aboard, one for each of the two 50-amp legs, in my easily-viewed breaker panel. I can attest to their utility to monitor electrical demands. I do not let the draws exceed 45 amps. I did have to move the water heater feed to the other bus to better balance loads. I am now able to run two 16,000 btu ACs, the water heater, the battery charger, and house loads other than the cooktop or microwave without exceeding 40 amps on either side. Turning the water heater off allows all other draws. The point is, an ammeter is very useful as stated.
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Old 10-20-2021, 08:51 PM   #11
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We've a good ol' 30 amp service on our 1977 Tollycraft

Main box has amperage and volt gauges/meters for easy reading. Whether on shore or gen set power the volts range from 115 to 120. Amp gauge reflects just how much power overall through the main breaker is being drawn.

I make it pretty simple: Keeping an eye on amp gauge and just don't let it go over 20 to 25 max amps [I actually like to see that gauge below 20 amps]. In other words... don't run too many burners on the stove, with water heater on and maybe fridge working or microwave running - or any mix thereof. It's really easy to not overload our 30 amp main breaker [same thing regarding a 50 amper] as long as an accurate "amperage gauge" does not show too much total-draw coming through the sub breakers.
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Old 10-20-2021, 09:00 PM   #12
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What is the service aboard the boat? I'm not talking about the shore power breaker or the actual power cord.
Is it only 30A aboard or is that all you can draw from the shore power outlet on the dock?

I am asking because if the actual boat service is more than 30A take a look on the dock to see if there is another shore power supply that you can use. Of course if the marina will allow it.

If so you may be able to run another 30A service aboard that would be dedicated to run some of those higher power items leaving the existing 30A service to power the more intermittent draws. This would take some thinking about but it may be a way to work out the fact that you may need more power than a single 30A service can produce.
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Old 10-20-2021, 09:41 PM   #13
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Portage Bay, that link to the meter got me to thinking that I might benefit from having it. Then I realized that I already have two Blue Sea analog ammeters aboard, one for each of the two 50-amp legs, in my easily-viewed breaker panel. I can attest to their utility to monitor electrical demands. I do not let the draws exceed 45 amps. I did have to move the water heater feed to the other bus to better balance loads. I am now able to run two 16,000 btu ACs, the water heater, the battery charger, and house loads other than the cooktop or microwave without exceeding 40 amps on either side. Turning the water heater off allows all other draws. The point is, an ammeter is very useful as stated.
If you're in the habit of monitoring your electrical load and understand why it's important to do so it matters not whether the meter is digital or analog. For many things I prefer analog. As I become familiar with a system I know where the needle needs to be and a quick glance is all that's required.

On the other hand the meter I provided a link to also has alarms. OP seems to be unfamiliar with managing loads and might appreciate the alarm function.

There have been some good suggestions here on load management. But OP has a real challenge living aboard full time on a single 30 AMP service during a Vancouver Island winter.

I used to have a class B motor home. It had auto load shedding installed. I have often wished there were a good marine version available.
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Old 10-21-2021, 06:07 AM   #14
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If you're in the habit of monitoring your electrical load and understand why it's important to do so it matters not whether the meter is digital or analog. For many things I prefer analog. As I become familiar with a system I know where the needle needs to be and a quick glance is all that's required.

On the other hand the meter I provided a link to also has alarms. OP seems to be unfamiliar with managing loads and might appreciate the alarm function.

There have been some good suggestions here on load management. But OP has a real challenge living aboard full time on a single 30 AMP service during a Vancouver Island winter.

I used to have a class B motor home. It had auto load shedding installed. I have often wished there were a good marine version available.
Now that I think about it, I actually considered using the meter you suggested to replace one of my analog meters. Who knows why, but the boat came to me with meters with differing scales, one 50-amp, the other 100-amp. It drove me nuts having to look closely at the 100-amp gauge to interpret the indicator. The 50-amp gauge is a Blue Sea. I installed a matching Blue Sea 50-amp gauge. Now, as you say, a glance at the position of the needles tells me everything I need to know. Plus, replacing one gauge rather than two was whole lot cheaper so for my needs, the analog gauges work well. However, I really like the alarm features of the electronic gauge!

I am one who closely monitors loads when I know demand is high with multiple high-draws are on line.
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Old 10-21-2021, 06:21 AM   #15
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For me, analog gauges are best. A piece of red tape at the max and I am happy.
An analog gauge/meter should be selected so the normal range is in the center 2/3rds. A quick glance and the world smiles on us.
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Old 10-21-2021, 07:39 AM   #16
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Now we're wandering off topic, but here goes.

I like OldDan's red tape hack. A work boat I ran years ago someone had done that.

With analog gauges for systems that shouldn't vary their value much, i.e. oil pressure or coolant temp, you can spin the gauge in it's mount so that 'normal' is straight up. One glance at the entire panel will tell you if anything is out of range.
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Old 10-21-2021, 08:23 AM   #17
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Analog = Gauges for Dummies"!! YEA Analog!!!

As you say - At a glance... can tell if all is well - or not!
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Old 10-21-2021, 09:05 AM   #18
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Analog are fine for traditional readout functions such as engine oil pressure, temperature, etc. And it's fine for a legacy house power system. But many cruising boats these days have multiple power sources (solar, shore, generator), power distribution options including inverter(s), and often multiple battery banks. There is no practical way to monitor these systems with analogue gauges.

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Old 10-21-2021, 09:20 AM   #19
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Analog are fine for traditional readout functions such as engine oil pressure, temperature, etc. And it's fine for a legacy house power system. But many cruising boats these days have multiple power sources (solar, shore, generator), power distribution options including inverter(s), and often multiple battery banks. There is no practical way to monitor these systems with analogue gauges.

Peter
Peter, that is a bold statement. Please explain your assertion. Last I knew, volts are volts, amps are amps. An analog ammeter will show amps. A digital ammeter will read the same, a bit more precisely but that level of precision is a nice to have but not necessary to make decisions. I am not averse to digital gauges but to say there is practical way to monitor systems is, I think, inaccurate. I do not have solar but evrything else you mention I do have inluding two alternators and two generators. My analog meters do me just fine.
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Old 10-21-2021, 09:55 AM   #20
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Analog are fine for traditional readout functions such as engine oil pressure, temperature, etc. And it's fine for a legacy house power system. But many cruising boats these days have multiple power sources (solar, shore, generator), power distribution options including inverter(s), and often multiple battery banks. There is no practical way to monitor these systems with analogue gauges.

Peter
And, That Is:

Exactly why Linda and I have and enjoy a very affordable, super comfortable and well capable, 1977 fun-time, play-toy, 34' tri cabin, twin screw, fast plane, cruiser boat... with all its original equipment maintained in good working condition.

"Simple is as simple does"!
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