Voltmeter on AC Panel, Why? - Novice question...

tomherrick · Guest

I've been looking at AC panels for our boats and see that some have a voltmeter built-in and some don't. I'm wondering why it's offered. Most AC devices have some leeway on the actual delivered voltage they require. Does the voltage at some marinas vary quite a bit, or is there some other reason I'm not aware of for wanting a voltmeter in the shore power mix?

thataway · Posted: Sun Jan 21, 2018 4:51 pm Post subject:

In my large boats and RV, I do have AC voltage meters. On my C Dorys I have not. On the end of many docks, especially ones where there may be heaters or air conditioners running, often the voltage is low. I have seen as low as 90 volts. Not a big problem for resistive loads, but many electronics and inductive loads do not do well with low voltages. I always carry a digital volt meter, and it takes a few seconds to check the voltages. I don't see a need for volt meter in a C Dory.

On the 12 volt DC side--that is an essential. Plus I also like to have an amps in and amps out, as well as current amperage draw, as well as voltage on my start and house batteries. I use a Link 10 +, The Victron 702 is comparable. Blue Seas and several others have good meters as well as Balmar.

AstoriaDave · Posted: Sun Jan 21, 2018 6:36 pm Post subject:

To put a sharp point on Dr. Bob's warning of the effects of low AC voltage on the AC supply: anything as low as 90 VAC is very likely to cause AC motors to burn out in relatively short order. How? AC power well below the rated voltage will cause the motor to run slower. A slowed motor will draw MORE current, possibly exceeding the rated current for the motor's wiring. Yes, MORE current. Counterintuitive, yes!?

journey on · Posted: Sun Jan 21, 2018 7:53 pm Post subject:

My opinion is that if you can afford it, get the one with voltmeters, both for the AC and DC panels. More information is really, really handy when you're troubleshooting an electrical system. Of course you can get the voltages with a portable voltmeter, but built in meters can tell you at a glance what is the situation.

You can also install separate voltmeters, available at any electrical supply store. Expensive in marine supply stores. The interior of a C-Dory is a relativity benign place for a voltmeter, so you don't need a special meter. Get mine from Newark Electronics. You only need a small 3" analogue meter.

Note that a lot of loads in a boat are constant POWER loads and POWER=VOLTAGExCURRENT, so as the voltage goes down, the current goes up. This may lead to a brownout or blowup. True for AC and DC. A voltmeter will tell you instantly what is the situation.

Boris

tomherrick · Guest

On further research, Dave, AC motors are the crux of that biscuit. Many other appliances can deal with wide voltage variation without much safety worries, but the AC motor requires a certain wattage to run and it'll take it any it can get it - increasing the amperage load to compensate for the voltage drop.

Now, as I probably won't be watching an analog voltmeter continuously, I wonder if the digital models have an alarm that can be activated below a user-determined setting...

All the best,

T

thataway · Posted: Sun Jan 21, 2018 8:55 pm Post subject:

You can buy digital 0 to 240 volt AC Meters on Amazon: Here

andone of many 12 volt DC gauges.

Or if you want to measure amps and voltage directly, then
Try this.

The problem with the factory volt meter on the AC panel, is that it is under the dinette, and difficult to see in most of the boats.

tomherrick · Guest

This investigation is two-fold: soonest for Laura Lee's West Wight Potter 19 to get it on the water for a week with marina stays, but also for a longer-term understanding of what I'll do for my Frankenstein 28' C-Dory "Classic"...

I'll check out your links in the morning, Bob.

Lots to learn...

Robert H. Wilkinson · Posted: Sun Jan 21, 2018 9:56 pm Post subject:

thataway · Posted: Mon Jan 22, 2018 12:45 pm Post subject:

There are step up transformers, but the current available is going to be less overall. The standard isolation transformers do not do this.

The Hughes Autotransformer is one which is used in RV's. (about $400) When voltage drops below 117 Volts, the voltage is stepped up a fixed 10%. However, since this uses more current, the total available current for other boats/RV's drops, as does their voltage drop further...thus many campgrounds "outlaw" this.

Charles makes a 50 amp 240 volt isolation transformer, which gives a 15% boost if the voltage drops below 210Volts. ($1200) There used to be a 30 amp 120 Volt AC model--but I cannot find this listed in any current sites. There may be used ones around.

Here is a circuit on U tube which alarms at 100 V AC. Modification could change it to what ever voltage you wanted. What this seems to do is to rectify the 110V AC to DC, and then use a fairly standard circuit for DC voltage protection. This might be done using other 110 v' rectifier with clean DC (not pulsed) and a DC voltage alarm, but would have to be calibrated..

A Macromatic VAKP120A under/over voltage relay could be fairly easily repurposed to make an alarm--total cost probably in $100 range.