50 Amp Cord vs 30 Amp Cord

[msd58]

I just noticed that I have been using 50 amp shore power cord in a 30 amp application. I have been doing it for 5 years, does in make a difference?

 

[Barnacle_Bill]

Re: 50 Amp Cord vs 30 Amp Cord

No, in fact 50 is better. Less resistance.

 

[Gone]

Re: 50 Amp Cord vs 30 Amp Cord

Your cord selection is okay as long as you aren't popping shore circuit breakers or having brown outs when you exceed the 30 amp source. Apparently you are not using 30 amps. The plugs for 30 and 50 amps usually are purposely different though adapters are available. Older marinas and RV parks cannot deliver even the 30 amps on busy weekends. The brownouts are death to A/C and microwaves.

CD

 

[msd58]

Re: 50 Amp Cord vs 30 Amp Cord

Your cord selection is okay as long as you aren't popping shore circuit breakers or having brown outs when you exceed the 30 amp source. Apparently you are not using 30 amps. The plugs for 30 and 50 amps usually are purposely different though adapters are available. Older marinas and RV parks cannot deliver even the 30 amps on busy weekends. The brownouts are death to A/C and microwaves.

CD

I am glad you brougt up incoming amps. How can I check to see what amps are coming into my boat? Or the voltage? All I have is basic household volt meter.

 

[Barnacle_Bill]

Re: 50 Amp Cord vs 30 Amp Cord

I use to have an AC clamp that plugged into my VOM. But I'm sure they have something easier to use these days. As for the voltage, just measure it at the end of the extension cord. Why all the concern after 5 years? Are you having problems?

 

[msd58]

Re: 50 Amp Cord vs 30 Amp Cord

No problems at all. 6500 BTU A/C runs fine. Microwave is fine. Fridge is fine. But If I move up to another boat that has a larger fridge, and A/C I want to make sure that theres going to be enough juice. I like the slip that I have and don't want to move.

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

You really don't care what the amps are perse.

What you are interested in is the voltage available at the appliance connector when the appliance is running. Much simpler to measure than amperage, but the reading is definitely amperage (vs line resistance) related. If you have a duplex outlet where the appliance is connected, just stick your meter leads in the other socket for your answer......and remember you are measuring lethal voltages....pay attention to where your fingers are.

The elect input spec for the appliance is usually marked on it somehwere and usually is rated within 10% of the mean value. Wiring to the appliance connector is usually sized for not more than a 2% drop at FLAmps......as I recall from the code.

Mark

 

[Gone]

Re: 50 Amp Cord vs 30 Amp Cord

You really don't care what the amps are perse.

What you are interested in is the voltage available at the appliance connector when the appliance is running. Much simpler to measure than amperage, but the reading is definitely amperage (vs line resistance) related. If you have a duplex outlet where the appliance is connected, just stick your meter leads in the other socket for your answer......and remember you are measuring lethal voltages....pay attention to where your fingers are.
Mark

Great advice!
I can only add that it would be good to know what the voltage drops to as the A/C is starting up. How low can an A/C tolerate at startup, Mark?

CD

 

[msd58]

Re: 50 Amp Cord vs 30 Amp Cord

I am not sure of the draw of the A/C or Fridge right now. I think am OK at this point it would just be it lets say U went to 10BTU unit or something like that.

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

Don't know. The inrush on a "motor start circuit" is probably 10x the running current. The nice thing about it is that it only lasts for a second so the wire has no time to get hot and raise it's resistance accordingly. It's primarily limited by the internal impedance of the generator.

For public utilities, that's really low. If you were running off an aux power unit it might tax it, depends on it and the load you are trying to run.

Just an FYI, i have operated a 14,000 BTU, 220v air conditioner off a 5kw gen powered by a 6 hp Briggs, on the end of a 100' 12/3 extension cord before and it seemed to run ok although the APU groaned when the compressor would kick in (starting surge). I had everything maxed out (if you go through the numbers)and only ran it until the house power came back on which was several hours and the compressor cycled.


One thing you want to be careful of is if you have two high wattage loads that could start simultaneously; like if you had a ref and freezer running off a 5kw gen (not the public utility) for example. If both attempted to start at the same time you would/could damage the alternator in your APU.

HTH

Mark

 

[msd58]

Re: 50 Amp Cord vs 30 Amp Cord

I guess what I am really asking is: there a basic meter reading I can take at the power source right at the dock from my dedicated shore power outlet is? What type of meter do I use? What type of fluctuations in voltage are acceptable? As I mentioned in my previous posting I may at some point go from a 6500 BTU unit to a 10BTU and possibly a larger than standard Norcld fridge.

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

Blind guess here....sorta.

You are docked at a marina. It supplies you with power from a public utility. You are connected via a 50 amp shore power cord and receptacle commonly known as an "entrance service" for all practical purposes.

From there you plug in your boat on a 30 amp 230v I assume (needs to be) power cord of some length. I further assume it's not over 50 ft long and is a 8 or 6 AWG wire size. 10 awg would probably be ok and a 10 would be much easier to deal with and that's probably what it is. The cable is marked with the wire size every 3' or thereabouts so you can look on it and see.

A BTU is right at .0003 Kw-hrs. So a 10,000 BTU ac will suck up 3 kw/hr assuming the efficiency is included in the number and it's pumping (compressor running) all the time. If not add 10% more (3.3kw).

Obviously when the compressor cycles off you are just running the fan and the load goes down to somewhere around 0.5 kw-hrs or less.

At 230V, 3.3kw will require a little over 14 amps on both legs of the 230 circuit.

A microwave is usually 115v and 1kw. That's another 7 amps on one leg of your 230 supply. So you run 7 amps of other things (lights, radio, fan, what have you) on the other leg to balance it out. (A refrigerator will also run about 7-8 amperes on 115v.)



Now you are demanding 14 + 7 = 21 amperes from your 50 amp available source through your 30 amp service cord to your boat.

Sounds like you are in great shape and 10 awg would do nicely out to 100'.

If you want to run the ref and the microwave and the ac then you are at 21 amps at peak (not surge) running amperes so you can still add your lights and things. Put your micro on one side of the 230 and the ref on the other to balance the load.

A TV/computer type thing is 250 watts. 10 ea 60 watt light bulbs is 600 w. A SS radio when not transmitting is maybe 100 w (guess). Transmissions are intermittent and not a constant load.

So now you have 250+600+100 = another kw. Split between both legs adds 3.5 amps per leg for a total now of 21+ 3.5= 24.5. Your #10 is still good to go.

Oh, when you go to a 10k btu ac get a 230 V unit. I think that is the size where you can buy either 115 or 230.

Is this what you are doing and the answers you are looking for? :/

Mark

 

[Gone]

Re: 50 Amp Cord vs 30 Amp Cord

I'll throw a WAG in here as well. Your voltmeter is sufficient to monitor the source.
100 volts would be the minimum that I would let the loads pull it down to. 90 volts unloaded and I would not even try to start the A/C as you are bordering on brownout. IMHO.

 

[msd58]

Re: 50 Amp Cord vs 30 Amp Cord

Thanks "dude" d

 

[seven up]

Re: 50 Amp Cord vs 30 Amp Cord

Greetings,

An electrical supply house will be able to help with the selection of a clamp meter for reading amperage flow if that's what your interested in. Most clamp meters will also read voltage too. Want a print out over time ? You can have that too. Best

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

From dude: Great advice!
I can only add that it would be good to know what the voltage drops to as the A/C is starting up. How low can an A/C tolerate at startup, Mark?

The starting surge of a motor is essentially the impedance of the generator (low as I said for public utilities) plus the resistance of the wire in the interconnect lines, plus the dc resistance of the wire in the motor, plus some motor inductance, which is minimal until a field is developed.

So the surge voltage, for a split second could go very low; value dependent on what I said. During that second of surge, the motor starts developing a magnetic field which starts it to spinning. As it starts spinning it starts developing what's called "counter EMF". This starts building "resistance" in the motor and it starts developing voltage across it until it hits rated rpms where the circuit stabilizes.

The inrush current develops the field, not the voltage and a half dozen situations would have a half dozen values of inrush current and resultant voltage.

But remember this all comes and goes in a split second and the motor is humming right along.

No you won't have a brownout; a wink yes, but not a brownout.......but you have a wink at home with house power when you or your neighbor have a high current load start. 8)

Mark

 

[Gone]

Re: 50 Amp Cord vs 30 Amp Cord

Residences are pretty well matched for the transformer load distribution. Not necessarily so in some marinas and RV parks. Too many outlets are loaded onto 1 transformer and on a busy weekend the current demand exceeds the transformer capability. Hence the low voltage or brownout. Especially if there are many units plugging in with 50 amp requirements (over 30 amps) and the the A/C "wink" becomes a pause.

CD

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

Don't agree with that either. No pause. Low distribution voltage maybe, because all you're dealing with is an increase in the winding resistance of the transformer and service entrance cable. Most power lines are setup high to start with so as you load the line down the 124v drops to whatever.

In the military we designed equipment to normal steady state limits of 108-118 vac. Then came abnormal SS limits of 102 to 124 and then emergency limits to something worse and I can't remember the exact values but they were there for a very short time (measuerd in seconds) whereas the others could be there indefinitely.

If power line voltage is low, someone usually complains to the power company where they make a survey and based upon that survey they provide service accordingly. The transformers and entrance service to the meters belong to the power co. not the trailer park.

Mark

 

[Texasmark]

Re: 50 Amp Cord vs 30 Amp Cord

But this is msd58's thread, not ours. Where'd he go? All this work for nuttin? :|

Mark

 

[Gone]

Re: 50 Amp Cord vs 30 Amp Cord

Up to the meter is correct, but there are no meters on the individual slips, just the main meter for the marina owner. How many outlets the owner eventually provides after the meter depends on how thin he wants to spread the service rather than pay for another transformer. Not being argumentative here, just a problem that I encounter with marina/RV parks. Industry associations are raising the standards so this happens less and less but some older facilities still have these issues.

CD