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Subject: Re: TWL: low marina voltage
From: Arild Jensen (elnav@XXX.XXX)
Date: Sun Sep 01 2002 - 01:33:40 EDT
Andy wrote:
Typically a marina would run this system down the dock and tap off of one leg and the neutral (and
> the ground) for the 125 volt, 30 amp receptacles, and tap off of two legs
> and the neutral (and the ground) for the 125/250 volt receptacles. Of
> course, you don't actually have 250 volts. You have 208 volts. Almost all
> 240 volt appliances will run on 208 volts. You should have 120 volts
> between the hot leg and the neutral.
REPLY
I beg to differ. An motor appliance rated as a 250V device will not work correctly at 208 sinc this amounts to a 16.8 percent drop below nominal.
Even a 240 V device is outside the normal limits at 208V
Any inductive load should have a low voltage cutout above this limit.
Carrier made a lot of air conditioners rated at 230 volts which would drop out at about 206 - 209V as a safety measure.
The reason I mentioned the 10% is that as long as the utility maintains the voltage within a plus or minus 10% tolerance they cannot be sued.
I agree that a 3% tolerance is much better and should be aimed for at the design stage.
In real life how much worse than the calculations does it actually get.
Unfortunately consumer appliances are typically made for a 120/240 single phase feeder arrangement. Not a 3 phase system.
Condo apartment owners who move their appliances from a house to the condo often get frustrated that oven and dryers do not seem to run hot enough. Reason the
heat elements see 208 not 240V.
And then when the untility sees maximum demand, the grid vottage droops down from the nominal.
While the 120 drops to 108 the 208 also drops a corresponding amount and ends up at mych less than 200. So now we have a real problem.
Andy wrote
> In my opinion, 10% voltage drop is way to much. The National Electric Code
> suggests no more than 3% voltage drop for branch circuits, and not more
> than 5% voltage drop for a combination of feeder circuits and branch
> circuits. This means no more than 2% voltage drop for feeder circuits.
> This is a suggestion by the Code, not a requirement.
REPLY
And like any suggestion, often gets ignored.
The reason being cost. It is much cheaper to supply a marina with 3 phase asnd let them worry about the 208 problem.
If the utility had to provide the required transformers their cost would go up.
Andy wrote:
Most transformers have taps that can be changed to increase the secondary voltage if the primary
> voltage is a little low, or decrease the secondary voltage if the primary
> voltage is a little high.
REPLY
This is not a load regulating selector switch. It is only intended to allow the utility to adjust for end of line droop compared to locations right next to the main substation
feeed point.
Essentially this is a set and forget switch that is adjusted only during installation to account for grid system factors.
A load tap changer is quite another thing. Such a contactor has to be able to handle switching under full load current.
To minimize current rating the switch should be installed on the primary side. However, in utilities which use high distribution voltages this poses other problems.
I worked for a uitlity that was in the process of upgrading all their older system to run 27.6 KV.
The new 27.6KV step down to 120/240V transformers were very different inside than the old 12.3KV or 8.2KV that were used in the old part of town.
Andy wrote:
> Just recently a dock neighbor was having some problems with the air
> conditioners on his boat. They kept turning off because of low voltage.
> He blamed the marina for having low voltage, but no one else was having any
> problems. Turned out his shore power cord and inlet receptacle on the boat
> were burned, probably due to a loose connection. This was causing his low
> voltage.
Which is why I suggested checking the drop from main panel to shore power pedestal to see if the drop was system induced or caused by the vessel shore cord. Such
a measurement will tell you a lot.
I would suggest that it wil not be long before most of the larger boats all come equipped with buck - boost type transformers to accommodate brown out conditions
ashore.
Cheers
Arild
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