Wednesday, October 23, 2013

Wiring Musing, 3-phase 240/208v power?

Continuing my 'reposting', here is kind of a composite post combining what I did in the Wooden Boat forum with an Email to the T&T mailing list - concerning the question around '240v' shore power measuring 208v. It is a common situation in many marinas, and boats which bring in 240v/50A shore power need to be aware of it, esp if they have motors which run off that 240v, e.g. an air conditioner compressor...


Why do some marina's only have 208v on their '240v' plugs?  Industrial sites often have power distributed via 3-phase, while to houses it comes in split-phase. Hence, when you measure total voltage in a house, you get 220-240, but because of 'phasing', when you measure across two legs of a 3-phase source it come out to around 208v.

In the case of the split-phase feed system two hot 120v wires are supplied with one natural. But a key is that the two hot wires are 180-degrees out of phase with each other. This is accomplished by the power company using a center taped transformer. If one measures voltage from any one of the hot legs to neutral one gets 120v. If one measured voltage between the two hot legs one gets 240v (120 + 120 = 240).

With three-phase power things are a little different. There are 3 Hot wires in a 3-phase feed but now the legs are no longer 180 degrees out of phase. They are now are 120 degrees (360 / 2 = 180 while 360 / 3 = 120). Under this condition one needs to use vector math when calculating the voltage of two 120v feeds that are 120 degrees out of phase, and as a result the 240v now becomes 208v. It does not impact any of the 120v loads as they still see only one hot and the neutral (and do not depend on any phase relationship to derive their voltage), but any 240v loads are impacted. Most concerning are motors as they can overheat. The other impact one will see is any 240v heating elements (again, the dryer) will produce less heat.

Look at larger motors (single phase), and you will often see there is an options to purchase 220/240v or 208v,. It allows them to be installed in industrial sites where the power is supplied via a 3-phase drop. Water heaters, and even appliances like dryers can be special ordered for 208v. (I use to have a summer job installing such in high rise condos.). This is also why some boats have issues running their ACs in marinas, the ACs are set up for 220/240v which the generator will produce, while the marina has the three phase wiring and delivers only 208v. Tends to cause the ACs to run hot, and even burn them out early.

I think it is not totally correct to deliver two phases of a 3-phase to provide '240v' marine service, but it is a lot cheaper. To correct it, the marina would have to install transformers and deliver power as it is delivered to your house. So, their install cost is higher.

Big problem comes in that some marina's put in the extra cost, and have a proper 220/240 service, while others do not... And there seems to be no real 'regulation', so it is up to the individual marina to decide :-)

If all you do is pull 120v loads, none of this is an issue as with either type of service you will see 120v between one of the legs and neutral. But in your case, you are looking to pull 220/240v, and that is the problem here.

Solution is simple: Many  isolation transformer have the ability to be configured to 220/240v, or 208v as the input. Making sure the correct 'tap' is selected will fix this, but be aware that if you ever go to a different marine, you need to verify if they have 'proper' 220/240v service, or the 208v service, and then adjust your transformer accordingly.

If you do not have an isolation transformer, or do and it does not have does not have the ability to select between 208 and 220/240v an alternative (though not that clean) is to install a 24-36v 'boost' transformer in front of your shore power feed. These are readily available at electrical supply houses. Again, you will need to be careful to switch it in and out depending on the shore power configuration.


  1. We ran into this many years ago at our fertilizer plant when the local utility very abruptly announced that from now on we would receive 208 V power. At the time I assumed it was a scam to make my meter spin faster (the same load on lower voltage would draw more amps). Perhaps there was more to it but whatever the cause the effect was that we had to buy a new 30 HP motor because the 25 HP motor that had previously been happy to spin the blender at 240 volts simply wouldn't start it on 208 volts.

  2. Hum.... No relief from the power company for this FORCED change in your service???