load measurements on standby generator ?


Old 03-07-06, 04:34 PM
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load measurements on standby generator ?

Hello group... long time lurker... first time poster...at least in a long time... here's my story/question

I've recently replaced my 50 year old military surplus 7kw gen set (Katrina just about killed it) with a shiny new Generac Elite 30 kw unit... all the bells and whistles..auto start and exercise...auto switch over, etc.. one thing it doesn't have that I miss(from my old mil set) is a 'percent of load' meter or some way of telling what kind of load is on the generator at any given time period. I know it's best to keep the load balanced on each leg and my old mil set had a switchable meter that showed the load (in percentage) that that leg was carrying...

Sooooo all you electrical gurus.... other than going out and taking 10 screws off the cover and clamping an amprobe on each leg...in a driving rainstorm/tornado/snowstorm.... is there a circuit I can build or a marketed 'hickey-thing' that I can attach (without voiding my 24 hour warranty) so I can monitor the load on the generator ???... I'd rather make the connections either at the ATS or on my breaker panel side of the ATS... that would protect the very delicate 'warranty' infringement aura. It would also be next to the breaker box where I could use circuit switching to balance (somewhat) the load..can't imagine it would matter much where the point of measure is on the cable as long as it's in a protected place. BTW I'm using 4/0 Alum to feed from the ATS to the generator...

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Old 03-07-06, 05:47 PM
Join Date: Oct 2005
Location: USA
Posts: 1,152
ATS means auto transfer switch? In any case, IF you are willing to "roll your own", I believe the best bet is to use a couple of current transformers (CT) and wire them into a couple of ammeters. You appear to have a 125amp generator. This is what a CT looks like. http://www.geindustrial.com/cwc/prod...=6&id=600v-cgt You simply run the 4/0 thru the big hole in the center, one leg (one wire) per CT. A CT basically converts a high current to a low, easily measured current using a set ratio. Lets say you get a 100:1 ratio CT. That means a 120 amp leg current will cause a 1.20 amp in the instrument side of the CT. That would be the little terminals in the picture. So, you mount the 1.5 amp full scale current meters in a convenient place, and run like 18awg wire to the actual CT, that will reside inside either your load center or the ATS.
This is a wee bit beyond many DIYer, you will have to decide that for yourself. There may be a better solution in the form of a kit, not sure. The beauty of the CT's is that you can monitor the current at a location far from the actual high current; like the control room in a power plant, as one example. There are some fine points in the CT installation, also a caution to never open the instrument loop during CT operation. But basically, that is how industry does it.
Old 03-07-06, 06:05 PM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Basically, what you need is the permanent equivalent of the clamp on meter.

You use a device known as a 'current transformer', which is essentially a donut that you slip over the cables that you wish to measure. This gets connected directly to a suitable meter. When current flows through the cable, a corresponding (but smaller) current flows in the meter circuit.

Common current transformers are rated in the form of 'xxx:5' amps, meaning that when XXX amps flows in the cable, 5 amps flows in the meter circuit. The meters are designed to reach full scale with 5amps flowing through them, but they all have different scales. For example, you might get a 3000:5 current transformer and connect it to a meter that points to the number 3000 when 5A flows through it.

I did a very brief search for current transformers and for 'kilowatt power meter' and found the following: http://www.powermeterstore.com/permanent/pml6200.php

I am sure if you do more looking you will find something both suitable and reasonably priced.

Old 03-07-06, 08:00 PM
Join Date: Oct 2005
Location: Milwaukee WI
Posts: 1,338
Seems you're comfortable with specs so here are some decent starting points:

http://www.laurels.com/juniorseries.htm Digital ammeter

http://www.ampsolution.com/ current transducers

If you're more skilled and determined than I am, you could calculate & calibrate the windings needed to make your own transducers.

If you're looking for a simple old analog switchable ammeter you might check a surplus store if you've got any nearby. Chances are somebody's got a load of them.

I'd be interested in how you resolve this as I've had a similar project in mind, so please post if you get anywhere.

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