ammeter for standby generator

I would like to build an ammeter for monitoring the load on a “whole-home” standby generator.

The generator is single-phase and has a rated maximum power of 12 kW (15 kW surge) and rated maximum load current of 50 Amps at 240 Volts and 100 Amps at 120 Volts.

FYI
The automatic transfer switch (ATS) has load sensing, as well as load shedding capabilities. There are two dry contactors on the ATS that control relays for the air-conditioning (terminals A-A) and a 90 Amp sub-panel (terminals B-B). Terminals A-A have precedence over terminals B-B, which shuts down the 90 Amp sub-panel so the air-conditioning can start. Air-conditioning is wired to the generator sub-panel.

Yes, this all work great. The problem is there is no ammeter to see how much load is on the generator.

I would like to accomplish two tasks; 1). balance the load on each leg of the generator, and 2). put as much stuff on the 90 Amp sub-panel as the generator will allow.

My current thought (no pun intended) is to use 2 current transformers wired to 2 panel meters.

Could it be this simple? Will a 100:5 CT wired to a compatible panel meter do the trick without any other circuitry? Will the panel meter bounce around at 60-cycles or is this controlled by the CT or meter?

Other than basic math, can I indicate get the total load on one panel meter? If yes, how?

In advance, thank you.

Yes a 100:5 current transformer wired to a 5amp ammeter (with a 100amp scale) will be fine.
You must of course use an AC ammeter, not a DC ammeter or it will "bounce around at 60-cycles"!!!

However, there are a couple of comments which I dont understand:

"Balance the load on each leg of the generator" What are the legs? it is a single phase generator with one output.

"Put as much stuff on the 90amp sub panel...." Do you mean you dont want to exceed 90amps, or the 100amp rating of the generator less the AirCon load?

JimB

Another thing I don't understand is what voltage you are using 120 or 240 as it is a 12KW generator and both 120V@100A and 240V@50A were both 12KW last time I checked.

Thanks for your reply.

In order to answer your questions about my comments let me first elaborate on the design of my power system.

The way I have configured the system is as follows; power from the utility company goes to the main load center, then the automatic transfer switch (ATS), then the generator load center. The main load center is NOT powered during utility power failure.

During the loss of utility power the generator, via ATS logic, powers the generator load center, which provides power to all the high-priority circuits. Additionally, the generator load center provides power to the air-conditioning and the 90 amp sub-panel.

Dry contactors (terminals A-A) on the ATS allow the air-conditioning contactors to close when the ATS logic determines adequate power is available. The same is true for the dry contactors (terminals B-B ), which allows power to close a 90 amp, 2-pole, general purpose relay, providing power from the generator load center to the 90 amp sub-panel.

Regarding your question about balancing the load; the generator provides 240 volts via two legs at 120 volts each. Although the generator is capable of providing a total of 100 amps, it can only provide 50 amps per leg. My goal is to provide the maximum current from the generator by placing equal loads (within reasonable variations by trial and error) on each leg.

Regarding your question about my comment’ “as much stuff on the 90 amp sub panel.” Under no circumstance do I wan to overload the generator by placing too much on the generator load center, so I will attempt to maintain its load below the maximum threshold.

Don’t let the 90 amp sub-panel fool you into thinking it is part of the power limit. It is used because it is a convenient size for the number of circuits (spaces) required.

Now, the ATS logic will prevent overloading the generator by opening the contactors to the air-conditioning contactor and the 90 amp sub-panel contactor. My goal here is to put as much on the generator load center and the 90 amp sub-panel without opening the ATS contactor to the 90 amp sub-panel.

For example, say the generator load center requirement is 50 amps and the 90 amp sub-panel requirement is 50 amps, for a total of 100 amps. In theory, the generator will power both panels, if neither leg exceeds 50 amps. If one leg exceeds the 50 amp limitation I loose the entire 90 amp sub-panel.

You could use 2 *clamp style transducers or 2*hall effect device or run a wire thru a ferrite ring (add the secondary coil first).

help needed here also

I also need help along the same lines. I need to install a meter for my generator which will output total amps of both legs of the generator summed together. I know I will need two current loop transformers but I don't know how to sum them together. If a 240 volt 15 amp widget is being sourced by the generator, both legs will indicate 15 amps current flow. If leg a is now also providing a 10Amp load (120V) then leg A will indicate 25Amps and leg B will indicate 15 Amps. The total will egual 25 amps........ 240V X 15Amps = 3600 Watts and 120V X 10Amps = 1200 Watts....So 3600 + 1200 = 4800 Watts

How do I get TOTAL WATTS produced by the generator, on a display that I can understand in I am nearing the threshold of total power possible to produce.....before the generator decides it has had enough and shuts down?

Thanks

Hi Digitech,

I'm fairly sure my generator will shutdown whenever an individual leg exceeds it threshold.

Total generator capacity is: leg A + Leg B = Total Generator Capacity.

Say you have a 12kw (12,000W/100A) generator, then: Leg A (50 A max) + Leg B (50 A max) = 100 amp maximum.

In your scenario, your widget has added a 15 amp load to both Leg A & B (30 amp total). If you add another 36 amps to Leg A (51A total), your generator will drop of line, even though total load is only 66 amps, which is well below the generator limit.

One meter, if both legs were balanced, would be ok for me as well.

The simplest way to do it is with three amp meters and three current transformers.
Two of the meters are 100 amp units and one is a 50 amp unit and they work by using three 100 amp current transformers.
Two of them read the independent 120 volt legs of the system and the third 100 amp CT is configured at a criss cross point so it can read both legs simultaneously and the 50 amp meter monitors them combined as if it was a single 240 volt load.

The reason that one needs to be a 100 amp CT is to read the two legs maximum currents but the meter itself is a 50 amp being its used to read a 240 volt circuit.
With this method all three meters read proportionally to the power they are handling.

US - Electronic Components Distributor | Newark.com carries current transformers and the related meters. They have a large selection of different devices to choose from as well.

You don't need three ammeters, just two monitoring each leg of the 240V line. The total power is simply the sum of the two currents times 120V.

Thus, in the example of the 15A, 240V load and the 10A, 120V load, you just multiply the total 240V leg amps (40) times 120V to give 4800W, the same as given by the separate 120V and 240V calculation.