MEP % load gauges

DieselAddict

Well-known member
Steel Soldiers Supporter
1,908
89
48
Location
Efland, NC
With the recent weather events a number of the members here have been contributing information about how their machines have been keeping the lights on. That is awesome. It makes me smile to read about this equipment improving the lives of their owners.

One thing everyone needs to be aware of when using the gauges and especially the % load gauge is this - gauges lie. The load gauge REALLY LIES. It can lead you to believe you generator is not loaded as much as it is. In case you don't want to read everything below the simple message here is do not rely on the load meter alone to determine that everything on your generator is fine. Get a clamp on amp meter and use it to better manage the load on your machine.

The reason is simple, the load meter is calibrated assuming for a power factor of 0.8. If you look at the gauge itself you'll see written on its face something like PF = 0.8.

If you are interested in what power factor is there are a number of really good tutorials on the web that will bore you with math we've all long forgotten. If you don't care so much for the math there is the cliff notes version..

Not all electrical loads are the same. Resistance loads (oven, incandescent lamps, and dryer) are considered "perfect consumers" they use POWER (Volts * Amps) exactly as the simplest version of the formula for power says it will. Awesome, nice and simple. PF = 1.

Motors, computers, and CFL/LED lights are not perfect consumers of power. These loads can have a PF < 0.7. In this case the basic formula for power can't be use to calculate load on your generator. More complicated measurements and math are necessary.

So.. What does this mean in regards to our generators load meter? The load meter circuit on the generators is very simple. More like the basic formula for power (V * A) kind of simple. It can't tell if the power being consumed is being consumed perfectly (PF = 1) or imperfectly (PF < 1). Since it can't tell the difference the designers make an assumption. They calibrate the system to what they think will be the average PF of the loads being attached to the generator. In this case 0.8.

The problem this causes is if your loads have a PF of 1 the load meter will read 20% low. If the PF of your loads is 0.7 the load meter will read 12.5% high. The only time the meter is correct (assuming its correct to start with) is when the PF = 0.8. Which will be practically never.

Ok. For most of us the PF of the average of all the stuff in our houses should be between 0.87 and 0.93. That means the error in the meter will be ~8% @ a PF of 0.87 and ~14% @ a PF of 0.93. Once you factor any additional error from the effects of time and usage, your load meter could be seriously lying to you.

This is why its important (in my opinion) to have a clamp on amp meter so you can measure the voltage and current directly. You can take some measurements to determine how truthful your load meter is.

This turned into a lot more than I wanted when I started typing. Hopefully the info is helpful. If anyone would like to discuss more about PF I'm happy to chat about it.
 

Triple Jim

Well-known member
Steel Soldiers Supporter
1,283
44
48
Location
North Central NC
Ok. For most of us the PF of the average of all the stuff in our houses should be between 0.87 and 0.93.
True enough. For those of us who occasionally run tests with resistive load banks of some sort, those will have a power factor of 1, so as you said, the meter will read 20% low. This is likely why a lot of reports of load tests come back with "it easily ran at 125% for an hour." It was really running at about 105%.
 

robson1015

Active member
475
48
28
Location
New Concord, Ohio
Is that why the 80% load mark on the gauge is bold green??? A resistant load PF=1 from say an oven/stove reading 80% is actually a 100% load...Is this correct?
 
Last edited:

Chainbreaker

Well-known member
1,220
395
83
Location
Oregon
True enough. For those of us who occasionally run tests with resistive load banks of some sort, those will have a power factor of 1, so as you said, the meter will read 20% low. This is likely why a lot of reports of load tests come back with "it easily ran at 125% for an hour." It was really running at about 105%.
OK, I'm having a hard time wrapping my brain around this...I think. So, if the meter is reading low on a resistive load wouldn't that mean that a PF 1 load is actually 20% higher than indicated? I would think that a 105% load would be actually a 125% load in reality. Or am I thinking about the gage relationship backwards?
 

DieselAddict

Well-known member
Steel Soldiers Supporter
1,908
89
48
Location
Efland, NC
OK, I'm having a hard time wrapping my brain around this...I think. So, if the meter is reading low on a resistive load wouldn't that mean that a PF 1 load is actually 20% higher than indicated? I would think that a 105% load would be actually a 125% load in reality. Or am I thinking about the gage relationship backwards?
You are correct, the load meter will read LOW with a resitive load. A ~100% on the meter load will really be ~120% on the engine. If you measure the amps with a meter you should be able to calculate the true load.
 

Daybreak

2 Star Admiral
Steel Soldiers Supporter
1,372
172
63
Location
Winchester, Virginia
You are correct, the load meter will read LOW with a resitive load. A ~100% on the meter load will really be ~120% on the engine. If you measure the amps with a meter you should be able to calculate the true load.
Howdy,
Yes, that is why when I use the load bank, I will throw a 6kw load to have the meter at 100% on a MEP-802A. The military load bank is a resistive load. That is why when I run the MEP-803A up to 133% on the meter, I am pushing 67.2 amps per leg from the load bank.
 

csheath

Active member
659
27
28
Location
FL
This is why I did THIS.
IMG_1421.JPG

A 40 amp resistance load was reading 110% on my old load gauge. 52 amps would peg it.

Those that have a working panel they don't want to modify should look at lonesouth's adaptation of these. He mounted them in a panel behind the storage door.
 
Last edited:

rustystud

Well-known member
8,487
141
48
Location
Woodinville, Washington

Kenny0

Member
114
15
18
Location
Leland, Iowa
https://www.amazon.com/Reliance-Con...4381&sr=8-8&keywords=Reliance++controls+meter

Gives a per leg reading and helps when trying to balance loads at the breaker box. Labeled on leg red and one black. Then colored each breaker red or black.

They make a smaller one that would be a better bet for a 802, but I'll leave finding it as an exercise to the reader.
I see that this meter reads watts consumed, this can be very different than amps produced by the generator. You can actually overload the generator using the watt meter as a guide. The digital volt meter and amp meter combination would be much more accurate and make sure you won't overload the generator.
 

steelypip

New member
753
3
0
Location
Charlottesville, VA
I've observed that my panel % Load meter indicates quite close to the as-configured standard. A purely resistive load (space heaters) does indeed indicate 80% load at 5KW. That said, I know that if I run my air compressor on the generator, the gauge will be a little more honest.

My bigger concern until I declare some power downtime and move some breakers around in the house load center, is that about 70% of the 120V are on L1, so I see a pretty significant load imbalance if I'm not running some big 240V loads and all the fridges and freezers are cooling down.

Why do you care? Because the gen set's voltage regulator can't separately regulate the voltage of the different legs. I end up a little over voltage on one leg and under voltage on the other. It's not a huge deal, because the delta-V between legs is something like five volts, but it is consistent and measurable.

The result is that for a longer run (a real power outage likely to last a day or more), I tend to spend a fair amount of time checking voltage on both legs in the house as I add loads, and sometimes have to make a trip or two out to the gen set to tweak the voltage up or down a little. In this scenario, the panel gauges can also lie somewhat, so it's important to know what's really going on.
 

csheath

Active member
659
27
28
Location
FL
When I first tested my unit it had a voltage drop only on L1 even with an even load applied. The higher the load the more the voltage dropped. L1 would drop from 123 to 111 under full load. I cleaned and recrimped a lot of connections and retested. It started similar then as I applied more load the voltage evened out and has remained even every since. Under actual use I see a little difference in current but the Voltage remains even on L1 and L3 all the way from no load to full now. I assume my previous readings were due to a bad connection.

Hertz will drop a little but that is to be expected. I set it around 61 unloaded and it ends up 59 under heavy loads.
 
Last edited:
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website like our supporting vendors. Their ads help keep Steel Soldiers going. Please consider disabling your ad blockers for the site. Thanks!

I've Disabled AdBlock
No Thanks