Generator Load Determination

[lesh]

Hello
I work in a 50 year old industrial facility. We have a 200kw diesel generator. It has been in service for ~15 years. It is connected to an e-power system, a subsystem of our main electrical system; certain devices are connected to this subsystem through automatic and manual transfer switches, i.e. e-lighting, boiler, water well etc. Recent questions have arisen on how much e-power is left. To try to sum connected loads I consulted the panel directories which, for the most part, call out lighting and may be out of date. I added all connected branch circuit currents and get somewhere in the neighborhood of 2k amps, connected to a generator with a 300A main!
Well this past weekend we had a power outage and the generator was online powering the e-system. I checked its ammeter and it indicated 102A. Certain loads were not operating, loads whose nameplates specify currents which would be simply added to the 102A. My question is how can I or should I accurately specify the current demand on this generator?
Thank You
johnk

 

[tkb]

Put a recording meter on the load side of the ATS and see what the load is.

 

[kingpb]

You need to do a load analysis which will entail determining not just the connected load, which it sounds like you already did, but also determine the running load.

There may be load shedding going on that you are not aware of. In which case, only those loads determined essential during loss of main power, actually run.

This type of exercise will help you get to know your system really well.

Watch the connected motors, as the starting current can have an significant impact on the generator.

 

[Hameedulla-Ekhlas]

Hello
I work in a 50 year old industrial facility. We have a 200kw diesel generator. It has been in service for ~15 years. It is connected to an e-power system, a subsystem of our main electrical system; certain devices are connected to this subsystem through automatic and manual transfer switches, i.e. e-lighting, boiler, water well etc. Recent questions have arisen on how much e-power is left. To try to sum connected loads I consulted the panel directories which, for the most part, call out lighting and may be out of date. I added all connected branch circuit currents and get somewhere in the neighborhood of 2k amps, connected to a generator with a 300A main!
Well this past weekend we had a power outage and the generator was online powering the e-system. I checked its ammeter and it indicated 102A. Certain loads were not operating, loads whose nameplates specify currents which would be simply added to the 102A. My question is how can I or should I accurately specify the current demand on this generator?
Thank You
johnk

Turn on all equipment for a while and measure the peak current in peak load.
Check your system for motors start current.
for more information see below link for best generator sizing

Link

by the way, can you make it clear this statement.
" I added all connected branch circuit currents and get somewhere in the neighborhood of 2k amps, connected to a generator with a 300A main"

can you explain the red words.

 

[dbuckley]

The really hard qurestion here is that although the running current is "only" 102A, the gap between the running current and the gensets maximum capability may be needed for motor starting. So you may calculate you have decent headroom, up the load accordingly, and suddenly find when someone callls an elevator (or whatever) the gensets shuts down through underspeed protection.

Thus a recording meter that measures inrush is the only reasonably safe apporoach.

Compare the readiungs on utility power to standby power to find any load that is only used when on utility power. Whilst doing this it also may be worth re-evaluating if anything is powered unnecessarily by the genset.

Finally, a 200KW genset may not actually be able to supply 200KW, because of standby / emergency load / some other weazel words that describe a genset's inability to provide nameplate power.

 

[lesh]

Hameedullah Ekhlas, Thank You for the reply.
To better explain myself- USING THE PANEL DIRECTORIES, SCHEMATICS I added all connected branch circuit currents and get somewhere in the neighborhood of 2k amps, connected to a generator with a 300A main. THIS WAS DONE ON PAPER, NO CURRENT MEASUREMENTS WERE DONE TO DETERMINE 2K AMPS. Sorry for CAPS.

 

[lesh]

dbuckley Thank You for the reply.
The lions share of the 102 amps is lighting load, no significant inrush. There are three motors of significant HP line start connected. Their nameplate current is 37.5 (x2) and 35. There are 5 VFD's connected (10HP motors @5.5A ea.) these also should not present too much inrush because of the internal soft-charge circuit. Beyond that there are 5, 2KVA (@3.5A ea.) control transformers. All said and done it appears as though the static load is about 257A. Considering inrush, headroom is now pretty much minimal which leads me into saying there is not a significant magnitude of ampacity left for future connection. But I'll be challanged to support that statement with fact (code).
Thank You
johnk

 

[zbang]

You will never get an accurate picture by adding up the overcurrent ratings, since you can easily have completely unused circuits included. If you can't do actual current, or better, wattage, readings, you have to look to each machine/device, count the lights included, etc. Check out the article 220 on load caclulations, that will give you a idea of the work involved.