Larger Load on Generator than Utility

[Elec_Man]

Hello everyone,

I received some drawings for a pump house and the utility only has a 30kVA service. It looks like they want to install a generator with an ATS to feed power to large pumps as well as the rest of the building (not fire pumps) when needed. Since these pumps would trip the main on the building they want to interlock the large pumps to only run when the generator is running. This doesn't seem right to me, should the generator with the larger pumps be its own system?

Please let me know if anything is unclear.

 

[kwired]

Seems pointless to me to try to connect them in any way to the utility supply if it can not run them.

I could see utility supplied controls to initiate starting the generation equipment then allowing these pumps to run when the generation is up and ready for it.

 

[Smart $]

Seems like this would be setup as a hybrid system. The generator setup as on-demand for pumps... connected gen-side of ATS. The down-line ATS would automatically turn on generator and transfer on loss of normal (POCO) power.

 

[GoldDigger]

Sounds like a job for a simple load shedding module only active when on grid power.

 

[Smart $]

Sounds like a job for a simple load shedding module only active when on grid power.

What loads would you shed?

How would you run pumps when normal power is NOT out?

 

[GoldDigger]

The OP stated that those particular pumps will only be run when the generators are running, whether the grid is up or not.
The loaf shed modules are controlled by a logic input. You just supply that input based on generator status (on means enable, off means shed) instead of based on grid status and total current load.
The normal load shed decision is based both on grid status and instantaneous load rather than just grid on/off.

 

[Smart $]

The OP stated that those particular pumps will only be run when the generators are running, whether the grid is up or not.
The loaf shed modules are controlled by a logic input. You just supply that input based on generator status (on means enable, off means shed) instead of based on grid status and total current load.
The normal load shed decision is based both on grid status and instantaneous load rather than just grid on/off.

Sounds like you are overcomplicating the scheme. All that is necessary is to connect pump starters on standby side of ATS, and provide for on-demand generator run.

 

[topgone]

Seems pointless to me to try to connect them in any way to the utility supply if it can not run them.

I could see utility supplied controls to initiate starting the generation equipment then allowing these pumps to run when the generation is up and ready for it.

You nailed it. What's the use of buying an ATS when your utility service cannot support running your pumps?

 

[kingpb]

Why not parallel the Gen with the utility?

 

[ActionDave]

You nailed it. What's the use of buying an ATS when your utility service cannot support running your pumps?

OP said because they want the rest of the building to work if the utility power is down.

 

[Elec_Man]

Thanks for all the replies everyone!

After a couple inquiries it looks like they want to have the option to run the larger pumps on the utility at a lower frequency (pumps are vfd controlled) so they do not draw enough power to trip the main. Would this be in violation with anything?

 

[Smart $]

Thanks for all the replies everyone!

After a couple inquiries it looks like they want to have the option to run the larger pumps on the utility at a lower frequency (pumps are vfd controlled) so they do not draw enough power to trip the main. Would this be in violation with anything?

Power is power. Running pump motors at lower frequency will not change the power they require.

As far as violation, quite likely, as all the NEC motor calculations are based on motor FLA table values... so even a motor pulling only 10% of rated capacity must still be calculated at 100%. Only alternative is to have interlocks (or some compliant method) to ensure not all pump motors run concurrently, if that's even an option.

 

[Elec_Man]

Power is power. Running pump motors at lower frequency will not change the power they require.

As far as violation, quite likely, as all the NEC motor calculations are based on motor FLA table values... so even a motor pulling only 10% of rated capacity must still be calculated at 100%. Only alternative is to have interlocks (or some compliant method) to ensure not all pump motors run concurrently, if that's even an option.

Oh my mistake, I guess this is why I am only a technologist. Thank you for your input and glad I asked here before asking the engineers at my firm.

 

[GoldDigger]

Power is power, but running the pumps at reduced speed can in turn lead to a reduced flow rate (lower performance) but at a lower power that the service can provide.
As long as they are aware that they will be getting reduced output, they can work with reduced input power.

 

[kwired]

Power is power, but running the pumps at reduced speed can in turn lead to a reduced flow rate (lower performance) but at a lower power that the service can provide.
As long as they are aware that they will be getting reduced output, they can work with reduced input power.

+1

If running at lower frequency which will result in lower speed, the pump will demand less power.

Hopefully owner/operator realizes they will have limitations when running on utility power.