Lift Station Service sizing

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hurk27

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I have a lift station the I have to wire and supply the service to, I am having problems with the engineers specs, and installed main breaker in the control cabinet.
The control cabinet has already been built, and ready for ship to site.

The problem is I believe the 100 amp main breaker is to small that they have installed.

We have two 10 hp pumps that for the most time will run one at a time, but if load dictates, can be both brought on-line, so the supply will have to supply both pumps, 3-phase is not available in the area so the engineers installed VFD's to supply 3-phase to the pumps through PLC control.

The pump motors are 10 HP with a FLA of 28.4 amps on the motor label @ 230 volts 3-phase

If you figure the amps at 240 volts single phase via the 746 watts per HP you end up with 31 amps per pump, (which I think is what their doing)
But at 28.4 FLA Im seeing just over 49 amps per pump, and this is not including panel heater, PLC and control loads, and loss across the VFD's?

I thinking I need a 150-200 single phase amp service, and at least a 150 amp main in the control panel?

I had problems before with the wiring in control panels from this engineer before, I'm just trying to come up with why they think a 100 amp 240 volt single phase service will hold this installation?
 

Jraef

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...
If you figure the amps at 240 volts single phase via the 746 watts per HP you end up with 31 amps per pump, (which I think is what their doing)
But at 28.4 FLA Im seeing just over 49 amps per pump, and this is not including panel heater, PLC and control loads, and loss across the VFD's?

I thinking I need a 150-200 single phase amp service, and at least a 150 amp main in the control panel?

I had problems before with the wiring in control panels from this engineer before, I'm just trying to come up with why they think a 100 amp 240 volt single phase service will hold this installation?

You're right to be concerned. The 1 phase current draw is higher than the 3 phase current by the sq. rt. of 3, or 1.732. So if the 3 phase motor FLA is 28.4, the 1 phase current drawn by the VFD will be 49.19A per VFD. But that's not the end of it. To run a 3 phase motor from a 1 phase supply, you have to DOUBLE the size of the VFD in order to supply that extra current AND to have enough capacitance to smooth the extra ripply from rectifying 1 phase. So for a 10HP 230V motor, you have to use a 20HP VFD. Then, you are required to size the circuit feeding a VFD at 125% of the VFD maximum input amp rating, not the motor. So a typical 20HP 230V VFD is probably going to have a maximum INPUT current of around 60A (just because they only make them in certain sizes). That means EACH VFD will need to have a circuit capable of 75A and so for a duplex lift station, you will need a circuit capacity of at least 150A, assuming no other significant loads.

But as long as the conductors are sized for this, there's nothing saying the breaker can't be smaller. They may end up spending a lot of time sending a service worker out to reset that breaker every time there is a storm and the lift station has to pump full-tilt-boogie with both pumps, but that technically isn't your problem. All you have to worry about is sizing the conductors properly. But you may want to mention this issue to the owners, they will have to suffer the consequences.
 
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Jraef

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As to why they thought a 100A would work?

I like the optimism concept... but most likely s because the consultant doesn't know what he's doing and /or got some bad advice. This issue is full of pitfalls and there are a lot of people who take on these projects without understanding them fully.
 

hurk27

Senior Member
You're right to be concerned. The 1 phase current draw is higher than the 3 phase current by the sq. rt. of 3, or 1.732. So if the 3 phase motor FLA is 28.4, the 1 phase current drawn by the VFD will be 49.19A per VFD. But that's not the end of it. To run a 3 phase motor from a 1 phase supply, you have to DOUBLE the size of the VFD in order to supply that extra current AND to have enough capacitance to smooth the extra ripply from rectifying 1 phase. So for a 10HP 230V motor, you have to use a 20HP VFD. Then, you are required to size the circuit feeding a VFD at 125% of the VFD maximum input amp rating, not the motor. So a typical 20HP 230V VFD is probably going to have a maximum INPUT current of around 60A (just because they only make them in certain sizes). That means EACH VFD will need to have a circuit capable of 75A and so for a duplex lift station, you will need a circuit capacity of at least 150A, assuming no other significant loads.

But as long as the conductors are sized for this, there's nothing saying the breaker can't be smaller. They may end up spending a lot of time sending a service worker out to reset that breaker every time there is a storm and the lift station has to pump full-tilt-boogie with both pumps, but that technically isn't your problem. All you have to worry about is sizing the conductors properly. But you may want to mention this issue to the owners, they will have to suffer the consequences.

Thanks Jraef
I thought this was kind of wrong, but I do think I miss figured the current, as the 28.4 is at the rating of the motor @ 230 volts, which gives you 11.3KW which ends up being 47 amps @ 240 volts single phase.

The VFD's are 20 HP AB not sure which model, the panel load might be about 1500 watts with the panel heater and cooling fan which would run at the same time, with the VFD's ramping the pumps up, we don't have to worry about start up current, but even so there is no room for the 125% extra head room, and like you said, this will be a future maintenance disaster.

I bid this job about 4 months ago, and was just hit with "we doing it next Wednesday" yesterday, well that is not going to happen, I can't even get the POCO on board that fast even if we didn't need a larger service.

How does the commercial go?

I Love logistics
 

Jraef

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Hurk,
Did anyone check to see that you can use the A-B VFDs like this? Some models, especially older ones, had phase loss detection in them and it could not be disabled, leaving it so that you could NOT use them as phase converters even though technically they were capable of it. Worth checking before getting them all installed and wired, only to find out it was a bad application of that product.
 

hurk27

Senior Member
Hurk,
Did anyone check to see that you can use the A-B VFDs like this? Some models, especially older ones, had phase loss detection in them and it could not be disabled, leaving it so that you could NOT use them as phase converters even though technically they were capable of it. Worth checking before getting them all installed and wired, only to find out it was a bad application of that product.

I'll keep you up dated when they deliver the control panel to the site, while they have several sites with VFD's for 3-phase from a single phase service, but I have not been involved with them as of yet., I not positive of the brand, just assumed that they would stick with A-B because all the other controls are A-B, including the PLC?
 

hurk27

Senior Member
As to why they thought a 100A would work?

I like the optimism concept... but most likely s because the consultant doesn't know what he's doing and /or got some bad advice. This issue is full of pitfalls and there are a lot of people who take on these projects without understanding them fully.

Well they come to the conclusion that I was correct and did resize everything to a 200 amp service.

That will mean 3/0 feeders from a 200 amp pedestal to a 200 amp transfer switch and from the transfer switch to a 200 amp main in the control panel.

The VFD's there using is the Allen Bradly Fire Flex 70 which he says allows single phase to 3 phase operation as per Allen Bradly engineers, but like was said, that part is not my problem.

Thanks for all the help:)
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Curious as to how the installation conforms to criteria established in NFPA 820 Table 4.2, and consequently, NEC 501 or 502 specifically as they relate to the conduits going between the pit and the first termination box.
 

david luchini

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Connecticut
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Curious as to how the installation conforms to criteria established in NFPA 820 Table 4.2, and consequently, NEC 501 or 502 specifically as they relate to the conduits going between the pit and the first termination box.

Hard to tell from the pictures, but it appears it may fall under "Individual Residential Pumping Units" from Table 4.2, and therefore be unclassified.
 

hurk27

Senior Member
Curious as to how the installation conforms to criteria established in NFPA 820 Table 4.2, and consequently, NEC 501 or 502 specifically as they relate to the conduits going between the pit and the first termination box.

One Indiana has never require a classification between the vapor break junction box and pit as long as the pit is a vented system (which it is, but the 4" stack hasn't been installed yet just the pipe and elbow) and there is a vapor flow barrier in the pipe going to the pits, basically kind of a sweeping P trap with electrical insulating oil in it, to stop air flow from the pit to the J box, it allows the pump and float conductors to be removed and installed without having to reseal the pipe, just kind of messy.

From what I'm understanding this might change next year.
 

Jraef

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Nice job by the way, and a nice spot to do it in from the looks of it. Were you not in Indiana I would have thought that was salt water behind it, hence the SS boxes. But I guess they just like to spend money eh?

I used to do these kinds of jobs in Washington State in really pretty settings like that. I'd be out on a Sunday drive in the countryside with my family and after a while of driving on back roads, my wife would turn to me and say "We're going to go look at another of your d@#^&* control panels again aren't we?" She was always right. I stopped after a while though because when she went back to work in a boring office, it made her jealous that I got to run around in nature for my job.
 
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