Fire Pump LRC

[kingpb]

Based on the one-line, does the utility transformer have to carry the LRC of fire pump and 100% of the service load, or just 125% of the fire pump plus 100% of the service load?

 

[don_resqcapt19]

It appears to me that 695.3(A) requires the transformer to be able to carry the locked rotor current of the fire pump. I think it must be able to supply the normal loads in addition to the fire pump LRC.

 

[G._S._Ohm]

And the LRC of the pressure maintenance pump motor(s)?

 

[texie]

It appears to me that 695.3(A) requires the transformer to be able to carry the locked rotor current of the fire pump. I think it must be able to supply the normal loads in addition to the fire pump LRC.

I think this is a little "gray". 695.5 makes it clear that a transformer does not have to be rated to carry the LRC, just the overcurrent device. The "gray" for me is, does this apply to a utility transformer?

 

[don_resqcapt19]

I think this is a little "gray". 695.5 makes it clear that a transformer does not have to be rated to carry the LRC, just the overcurrent device. The "gray" for me is, does this apply to a utility transformer?

That section only applies "Where the service or system voltage is different from the utilization voltage of the fire pump motor". That would not apply to the utility transformer.
That does bring up a question, if this second transformer is not sized to carry the LRC, what good does it do to have the utility transformer sized to carry that current?

 

[Haji]

That does bring up a question, if this second transformer is not sized to carry the LRC, what good does it do to have the utility transformer sized to carry that current?

There may not be a problem with excessive voltage drop with the controller of the fire pump (the controller is mechanically held), if the second transformer is not sized to carry the LRC. But the same can not be said about other loads such as a fire lift motor controller powered by utility transformer.
Of course there is a limit to voltage drop beyond which the motor would stall and NEC is not a design manual.

 

[mayanees]

Based on the one-line, does the utility transformer have to carry the LRC of fire pump and 100% of the service load, or just 125% of the fire pump plus 100% of the service load?

I think yes, the LRC of the fire pump and 100% of the service load, at 15% VD per NEC 695.
John M

 

[kingpb]

Thanks all, muy gut told me it had to carry the LRC, but it seemed to me the wording was vague in that area, which I think some of you recognized as well.

If it wasn't capable of the LRC, I would think you might have a hard time with the limit of 15% VD. Maybe that's how they back into it, after all the NEC cannot dictate to utilities.

I am going with the LRC approach. It's what makes sense to me.

 

[steve66]

, after all the NEC cannot dictate to utilities.

Exactly. Around here, the utility can size their transformers any way they want. The NEC doesn't apply.

 

[mayanees]

Exactly. Around here, the utility can size their transformers any way they want. The NEC doesn't apply.

But, I think as a consultant we need to put on the design documents, a minimum sized transformer required to start the fire pump. If the Utility doesn't provide the specified size, the fire pump might not start, and that's a problem! But at least the design called out a minimum size. I did one of these a long time ago, and I used the Utility provider's source contribution in order to do a transient motor starting analysis. Then I included all that info on the design drawing... cya.

John M

 

[Skokian]

Transformer sizing must meet both voltage drop requirements.

Transformer sizing must meet both voltage drop requirements.

Sizing the transformers must be such that it meets both the 15% starting current voltage drop at the fire pump controller inlet terminals and the 5% 115% of FLA voltage drop at the motor terminals (or the fire pump controller load terminals). The voltage drop calculations must be done at the appropriate power factors, namely 30% to 40% during starting, and 85% during running. The starting voltage drop calculations must be done vectorially. The running drop can make use of the 85% PF column in Table 9, Chapter 9.

The vector calculations are illustrated in NEMA publication: NEMA Standards Publication ICS 14-2007 Application Guide for Electric Fire Pump Controllers:

http://search.yahoo.com/search?p=NEMA+ICS+14

Hope this helps.

 

[G._S._Ohm]

This is becoming a can of worms?

Decision Theory would say, list all the possibilities.

Then check what all is covered by the rule, what cases are not covered, and what is still a gray area (hopefully, none). And there have been lawsuits based on the placement of a single comma.

If the rule usually "works" it might be due to luck since PoCo is not bound by it.

While we're at it, is there any kind of feedback loop between the NFPA and the PoCo?

 

[kingpb]

Transformer taps are used to help reduce effect of voltage drop. Minimal improvement in VD at FP due to over sizing the cable. Slight improvement in VD would not justify the cost of larger cable.

The load on the main SWBD was assumed to be 90% of the full rating of the existing transformer, 500KVA; verification needs to confirm actual load. The transformer shown has been increased to standard 750KVA for this project.

The increase from 500KVA to 750KVA is within reason; i.e. 250KVA needed for starting 100Hp motor. The first one line shows the VD upon starting the FP, the second is running at FLA.


The utility could leave the 500KVA in place. The VD on starting will be around 9% at the ADMIN bldg, but then recover to around 2% after starting. The 9% VD in my opinion could be deemed acceptable since the only time it would occur is if a fire were to happen. At that point I would be more concerned with the fire then the lights flickering.