Fire Pump & Uility XFMR Installation

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NECuser

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I am working on a design for a facility according to the following:
1. Building loads (including a 100 HP fire pump) which the utility company determined that a 750 KVA utility transformer is required. The utility company was informed of the fire pump size when they sized the padmount transformer.
2. Installation of a 100 HP fire pump which will be connected to the utility transformer and a backup generator.

In review of NEC 695, I am having trouble determining whether I need to request for the utility transformer to be increased to a larger size to carry the locked rotor current of the fire pump.
 
Voltage drop (VD) determination

Voltage drop (VD) determination

NEC 695 Fire Pumps calls for 15% VD on starting and 5% while running.
The proper way to determine if the transformer is large enough is to do a voltage drop calculation that includes the normal building load, and the starting of the fire pump. You really need to know the Utility source contribution for the primary of the transformer, along with transformer impedance and cabling information.
There are some rules-of-thumb for sizing the transformer, like using a 3X ratio of transformer-to-fire pump size, but the safest way is to do the calculations. And the easiest way is to model the system in a power systems software program like SKM and do a Transient Motor Starting calculation, or run Voltage Drop using the fire pump locked-rotor current for the start (15% allowable), and the motor fla for the running VD (5%).
Using the 3X rule-of-thumb and the numbers you provided: 750 kVA XF and 100 hP fire pump would suggest that if the building load was 450 kVA, that would leave 300 kVA for the 100 hP motor, which would probably be adequate. (kVA ~=hP) But I suspect the utility is providing a 500 if your load is 450!
 
Let me rephrase my question.

  • Connected Load is 1065077 VA or 1281 A
  • Demand Load is 1015361 VA or 1221 A
  • The loads stated above do not include a 100 Horsepower Fire Pump. Per NEC Table 430.250 the Full Load Amps for this fire pump at 480Y 3-phase is 124A.

I am having trouble interpreting NEC to determine what size service I need to request from the utility company. My understanding is I can do 1221 A + 124 A = 1345 A. Then, I would round up and request a 1600A service from the utility. Please confirm this is correct and that I do not need to request additional spare amperage for the fire pump.
 
NECuser said:
Let me rephrase my question.

  • Connected Load is 1065077 VA or 1281 A
  • Demand Load is 1015361 VA or 1221 A
  • The loads stated above do not include a 100 Horsepower Fire Pump. Per NEC Table 430.250 the Full Load Amps for this fire pump at 480Y 3-phase is 124A.

I am having trouble interpreting NEC to determine what size service I need to request from the utility company. My understanding is I can do 1221 A + 124 A = 1345 A. Then, I would round up and request a 1600A service from the utility. Please confirm this is correct and that I do not need to request additional spare amperage for the fire pump.
Click to expand...
You have to use the Locked Rotor Amps (LRA) not the FLA when sizing for a fire pump. And you need to calculate for less than 15% VD at the fire pump terminals from all wiring including service
 
I will inform the utility that we are requesting a 480Y/277V 3Phase 2000 A service.

As an FYI, we calculated this as follows:

  • The connected load is 1065077 VA or 1281 Amps.
  • Per NEC allowance, this load can be reduced to a Total Estimated Demand load of 1015361 VA or 1221 A.
  • These loads above do not include the 100HP fire pump which will also need to be fed by this service.
  • Per NEC, we need to use the Locked Rotor Current for sizing the fire pump. Per NEC Table 430.251(B), a 100 HP pump has an LRC of 725 A.
  • 1221 + 725 = 1946 Amps. Thus, we are requesting a 2000 Amp service.

The utility company also requested the following in order to size the utility transformer:
Does the utility need to calculate the load of the fire pump based on the 700+ kw of the locked rotor current or should they base the load on 75 kw which is what a 100 HP is?
 
The utility can normally size their transformer however they see fit. They are not bound by the NEC.
 
NECuser said:
Let me rephrase my question.

  • Connected Load is 1065077 VA or 1281 A
  • Demand Load is 1015361 VA or 1221 A
  • The loads stated above do not include a 100 Horsepower Fire Pump. Per NEC Table 430.250 the Full Load Amps for this fire pump at 480Y 3-phase is 124A.

I am having trouble interpreting NEC to determine what size service I need to request from the utility company. My understanding is I can do 1221 A + 124 A = 1345 A. Then, I would round up and request a 1600A service from the utility. Please confirm this is correct and that I do not need to request additional spare amperage for the fire pump.
Click to expand...

If the utility is the sole source of power for the fire pump, than the NEC (2011 at least) does say the source needs to be able to carry the locked rotor current for an indefinite amount of time. Does that mean you need to add the locked rotor current to your other loads to size the service? I'm not sure. I would expect that when the fire pump starts, many of your other loads will shut down.
 
see my post 2!

see my post 2!

As an engineer requesting service from the Utility, I would do a Load Flow analysis that uses the demand load of 1221 amps and the fla of the fire pump to verify the 5% VD criteria is met, then use the demand load number and the locked rotor current for the 15% VD check.
You're likely supplying the Utility with a load letter, and that's where I would list everything including the fire pump, along with a calculation showing the size transformer you recommend. Then you're on the record with the recommendation. They're not obligated to comply, and as long as the fire pump starts without issue, it stops there. I
I've never seen anyone require a measurement of the voltage during startup.
I did have one case where I designed a 480V, dual 75 HP pump station with across-the-line starters because it was a retrofit of a given space and that's all that would fit. The Pump Station was for the City of Dover who was also the Utility provider.
I did Voltage drop for one 75 HP running and one starting, and in order to stay under 15% it needed a 500 kVA transformer. (I use the 15% VD even for non-fire pump starts as a reasonable cutoff to ensure starts)
The Utility didn't have a 500, so they installed a 300, as it fit the same base. The station kicked off-line as soon as the first motor tried to start.
I faxed (15 years ago!) the voltage drop report to them in support of my 500 kVA recommendation, and they gave in and installed a 750 because that's all they had, but it worked.
So you can request a 480V, 2000-amp service all you want, but without a load list and a legitimate voltage drop report you won't get it.
Rule-of-thumb, it looks like it should be a 1500 kVA unit. But I would get the Utility contribution at the primary of the transformer for the VD calculation for an accurate calculation.

What typically happens is that you show 1221 amps demand, and it's actually half of that, in their experience. And if it's actually as high as you're saying and the fire pump won't start during testing, the Utility sets a larger one an hour later! But as the specifying electrical engineer I like to be on the record with a size recommendation.
 
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