Supply to A Fire Pump

[jwilson3]

The 2002 NEC "Handbook" provides an example of how to size a transformer and select overcurrent device ratings for a 4160v supply to a 480v 100hp fire pump and 1.5hp jockey pump. It's covered in NEC695.5(A)&(B) and is a straight forward process to select a 150kva transformer.

Where two 4160v sources are needed (normal & backup) 695.5(C) is used. The transformer size is selected per 695.5(C)(1) in the same manner as above. The OC devices are sized per 695.5(C)(2), also as above, to carry the locked rotor of both pumps.[100a on pri. and 850a on sec.]

NEC 695.5(C)(2) has the further requirement that the transformer size be "coordinated" so that the OC devices provide proper protection per 450.3 and Table 450.3(A)
The 150kva size is now inadequate since the 100a pri device is 481% of rating and the sec. device is 470%, therefore a 300kva transformer must be used.

Why is the transformer rating required to be greater when served from two sources than when served from one?

 

[peebee]

695.3(B)(2) and 695.5(C) are intended to apply to large campus-style installations. In a typical small commercial building, you're allowed to use a second service entrance tapped off the single utility transformer to serve the fire pump. But prior to the 2002 NEC, you were not allowed to use the customer-owned transformer typical of campus installations to serve the fire pump -- the NFPA, strictly speaking, required a second transformer.

But in 2002 this was revised. Now you can use the single customer owned transformer to serve the pump and the building (so long as that's OK with the AHJ). So this is really a bonus to you. But that transformer needs to be protected the same as any other transformer now, as it's serving general purpose loads, not just fire pumps. So, in order to continuously carry the locked rotor of the pump but also have the transformer adequately protected, you need to increase the size of the transformer.

 

[jwilson3]

To peebee: Thanks. What you say makes sense, particularly if the transformer is serving "other" building load that is somewhat greater than the pump load -- say 500kva builkding load with a 100hp pump.
My problem with the wording of 695.5(C) is that even if there is no "other" load on the transformer, it still requires the transformer to be upsized.

 

[rbulsara]

jwilson3:

You are confused (which is easy to be with Code wordings).

Firstly, your sentence "Where two 4160v sources are needed (normal & backup) 695.5(C) is used." is not true. 695.3.B permits having a two seprate sources, which may or may not require a trasformer. If a transfomer is required normally 695.5.A and B apply unless you are feeding 'other' loads on a campus style setting.

As peebee mentioned 'feeder sources' only apply to campus style buildings so there is a building and there will be other loads.

695.3.B(2)does not require that you have a transformer ( you can have 480V services to the buildings). 695.5 (C) is there to permit you a buiding service transformer, which obviously will be serving other building loads, to feed a fire pump.

If you have a dedicated transformer for a fire pump, say a second transformer tapped off the primary, 695.5.(C). does not apply as you are not serving 'other' loads. 695.5.(A )and (B) will be applicable then.

Code may be confusing, but they are not intended to override engineering principals and contradict its own provisions. If one size transformer works in one circuit, it has to work in other ciruit.

 

[jwilson3]

rbulsara,
You are correct, I am confused. Confused by a poorly organized and worded part of the NEC.
I agree that "sources" may not require a transformer, but a transformer was included in my example.
I reluctantly agree, with some reservation, that 695.5(A)&(B) can apply to single or multiple sources, but only if the transformer(s) are dedicated to fire pump and associated loads only and 695.5(C) applies only to multiple sources when "other" loads are also served.

For clarification I feel this section should be reorganized as follows:

695.5 Transformers
(A) Fire Pump Service Only
(1) Size
(2) Overcurrent Protectio
(B) Fire Pump and Other Load Service
(1) Size
(2) Overcurrent Protection

I recognize engineering principles are paramount, but often engineering principles are obscurred by poorly worded code articles. That's why I was interested in the reasoning related to these articles.
Thanks