Conductor sizing for new fire pump service.

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GoldDigger said:
If your connection is to the service conductors (upstream of normal service disconnect) it is not a tap and no tap rules apply. Instead you follow the rules for service conductors, including keeping them outside the building to the extent possible.
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I didn't see a service disconnect in his drawing.
 
jeremy.zinkofsky said:
Then you still size the conductors to that approximate load. It doesn't matter who is sizing what or who is providing the XFMR. When you make the connection to the service, the cables you bring will have to be able to handle the same ampacity of the ones provided by the utility. So you will have to do the same calculation that the Utility will do.

Just make sure that you are adhering to NEC 240.21
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So the service to my house has to carry the full available current at the street?!?! That's essentially what you're saying.
 
gadfly56 said:
So the service to my house has to carry the full available current at the street?!?! That's essentially what you're saying.
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I was talking about the cables from the secondary of the Utility XFMR, the cables that terminate in your houses meter. Not the power lines.
 
jeremy.zinkofsky said:
I was talking about the cables from the secondary of the Utility XFMR, the cables that terminate in your houses meter. Not the power lines.
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You had said:

"When you make the connection to the service, the cables you bring will have to be able to handle the same ampacity of the ones provided by the utility".

This suggested that you'd need to handle the full potential current as if you loaded the transformer to 100%. A utility transformer might handle three or more residential connections. By that reasoning, each connection would need to be able to handle the full output of the transformer. Clearly this doesn't happen.
 
gadfly56 said:
You had said:

"When you make the connection to the service, the cables you bring will have to be able to handle the same ampacity of the ones provided by the utility".

This suggested that you'd need to handle the full potential current as if you loaded the transformer to 100%. A utility transformer might handle three or more residential connections. By that reasoning, each connection would need to be able to handle the full output of the transformer. Clearly this doesn't happen.
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You're right in that situation. But what I was refering to was in regards to the system that Fitzdrew is designing, which is most likely a commercial facility with it's own service XFMR.
 
jeremy.zinkofsky said:
You're right in that situation. But what I was refering to was in regards to the system that Fitzdrew is designing, which is most likely a commercial facility with it's own service XFMR.
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Are you going by 240.21(C)(3)? I don't think that's the right section. I think these would be considered service taps, not feeder taps.
 
gadfly56 said:
Are you going by 240.21(C)(3)? I don't think that's the right section. I think these would be considered service taps, not feeder taps.
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It makes sense to classify them as service taps because the drawing shows them going straight to a wireway fromthe service XFMR, but I question if the drawing is showing enough. Where is the main service disconnecting means? I assumed that there is one somewhere between the wireway and the XFMR. But if (for some reason) there is not, then you are correct.
 
jeremy.zinkofsky said:
It makes sense to classify them as service taps because the drawing shows them going straight to a wireway fromthe service XFMR, but I question if the drawing is showing enough. Where is the main service disconnecting means? I assumed that there is one somewhere between the wireway and the XFMR. But if (for some reason) there is not, then you are correct.
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Well for sure, the jockey pump and fire pump are going to be installed as service tapped equipment. I figured it was 3 for 3.
 
If fitzdrew can confirm that these are service entrance conductors then we can easily put this discussion regarding cable sizing to bed by looking at 230.42(A)
 
Connections

Connections

Most if not all Fire Pump controllers are listed as a "Service Entrance". They have a Service Disconnect along with a Circuit Breaker inside them. I have never seen a "Jockey Pump" controller listed as a service entrance (not that they do not exist). It needs to be fed off a breaker from the PH-H panel.
 
Fire Pump Voltage Drops.

Fire Pump Voltage Drops.

Note that the fire pump wiring must be sized to allow no more than a 5% voltage drop when the motor is running at 1.15% of motor rated FLA. This calculation can be done at 85% Power Factor using the last six columns in Table 9 in Chapter 9 and can now be measured at the controller outlet terminals instead of the motor terminals.

However, the maximum 15% voltage at the controller inlet terminals must be calculated using the complex impedances according to the "X" and "R" values given in first eight columns of the same Table and calculated at a 0.3 to 0.4 Power Factor which is what the motor is at locked rotor.

In both cases the impedance of the service conductors (No. 1 in the diagram) must be taken into account as well as the impedance of the utility transformer. The transformer upstream impedance can normally be taken as zero (E.G.: ignored under the assumption of an "infinite bus"). Since the fire pump is the largest load (larger than all of the other loads combined), the utility transformer impedance will probably be a significant factor.

The utility should be able to give at least an estimate of the X to R ratio at the transformer secondary for adding to the locked rotor voltage drop (15% max.) at locked rotor. The transformer rated (nameplate) impedance can usually be used for the 115% FLA voltage drop (5% max.) calculations.

Hope this helps some.
 
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