Conductor sizing for new fire pump service.

[Fitzdrew516]

I’m in the middle of designing a service to a new fire pump building. I have attached a PDF for reference. Does this sizing seem ok or does anyone have any issues with this set up?

Note 2 - will be sized as normal for the 60A panel.

Note 7 - will indicate wire size for 125% of the FLC of the jockey pump. (480, 3p, 1.5 HP Pump = 3A FLC (per 430.250)*1.25 = 3.75A = #12G wire)

Note 8 - will indicate wire size for 125% of the FLC of the fire pump. (480, 3p, 75HP Pump = 96A FLC (per 430.250)*1.25 = 120A = #1G wire)

Note 1 (service conductors) –Here’s where I’m not so sure. I’m assuming I would just add the 60A+120A+3.75A and then size the wire for that which would equal 183.75A or 3/0 wire. Note – voltage drop on this size is under the required 15%. This seems like the common sense way to size things, but I can’t find anything that verifies my thought process and I know fire pumps can be tricky.

If anyone could provide some insight I’d appreciate it.

Thanks,
- Drew

 

[jeremy.zinkofsky]

Shouldn't there be a service disconnect?

The service entrance conductors should be sized according to the max available current from the service XFMR.

For example:

Let's say that the utility XFMR is 150kVA 480/277V.
Secondary full load current is:
150kVA/(480V * 1.732) = 180A
Which would require you to use 3/0 AWG feeders.

As you see, it makes sense to add up the loads (and you would have to to size the XFMR). But the proper way to size feeders is always to base it off of what amperage is available i.e. XFMR full-load or Overcurrent device rating.

 

[Fitzdrew516]

Shouldn't there be a service disconnect?

The service entrance conductors should be sized according to the max available current from the service XFMR.

For example:

Let's say that the utility XFMR is 150kVA 480/277V.
Secondary full load current is:
150kVA/(480V * 1.732) = 180A
Which would require you to use 3/0 AWG feeders.

As you see, it makes sense to add up the loads (and you would have to to size the XFMR). But the proper way to size feeders is always to base it off of what amperage is available i.e. XFMR full-load or Overcurrent device rating.

Sorry - I should have been more clear. Each control panel has it's own disconnect. I'm utilizing the 6 disconnect rule in this application.
Also - Fire pumps are treated a bit differently. The wire is not sized to the rating of the OCPD. The wire is sized at 125% of the FLC, the OCPD however is sized based on the Locked rotor current.

- Drew

 

[jeremy.zinkofsky]

Sorry - I should have been more clear. Each control panel has it's own disconnect. I'm utilizing the 6 disconnect rule in this application.
Also - Fire pumps are treated a bit differently. The wire is not sized to the rating of the OCPD. The wire is sized at 125% of the FLC, the OCPD however is sized based on the Locked rotor current.

- Drew

Right, but we're talking about sizing the service cables and not the motor feeder cables. If you size the cable appropriately based on the XFMR secondary's full load amperage and the cable gets overloaded, the XFMR will get overloaded as well. In that scenario, the XFMR has not been sized appropriately and you have bigger problems.

 

[jeremy.zinkofsky]

How did you size the secondary cable of XFMR 'T3' in your drawing? The Utility XFMR will be sized using the same approach.

 

[Fitzdrew516]

Right, but we're talking about sizing the service cables and not the motor feeder cables. If you size the cable appropriately based on the XFMR secondary's full load amperage and the cable gets overloaded, the XFMR will get overloaded as well. In that scenario, the XFMR has not been sized appropriately and you have bigger problems.

In this application they're the secondary conductors of the utility transformer. The utility is not governed by the NEC. All you do is tell the utility what your approximate load is going to be, then they size the xfmr. If it were a house transformer I would agree with you as far as sizing for the secondary. This application is pretty different in many aspects so I'm trying to make sure I'm covering all bases - I just don't know 100% if I am.

 

[jeremy.zinkofsky]

In this application they're the secondary conductors of the utility transformer. The utility is not governed by the NEC. All you do is tell the utility what your approximate load is going to be, then they size the xfmr. If it were a house transformer I would agree with you as far as sizing for the secondary. This application is pretty different in many aspects so I'm trying to make sure I'm covering all bases - I just don't know 100% if I am.

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

 

[Fitzdrew516]

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

Yeah. I agree. What I'm asking is do I size the load to 125% of the Fire/Jockey pumps or would that only be a consideration for the wire that feeds directly from the control panel to the motor itself?

 

[jumper]

Not sure if this is your answer.

695.6

(B) Conductor Size.
(1) Fire Pump Motors and Other Equipment. Conductors
supplying a fire pump motor(s), pressure maintenance
pumps, and associated fire pump accessory equipment shall
have a rating not less than 125 percent of the sum of the fire
pump motor(s) and pressure maintenance motor(s) full-load
current(s), and 100 percent of the associated fire pump accessory
equipment.

 

[Fitzdrew516]

I guess a question regarding 240.21 would be - The OCPD for the control panels are sized for the locked rotor current current of the pumps - Now would you need to size the wire from the wireway to the controllers based on the OCPD of the controllers? I know that's how you would normally approach the tap rule, but feeding 600 amp wire off of some 3/0 service conductors seems asinine.

 

[Fitzdrew516]

Not sure if this is your answer.

695.6

(B) Conductor Size.
(1) Fire Pump Motors and Other Equipment. Conductors
supplying a fire pump motor(s), pressure maintenance
pumps, and associated fire pump accessory equipment shall
have a rating not less than 125 percent of the sum of the fire
pump motor(s) and pressure maintenance motor(s) full-load
current(s), and 100 percent of the associated fire pump accessory
equipment.

Right - I'm following that rule when sizing to the pumps them self, but I guess the question is do the service conductors need to be sized to account for that extra 25% in this case?

 

[jeremy.zinkofsky]

Yeah. I agree. What I'm asking is do I size the load to 125% of the Fire/Jockey pumps or would that only be a consideration for the wire that feeds directly from the control panel to the motor itself?

That is only for the cable that feeds the motor directly. Your drawing shows all of the equipment going to a wireway, where (I'm guessing) it will be tapped. That cable (Keyed Note #1) needs to be sized to carry the maximum load of all of the equipment and in accordance with the tap rule.

How are the pumps protected? Are there fuses in the control panel or a circuit breaker?

 

[jeremy.zinkofsky]

I guess a question regarding 240.21 would be - The OCPD for the control panels are sized for the locked rotor current current of the pumps - Now would you need to size the wire from the wireway to the controllers based on the OCPD of the controllers? I know that's how you would normally approach the tap rule, but feeding 600 amp wire off of some 3/0 service conductors seems asinine.

For the configuration you have shown in your drawing, you have to adhere to 240.21. If you add in a Main Distribution Panel, then you can size the feeders to the fire pump control panels based on the OCP device in that MDP. But since you are tapping from one point, the cables need to be able to carry the max current. which leads us back to where we started with the Utility XFMR secondary full load current. You are giving the Utility an approximated load, that same load will have to be carried by the XFMR's secondary conductors no matter what. If the approximated load given to the Utility included that extra 25% percent then you have answered your own question, the service conductors must be sized to include the extra 25% as well.

 

[Fitzdrew516]

How are the pumps protected? Are there fuses in the control panel or a circuit breaker?

The panel's schematics show CB's at the main disconnect.

 

[jeremy.zinkofsky]

The panel's schematics show CB's at the main disconnect.

What are they rated for?

 

[Fitzdrew516]

For the configuration you have shown in your drawing, you have to adhere to 240.21. If you add in a Main Distribution Panel, then you can size the feeders to the fire pump control panels based on the OCP device in that MDP. But since you are tapping from one point, the cables need to be able to carry the max current. which leads us back to where we started with the Utility XFMR secondary full load current. You are giving the Utility an approximated load, that same load will have to be carried by the XFMR's secondary conductors no matter what. If the approximated load given to the Utility included that extra 25% percent then you have answered your own question, the service conductors must be sized to include the extra 25% as well.

I agree with you on the extra 25% - That's how I was leaning in the first place, however, I don't necessarily agree with the logic behind the 240.21 portion of this conversation. The reason feeder taps normally need to be sized at the terminating device's rating is so that they are protected by overload. With fire pumps there is no overload protection because the idea is that you'd rather burn up the feeders than trip the motor. Also - the OCPD in the fire controllers should be sized based on locked rotor current not actual load. To me it doesn't make sense to run 3/0 in from your service, then tap off to (2) sets of 300's to satisfy the rating of that OCPD only to then run 1/0's to the fire pump motor.

Normally I would agree with you regarding the tap rule, but in a fire pump application I'm not sure if that's the right approach or not.

 

[Fitzdrew516]

What are they rated for?

It doesn't give me that info

 

[jeremy.zinkofsky]

I agree with you on the extra 25% - That's how I was leaning in the first place, however, I don't necessarily agree with the logic behind the 240.21 portion of this conversation. The reason feeder taps normally need to be sized at the terminating device's rating is so that they are protected by overload. With fire pumps there is no overload protection because the idea is that you'd rather burn up the feeders than trip the motor. Also - the OCPD in the fire controllers should be sized based on locked rotor current not actual load. To me it doesn't make sense to run 3/0 in from your service, then tap off to (2) sets of 300's to satisfy the rating of that OCPD only to then run 1/0's to the fire pump motor.

Normally I would agree with you regarding the tap rule, but in a fire pump application I'm not sure if that's the right approach or not.

I think i may have stated that wrong. I am not saying that you should splice a 300 with a 3/0 only that you should size the service cables to the secondary of the Utility XFMR, that's all. The tap rules do get tricky, but it is best to not worry about what each breaker is feeding only it's rating. Check out the portion of 240.21 regarding XFMR secondary taps.

 

[GoldDigger]

I think i may have stated that wrong. I am not saying that you should splice a 300 with a 3/0 only that you should size the service cables to the secondary of the Utility XFMR, that's all. The tap rules do get tricky, but it is best to not worry about what each breaker is feeding only it's rating. Check out the portion of 240.21 regarding XFMR secondary taps.

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.

 

[gadfly56]

I guess a question regarding 240.21 would be - The OCPD for the control panels are sized for the locked rotor current current of the pumps - Now would you need to size the wire from the wireway to the controllers based on the OCPD of the controllers? I know that's how you would normally approach the tap rule, but feeding 600 amp wire off of some 3/0 service conductors seems asinine.

No, the conductors are sized to the FLC, not the LRC.