Fire Pump Equipment Ground Conductor

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ZWire85

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Good morning and go easy on me... new to "complex" Fire Pump installations! Need some "grounded/ing" help. Essentially the system is setup as follows:
  • A 480V Service Transformer has the secondary lugs hits two separate service rated switchboards.
    • One Switchboard is building loads.
      • We can ignore this service for the rest of the question.
    • The other service switchboard is the fire pump service disconnect. It houses a 1000A main breaker for short circuit protection and sized to carry locked rotor for a 125 HP motor (again, 480V, 908A per 430.251).
      • We bring a supply side bonding jumper to this OCP device / panel. The neutral ground bond occurs here as well, and we run an Electrode Grounding Conductor to ground from here.
  • However... that fire pump service panel does not house the ATS or the Controller!
    • From the fire pump service disconnect mentioned above we run to a separate ATS.
    • From the separate ATS... we run to a separate fire pump controller.
    • From the separate fire pump controller, we finally hit the 125 HP fire pump.
Now, in my past lives, that whole cluster was one panel. Disconnect, ATS, Controller... Easy peasy, we run the grounded conductor sized per 250 as a supply side bonding jumper (think I got that term right) and we are all done. And the electrode grounding conductor, but regardless... one stop shop and easy.

Here though, the separate 1000A OCP / panel is the "service disconnect". Neutral / Ground bonding occurs there. NOW downstream we now have an equipment grounding conductor sized per 250.122... which intuitively doesn't make sense to me because of the function of that breaker... but that's how I am reading the code.

What was running as #4 into the service now becomes a 2/0 from the service to the ATS... basically the size of the conductors we are running for the pump (3/0 for a 156 FLA before 125% added)!

Is this a equipment ground sized per 250.122 or a SSBJ sized per 250.102?

Thanks for any headache you can stave off!
 
After your 1000 amp service disconnect 250.122 should apply (a 2/0 EGC should be all you need uUnless we are looking at separate structures or an oversize phase conductors)

The neutral supplying the 1000 amp service should be sized per 250.102.
 
Thank you for the confirmation. That's what I was reading, and I suppose it makes sense given the conductor needs to carry a large load given the OCP device.

It just seems strange because the Disconnect / OCP is a optional but permitted addition to the system. If I didn't include that disconnect, I would have just run a #4 all the way to controller.
 
I'm curiousa
ZWire85 said:
Thank you for the confirmation. That's what I was reading, and I suppose it makes sense given the conductor needs to carry a large load given the OCP device.

It just seems strange because the Disconnect / OCP is a optional but permitted addition to the system. If I didn't include that disconnect, I would have just run a #4 all the way to controller.
Click to expand...
I'm curious as to how you arrived at that number ??
 
Presuming no separate ATS and disconnect, and instead it was just the fire pump controller... The conductors from the secondary of the transformer to the Fire Pump controller would be sized based on 125% of Fire Pump FLA. 125 HP is 156 Ampere, plus 125% is 195A minimum circuit ampacity.

So the ungrounded Conductors would be 3/0 (200 ampacity) from Transformer to this panel. The associated SSBJ (and basically the Grounding electrode conductor ) would be #4 Copper up until that controller based on Tables 250.102 (and Table 250.66 respectively for the grounding electrode conductor).

If no upstream disconnect / OCP was included in the system in our example, as they are optionally permitted per 695.4(B)(1)) the conductor would essentially remain unchanged up to the fire pump controller, this #4 would have been fine all the way.

However you install a 1000 ampere OCP device, and the #4 SSBJ conductor enters the disconnect but leaves as a 2/0 equipment grounding conductor per 250.122. By essentially adding something that could cause a trip in the circuit (some level of protection), you change the grounded conductor function and require the increasing the conductor size.

Leaving the system without that OCP though, you can use a smaller conductor.

I get the conductors serve different function, its just the fire pump application really highlights that.
 
Aside from the grounding/bonding question, I'm wondering what your physical lay out of this is. These days it is highly unusual to have a separate ATS and fire pump controller. In any event the ATS would have to be rated for fire pump duty (most are not) and also the ATS and the controller must be located in the fire pump room.
 
I do not have access to the entire drawing set, but what I've seen so far is what you might expect for a typical arrangement for a city. MV utility service comes into a transformer vault, steps down to MV. We hit a service switch in that room, then the conductors fly through the building to the fire pump room (same floor) where the ATS, controller, and fire pump are. Its a dedicated room, just has separate equipment.

This was only a preview set though, maybe they combine the ATS an Fire Pump Controller in the final. I am wondering if its a ISC thing and they needed a beefier model?
 
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