fire pump ground wires

[Grouch]

Hi all,
For a high rise building, i have a fire pump on the rooftop (say the 20th floor). It's connected to a transfer switch. The normal feed to the transfer switch comes from the basement, via a fused service disconnect switch (sized at 600% of the pump FLA) tapped off the main switchboard. And the emergency feed to the transfer switch comes from a breaker (sized at 150% of the pump FLA) in the generator.

Here are my 2 questions:
1) What size ground wire do i use in the feeder that runs from the load side terminal of the transfer switch to the fire pump? I would imagine it's sized based on the fuses inside the service disconnect switch, rather than the breaker from the generator, since during either normal or emergency operation, the largest sized OCPD is in the service disc. switch (600% versus 150%).

2) What size ground wire do i use in the feeder from the switchboard tap to the service disc. switch? These are tap conductors, so how do i size the ground wire if there's ground bus bars inside the switchboard?

Thanks!

 

[dkidd]

Is the fire pump service disconnect in the same lineup as the normal service disconnect(s)?

If not, size it from the size of the conductors to the fire pump disconnect as if they were service, Table 250.102(C)(1) (2014)

 

[texie]

Before answering the direct question, I have questions on the layout of this. Isn't the ATS an integral part of the fire pump controller? If not, is it located in the fire pump room and is is listed for fire pump service? If this ATS is not a part of the FP controller things get a lot more complicated from a code standpoint.

 

[Grouch]

I made a quick sketch... hopefully this clears it up. and yes, it's a combo ATS / controller. Let me know if you can see the attachment...

 

[infinity]

2) What size ground wire do i use in the feeder from the switchboard tap to the service disc. switch? These are tap conductors, so how do i size the ground wire if there's ground bus bars inside the switchboard?

Thanks!

This is not a tap these are SEC's so there is no EGC just a grounded conductor sized based on T250.66.

 

[Grouch]

This is not a tap these are SEC's so there is no EGC just a grounded conductor sized based on T250.66.

Interesting... but I thought a fire alarm disconnect switch is not considered a service switch... just a regular disconnect switch. if it is a service switch, then I see what you mean that these are SEC's, and not a tap. (whether it's a service switch or not, I realize that's another topic! ).

 

[infinity]

Interesting... but I thought a fire alarm disconnect switch is not considered a service switch... just a regular disconnect switch. if it is a service switch, then I see what you mean that these are SEC's, and not a tap. (whether it's a service switch or not, I realize that's another topic! ).

It's usually connected ahead of the service disconnect(s) so it is also a service disconnect which means that there is no EGC in with the service entrance conductors just a grounded conductor. Since it's a grounded conductor it would need to be identified according to Article 200 which typically means either white or gray not green.

 

[Grouch]

It's usually connected ahead of the service disconnect(s) so it is also a service disconnect which means that there is no EGC in with the service entrance conductors just a grounded conductor. Since it's a grounded conductor it would need to be identified according to Article 200 which typically means either white or gray not green.

Got it. Thanks.

what about my 1st question? I assume the EGC is sized based on the largest OCPD, depending on whether the ATS switch is on normal or emergency.

 

[Grouch]

This is not a tap these are SEC's so there is no EGC just a grounded conductor sized based on T250.66.

wait wait... doesn't Table 250.66 give sizes for groundING conductors? you're saying groundED conductor, as if for a neutral. wouldn't this wire be green, instead of white or gray as you mentioned in another post?

 

[infinity]

Got it. Thanks.

what about my 1st question? I assume the EGC is sized based on the largest OCPD, depending on whether the ATS switch is on normal or emergency.

That is a great question normally you would use the OCPD ahead of the circuit to size the EGC. I don't know of any special requirements for fire pumps where the protection is set at 600% and the conductors are sized at 125%. The EGC wouldn't need to be larger than the ungrounded conductors.

 

[infinity]

wait wait... doesn't Table 250.66 give sizes for groundING conductors? you're saying groundED conductor, as if for a neutral. wouldn't this wire be green, instead of white or gray as you mentioned in another post?

T250.66 is for GEC's, the grounded conductor brought to a service disconnect is sized to the minimum in T250.66. Look at 230.23(C) & 250.24(C)(1) {2008 NEC}.

 

[Grouch]

T250.66 is for GEC's, the grounded conductor brought to a service disconnect is sized to the minimum in T250.66. Look at 230.23(C) & 250.24(C)(1) {2008 NEC}.

I got it now. thank you for the code sections. I have the NEC handbook so the illustrations in those sections helped a ton.

 

[tom baker]

A comment. "Ground" is not a defined term. Electricians get confused about grounding and bonding, using the term ground loosely. What color is it and what does it connect to?