Fire Pump questions

skytop

Member
Location
New York
Occupation
Engineer
I have 2 questions for fire pumps.

if you have one 2000 amp service switch feeding a building, plus a tap before this service switch to feed a fire pump... let's assume it's a 400 amp disconnect switch to feed the fire pump... is this considered a 2000 amp service or a 2400 amp service? I don't know if the fire pump switch is included.

The service entrance conductors to which the fire alarm tap is connected to... those service entrance conductors are sized at 125% of the continuous load, plus 100% of the non-continuos load, plus motor loads... plus 125% of the fire pump current? is the fire pump current included? Not sure if when the fire pump is on, all other loads might be off, or do you count as everything being on?
 

tom baker

First Chief Moderator
Staff member
Lets back up a bit. What is the HP of the fire pump? Does the fire pump disconnect have overcurrent protection and if so, can it carry the locked rotor current of the FP?
Why not take the tap and go straight to a listed fire pump controller. Much easier. Fire pumps normally don't have overcurrent protection, but there is SC and GF protection built into the FP controller
Also did you account for voltage drop to the FP? Art 690 is the only art to require voltage drop compliance
 

skytop

Member
Location
New York
Occupation
Engineer
25 HP fire pump. The service is 120/208 3ph 4w. Sizing the the fire pump disconnect switch to handle the locked rotor current, it's 400 amps. What i meant is, there's a tap on the line side of the 2000 amp service switch. That tap feeds the fire pump disconnect switch... it's a 400 amp switch fused at 400 amps. This 400 amp switch then goes on to feed the fire pump controller. Not sure if that clarifies it. would this be a 2000 amp service or a 2400 amp service?
 

skytop

Member
Location
New York
Occupation
Engineer
This is just a theoretical installation... but let's assume 5 sets of 600 MCM... which gives you 2,100 amps. I'm not sure however if i should be adding the fire pump current at 125%... that's my 2nd question in my OP.
 
Location
MD, USA
Occupation
EE
I have 2 questions for fire pumps.

if you have one 2000 amp service switch feeding a building, plus a tap before this service switch to feed a fire pump... let's assume it's a 400 amp disconnect switch to feed the fire pump... is this considered a 2000 amp service or a 2400 amp service? I don't know if the fire pump switch is included.

The service entrance conductors to which the fire alarm tap is connected to... those service entrance conductors are sized at 125% of the continuous load, plus 100% of the non-continuos load, plus motor loads... plus 125% of the fire pump current? is the fire pump current included? Not sure if when the fire pump is on, all other loads might be off, or do you count as everything being on?
230.42 tells us that service conductors "shall have an ampacity of not less than the maximum load to be served." If you're asking how the FP plays into the service calc, 230.42 points to Art 220 Part III, which in turn points to Art 430 for motors. If your FP is the largest motor, calculate at 125%, otherwise at 100% of FLC.

LRC does not factor into the calculation of the service conductors - 695.4(B)(2)(a)(1) clarifies that: "...The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuit(s)."

I'm not certain that I understand the 2nd question, but it sounds like 230.42 applies as well.
 

skytop

Member
Location
New York
Occupation
Engineer
I don't follow unfortunately. This is what I mean, please see attachment. Is this considered a 2000 amp service or a 2400 amp service? it has a 2000 amp service switch for the building loads, plus a tap on its line side going to a 400 amp switch (fused at 400 amps) feeding the fire pump. And yes, the 400 amp for the fire pump was sized at its locked rotor current.

These are just theoretical numbers, just throwing numbers out there.

service.jpg
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
When there are multiple service disconnects the service size is based in the size of the SEC's.
 

skytop

Member
Location
New York
Occupation
Engineer
230.42 tells us that service conductors "shall have an ampacity of not less than the maximum load to be served." If you're asking how the FP plays into the service calc, 230.42 points to Art 220 Part III, which in turn points to Art 430 for motors. If your FP is the largest motor, calculate at 125%, otherwise at 100% of FLC.
I follow! so the fire pump current is in fact added to the service entrance conductors, at 125% if it's the largest motor. This actually answered my 2nd question.
When there are multiple service disconnects the service size is based in the size of the SEC's.
is this stated in article 230? I'm sure it is, but which section if you have multiple service switches?
 
Location
MD, USA
Occupation
EE
I follow! so the fire pump current is in fact added to the service entrance conductors, at 125% if it's the largest motor. This actually answered my 2nd question.

is this stated in article 230? I'm sure it is, but which section if you have multiple service switches?
When you say service - do you mean the amperage of the service equipment and service conductors? The quantity of service disconnects in the gear is irrelevant to the service size...you can have six 800A service disconnects in the gear and still have a 2000A gear with 2000A CTs - it all starts with the load.

The way I see it is you do the service calc and size your service equipment based on your POCO's CT/service requirements.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Is that in article 230? which section exactly when theres multiple switches?
230.42 applies in any situation (conductors must carry load); 230.90 allows multiple service disconnect ratings to
exceed the ampacity rating so long as the conductors are sized for the load.
 

skytop

Member
Location
New York
Occupation
Engineer
I see. ok, 230.42 and 230.90 helped me a lot!

so, (these are all made up numbers) let's say I have four 600 amp service switches (each fused at 600 amps... one can be for the fire pump), and my service entrance conductors are 5 sets of 600 mcm (copper, so 420 amps at 75 deg C). My total calculated demand load is 1900 amps on the service entrance conductors.... that includes 125% of continuous loads, 100% non-continuous, motors, 125% of largest motor, etc.

If someone (engineer, contractor, inspector, etc) asks me what my service size is, is it correct to say my service size is 2400 amps (based on the OCDP's in the switches), OR 2100 amps (which is the maximum current allowed by the service entrance conductors)? I think my answer would be 2100 amps.
 
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skytop

Member
Location
New York
Occupation
Engineer
When you say service - do you mean the amperage of the service equipment and service conductors? The quantity of service disconnects in the gear is irrelevant to the service size...you can have six 800A service disconnects in the gear and still have a 2000A gear with 2000A CTs - it all starts with the load.

The way I see it is you do the service calc and size your service equipment based on your POCO's CT/service requirements.
Right... so in your example, the service size is 2000 amps. Not 4800 which would be the total rating of your OCPDs in your service disconnects (assuming they are all fused at 800 amps).

Does the same go for my example in post #13, right above? Let's say it has 2500 CTs. my service size would still be 2100 amps.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If someone (engineer, contractor, inspector, etc) asks me what my service size is, is it correct to say my service size is 2400 amps (based on the OCDP's in the switches), OR 2100 amps (which is the maximum current allowed by the service entrance conductors)? I think my answer would be 2100 amps.
I don't see how you can have a service size that is larger than the conductors feeding it so the answer would be 2100 amps.
 

skytop

Member
Location
New York
Occupation
Engineer
I see. ok, 230.42 and 230.90 helped me a lot!

so, (these are all made up numbers) let's say I have four 600 amp service switches (each fused at 600 amps... one can be for the fire pump), and my service entrance conductors are 5 sets of 600 mcm (copper, so 420 amps at 75 deg C). My total calculated demand load is 1900 amps on the service entrance conductors.... that includes 125% of continuous loads, 100% non-continuous, motors, 125% of largest motor, etc.

If someone (engineer, contractor, inspector, etc) asks me what my service size is, is it correct to say my service size is 2400 amps (based on the OCDP's in the switches), OR 2100 amps (which is the maximum current allowed by the service entrance conductors)? I think my answer would be 2100 amps.
I do have one more question... using my example above, all the numbers are the same except for the service entrance conductors... let's now size it at 7 sets of 600 mcm (copper). if someone asks me what my service size is, is the answer 2940 amps (7 sets of 600 mcm), or 2400 amps (the limit of the OCPDS's)? I'm assuming the answer is 2400 amps, but I'm not 100% sure. Based on post #8 though, the answer would be 2940 amps... when you have multiple switches, the service size is based on the SEC's.
 
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