FIRE PUMP FED BY GENERATOR

[PE (always learning)]

This is my last post for the week, I promise!

I have a question pertaining to the instantaneous setting of a breaker that feeds a fire pump from a standby generator. I've been hired to do a power systems study and I need to provide settings for this breaker in question that I mentioned above. After reading through NEC 695.4(B), I found this statement, "Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short circuit protection only."

Let me know if I'm interpreting this correctly please. It is my understanding that they want you to set the instantaneous portion above the locked rotor current, but still abide by 430.62 which makes you follow table 430.52, which only allows you to set the instantaneous portion at 800% of the full load current. This is so you protect against short circuits and don't destroy the generator in the event that a fault occurs and continue power to other sources that the generator may feed.

In my scenario, 800% of the full load current doesn't get me above the inrush current being put out by my fire pump motor. I have set my instantaneous beyond the suggested 800% maximum shown on table 430.52. In my scenario, the only other loads that are fed by the generator are IT equipment loads and the emergency lighting is fed by battery backup. I feel that the importance of my fire pump running outweighs losing power to IT equipment. Would it be acceptable in this scenario to disregard the 800% max required from 430.52?

 

[hillbilly1]

On all of the ones I’ve done, the engineers had us tap ahead of the main breaker on the generator, and also shuntrip the main to drop all other loads if fire pump is running.

 

[mayanees]

Use the fire pump motor nameplate to find the motor's type or code (A-V) for determination of the expected inrush. Then use exception number 1 to adjust the instantaneous to a higher level that exceeds 800% if necessary to accommodate the kVA/hp from the table.

 

[Jraef]

Use the fire pump motor nameplate to find the motor's type or code (A-V) for determination of the expected inrush. Then use exception number 1 to adjust the instantaneous to a higher level that exceeds 800% if necessary to accommodate the kVA/hp from the table.

Yes, it appears you are not reading the exceptions in 430.52.

 

[PE (always learning)]

Yes, it appears you are not reading the exceptions in 430.52.

Ahh, sorry I get tunnel vision when looking at these codes sometimes. The exception you are talking about was on the next page of the NEC handbook I'm looking at. I see where it says, "Where the values for branch circuit short-circuit protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of settings of adjustable circuit breakers, a higher setting that does not exceed the next higher standard ampere rating shall be permitted."

In my scenario, I have limited information, all I know is that the fire pump is rated for 150 HP (based on construction drawings), which gives me a FLA of 180.9 amps at 480V, 3 phase. I know there are two jockey pumps which should be around 7 HP a piece. I have adjusted the breaker instantaneous setting to around 2400 amps, which is around 13 times the FLA. I could set the breaker at a lower setting and just barely be above the inrush curve, but I don't feel comfortable doing that. Would 13 times the inrush be acceptable or is that overkill? Again, I feel that letting the generator run even if there is a fault is justified in this situation because breaking continuity does not affect any other life safety loads, it only affects the some IT equipment.

Best Regards

 

[PE (always learning)]

I see the statement further ahead on this exception that says, "Where the setting specified in Table 430.52 is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current for other than Design B energy-efficient motors. I've got my breaker set at about 13.2 times the motor FLA, which is just above the 1300 percent they mention. Maybe I'm thinking about this too hard, but am I violating anything here?

 

[topgone]

I hope I'm not late in chiming-in! FYI, the overload protection on fire pumps ought not to be activated. It is required that fire pumps should be run to destruction, hence you are only allowed to provide for instantaneous trip only (protection against short-circuits)! I suggest you use an MCP (mag-only trip) circuit breaker.

 

[PE (always learning)]

I hope I'm not late in chiming-in! FYI, the overload protection on fire pumps ought not to be activated. It is required that fire pumps should be run to destruction, hence you are only allowed to provide for instantaneous trip only (protection against short-circuits)! I suggest you use an MCP (mag-only trip) circuit breaker.

If you look at my time current curve, I have set the long time portion of the breaker at 400 amps, which is well above 125% of the FLA (minimum requirement for Fire pumps).

 

[mayanees]

That looks more like a breaker that feeds a panel that feeds a fire pump controller (FPC). Is that breaker part of a listed FPC? I would more expect to have that instantaneous maxed out, and an FPC with an MCP that is set to 13x.

 

[PE (always learning)]

This breaker is located at the generator. It feeds the fire pump controller, which has an automatic transfer switch.

 

[topgone]

If you look at my time current curve, I have set the long time portion of the breaker at 400 amps, which is well above 125% of the FLA (minimum requirement for Fire pumps).

Here are the provisions that specifies what a power source to fire pumps should be that I'm talking about:

695.4 Continuity of Power. Circuits that supply electric motor–driven fire pumps shall be supervised from inadvertent disconnection as covered in 695.4(A) or (B).

Further down the NEC article 695 stipulates what protections are allowed:

695.4(B)(2)(2) Overcurrent protection shall be provided by an assembly listed for fire pump service and complying with the following:

a. The overcurrent protective device shall not open within 2 minutes at 600 percent of the full-load current of the fire pump motor(s).​

b. The overcurrent protective device shall not open with a re-start transient of 24 times the full-load current of the fire pump motor(s).​

c. The overcurrent protective device shall not open within 10 minutes at 300 percent of the full-load current of the fire pump motor(s).​

d. The trip point for circuit breakers shall not be field adjustable.​

On your TCC, these are what I observed:
a) At 600% of FLA (1,086A), the curve indicates it will trip in about 20 seconds! The provisions says not less than 2 minutes at 600%.
c) At 300% FLA (543A), your curve trips at around 300 seconds or 5 minutes! The code says not less than 10 minutes at 300%.
and last but not least, d) you said you "I have set the long time portion of the breaker at 400 amps", does that mean other people could change these settings in the field? If yes, then it is violating the code, IMHO.

 

[PE (always learning)]

Here are the provisions that specifies what a power source to fire pumps should be that I'm talking about:

Further down the NEC article 695 stipulates what protections are allowed:

Firstly, thank you for the response.

Look at 695,4(B)(2)(2)(b) On-Site Standby Generators. This is what applies to my scenario. This sections states, "Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short-circuit protection only.

Section 430.62 then redirects you to table 430.52, which I have referenced for my settings.

Also, this is a generator breaker which is behind a locked enclosure.

Finally, I'm not sure you're looking at the same TCC as me. Did you look at the one I posted? Your evaluation of my settings seems off.

 

[PE (always learning)]

Also, as far as long time settings go, I have set this breaker to the highest long time setting possible.

 

[topgone]

Firstly, thank you for the response.

Look at 695,4(B)(2)(2)(b) On-Site Standby Generators. This is what applies to my scenario. This sections states, "Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short-circuit protection only.

Section 430.62 then redirects you to table 430.52, which I have referenced for my settings.

Also, this is a generator breaker which is behind a locked enclosure.

Finally, I'm not sure you're looking at the same TCC as me. Did you look at the one I posted? Your evaluation of my settings seems off.

I just revisited the TCC you've posted.
I am certain my read outs are on the dot! Please read up from the current of about 1085A and you'll see your curve crosses at about 20 seconds! Also, at 542A, the curve crosses at 300 seconds!

 

[PE (always learning)]

On your first curve markup, the red line is around the 2000 amp marker, not 1085 amps. If you look at the breaker setting it even says that the instantaneous is set to around 2400 amps with a minimum opening time of around 2000 amps.

So if you actually follow up from where 1085 amps is located, the breaker would hit a minimum opening time of around 50ish seconds, so you are technically correct that I don't meet the requirement listed for (a), but this brings me back to my original point, this breaker is coming from the standby generator (technically inside the generator enclosure). The rules for the generator are quite different from that of the utility feed.

If you look at 695,4(B)(2)(2)(b) On-Site Standby Generators. This is what applies to my scenario. This sections states, "Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short-circuit protection only.

On-site standby generators and the associated over current devices are only required to carry the normal starting and running currents. This is different from the utility source which is required to carry locked rotor current indefinitely.

Look at attached TCC for reference as well. Let me know if I'm missing anything.

 

[PE (always learning)]

Unfortunately, this confusion has resulted in the misperception that the generator should be designed with a “run to failure” mentality, meaning the generator should run to destruction while trying to carry fire pump locked rotor amps indefinitely. This mindset is potentially life threating considering the same generator carrying the fire pump will often power the building’s emergency system (exit lights, egress lighting, stairwell pressurization), which is the system designed to get people out of a burning building.

 

[topgone]

Unfortunately, this confusion has resulted in the misperception that the generator should be designed with a “run to failure” mentality, meaning the generator should run to destruction while trying to carry fire pump locked rotor amps indefinitely. This mindset is potentially life threating considering the same generator carrying the fire pump will often power the building’s emergency system (exit lights, egress lighting, stairwell pressurization), which is the system designed to get people out of a burning building.

Okay, looked at the TCC again and found out that I did miss the 1085A point and placed the vertical line at the 1800A point, my apologies! But I got a tripping time of 70 seconds (1.17 minutes) when I adjusted the vertical line to about 1000A roughly! The 300% FLA trip time is however correct (300 seconds or 5 minutes). Both tripping times failed to comply with the code provisions which states that the protection should not trip earlier than 10 minutes at 300%FLA and not earlier than 2 minutes at 600%!

The thing I'm trying to point out is that engineers are bound to follow what the code says, at the very minimum! The overload protection for fire pump motors are not allowed, the power source protection are allowed to have their overcurrent protections set at those 300% nd 600% load FLA's trip times, protections are not allowed to trip at restarts of 24X the FLAs, and most importantly those equipment are listed for fire pump service! The cost of losing a fire pump motor is always much very smaller compared losing the whole facility. Believe me, those engineers before us spent sleepless nights trying to figure out what's best!

 

[PE (always learning)]

Okay, looked at the TCC again and found out that I did miss the 1085A point and placed the vertical line at the 1800A point, my apologies! But I got a tripping time of 70 seconds (1.17 minutes) when I adjusted the vertical line to about 1000A roughly! The 300% FLA trip time is however correct (300 seconds or 5 minutes). Both tripping times failed to comply with the code provisions which states that the protection should not trip earlier than 10 minutes at 300%FLA and not earlier than 2 minutes at 600%!

The thing I'm trying to point out is that engineers are bound to follow what the code says, at the very minimum! The overload protection for fire pump motors are not allowed, the power source protection are allowed to have their overcurrent protections set at those 300% nd 600% load FLA's trip times, protections are not allowed to trip at restarts of 24X the FLAs, and most importantly those equipment are listed for fire pump service! The cost of losing a fire pump motor is always much very smaller compared losing the whole facility. Believe me, those engineers before us spent sleepless nights trying to figure out what's best!

But, I did comply.

Are you reading what I'm saying about the connection from the generator to the fire pump.

On-site standby generators and the associated over current devices are only required to carry the normal starting and running currents. This is different from the utility source which is required to carry locked rotor current indefinitely.

 

[hillbilly1]

But, I did comply.

Are you reading what I'm saying about the connection from the generator to the fire pump.

On-site standby generators and the associated over current devices are only required to carry the normal starting and running currents. This is different from the utility source which is required to carry locked rotor current indefinitely.

That’s was my understanding of it too, but on all of prints that I’ve installed generators from, the engineers show it tapped ahead of the main. Technically it will not destroy the generator anyway, because the generator controller will shut down on overload.

 

[PE (always learning)]

That’s was my understanding of it too, but on all of prints that I’ve installed generators from, the engineers show it tapped ahead of the main. Technically it will not destroy the generator anyway, because the generator controller will shut down on overload.

From what I was reading, if you are sized over 125% of the FLA in the overload region, the NEC considers the break to have instantaneous only protection. My breaker is adjusted well over the 125%.