Overcurrent protection, and breaker rating

[pzrapnbeast]

hmm, according to my calculations Isc is actually 65kA, and that's not even considering the motors contribution...

Higher rated breaker or current limiting fuses.

That's close to the infinite source at the transformer but after accounting for the cable you get about 35.6kA at the panel.

 

[pzrapnbeast]

Use the tables located on page 9-3 (pdf page 253) to determine series ratings for your equipment.

 

[pzrapnbeast]

what side of the breaker is the short circuit current available? if it is at the upstream wouldn't the Main CB be sufficient ,The main panel CB (SQUARE D POWERPACT PG 1200 CAT NO. PGA36120)is rated for 35KA, we do we have to change the remaining breakers in the panel?

Short circuit ratings exist for the equipment itself which includes everything inside of that equipment. This means all breakers must be either fully rated or series rate above the available short circuit current.

http://static.schneider-electric.us/assets/digest/schneider-electric-digest-176.pdf

page 9-3

 

[jim dungar]

Use the tables located on page 9-3 (pdf page 253) to determine series ratings for your equipment.

Series ratings cannot be used in the OP application.
There are two many branch circuits with large motors on them. For example, effectively Branch #2 motor is connected between the Main breaker and Branch breaker #1, this is a violation of NEC 240.86(C).

 

[jado85]

240.86 Series Ratings.
(C) Motor Contribution. Series ratings shall not be used
where
(1) Motors are connected on the load side of the higherrated
overcurrent device and on the line side of the
lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent
of the interrupting rating of the lower-rated circuit
breaker.

Series ratings cannot be used in the OP application.
There are two many branch circuits with large motors on them. For example, effectively Branch #2 motor is connected between the Main breaker and Branch breaker #1, this is a violation of NEC 240.86(C).

Is this why why we need to change the lower rated breaker, to account for the motor contribution?
Rough estimate on the 150 HP Load is 190A, how can I get the motor contribution from that? how does having 3 motor of the same size affect the sizing of thatbreaker?

 

[jim dungar]

Is this why why we need to change the lower rated breaker, to account for the motor contribution?
Rough estimate on the 150 HP Load is 190A, how can I get the motor contribution from that? how does having 3 motor of the same size affect the sizing of thatbreaker?

The NEC only allows the total 'on-line' motor load to be 1% of the lowest rated AIC.

This is partly why series ratings are not often used on panels >400A (where motor loading is likely to be higher).
There is no specific guidance given, but it would be convenient, if not prudent, to use the NEC table FLA rather than nameplate amps.

 

[jado85]

The NEC only allows the total 'on-line' motor load to be 1% of the lowest rated AIC.

This is partly why series ratings are not often used on panels >400A (where motor loading is likely to be higher).
There is no specific guidance given, but it would be convenient, if not prudent, to use the NEC table FLA rather than nameplate amps.

based on that table we are still at 1%.
at this moment 1'm are pricing 2 options,
1. add a NEMA3 enclosure with CB, 1200AS/1200AT 480V 80% Rated 65 kA 3 Pole UL, Group Mounted Electronic Trip Circuit Breaker: Type PJ
2.add a NEMA3 enclosure with fuses ,1200A/ 3 Pole 1200A Trip, Fused Power Circuit Device, Electric Trip with Ground Fault Protection(Inverted Feed), Type BP

I was surprised, when the Fused option came back with double the price of CB option. Any thoughts on why the dues option requires a larger enclosure? are there any requirements for spacing..etc?

 

[templdl]

Series reactors typically only affect the short circuit values.

Isn't the availablr short circuit current availability the issue here?
Or am I missing your point as I provided many LV air core reactors for MCCs and a number of large ones for MV application so large that a gle enclosed reactor was so large that special clearance were required to ship by 3 trucks all to address the reduction a SSC availability.

 

[kingpb]

Isn't the availablr short circuit current availability the issue here?
Or am I missing your point as I provided many LV air core reactors for MCCs and a number of large ones for MV application so large that a gle enclosed reactor was so large that special clearance were required to ship by 3 trucks all to address the reduction a SSC availability.

I agree, air core reactors are used as you describe. Only issue with them (besides size) is to be careful regarding voltage drop during starting of large equipment.

 

[jado85]

I was told that we need to change the breaker ratings in "2BPP6" to match the Isc(31000A), and I'm having trouble understanding why? what would happen if they stay at the current 18KA rating.

Anything I could do to reduce Isc beside using fuses?

first,Replacing all the Underated CB's with properly rated CB's fixes that issue. Many thanks to all who helped with this issue.

2nd, protecting the XF 2ndary conductor.

1.What about the transformer 2ndary, say I have a similar XF(see first post) that runs 200 ft and terminates at an appropriately sized OCPD, is that against any rules?
if not what is protecting that conductor from XF to the OCPD?

Understand that I will also need GFPD too!?

2. say that I want to apply rule 240.92.C.1, adding a CB within 100ft of the XF, do I add the Ampacity of all the conductors leaving the transformer or just the Ampacity of the feed I'm concerned with?

ideally I would like to know what would be the correct installation of the XF with its proper OCPD and GFP/SCP(by applying 240.92.C.1, or GF relay)

 

[don_resqcapt19]

...

2nd, protecting the XF 2ndary conductor.

1.What about the transformer 2ndary, say I have a similar XF(see first post) that runs 200 ft and terminates at an appropriately sized OCPD, is that against any rules?
...

If the transformer is outside and the secondary cables terminate at an OCPD outside the building or inside nearest the point of entrance, then 240.21(C)(4) would permit the 200 feet of cable between the transformer and the OCPD.
If the transformer is inside, then you are limited to 25' of wire between the secondary terminals and the terminals on the OCPD.

 

[jado85]

If the transformer is outside and the secondary cables terminate at an OCPD outside the building or inside nearest the point of entrance, then 240.21(C)(4) would permit the 200 feet of cable between the transformer and the OCPD.
If the transformer is inside, then you are limited to 25' of wire between the secondary terminals and the terminals on the OCPD.

that means I need to use 240.92 (C).(1).(2) or (3) to provide the GFP
Is there a maximum distance if it is outdoor?

 

[don_resqcapt19]

that means I need to use 240.92 (C).(1).(2) or (3) to provide the GFP
Is there a maximum distance if it is outdoor?

If this is a supervised industrial location, then the rules I cited do not apply. You can apply the rules in 240.92, and those rules permit longer lengths. The rules in 240.92 do not require GFP of the type required by 215.10. If 215.10 applies you would have to proved GFP.

 

[jado85]

If this is a supervised industrial location, then the rules I cited do not apply. You can apply the rules in 240.92, and those rules permit longer lengths. The rules in 240.92 do not require GFP of the type required by 215.10. If 215.10 applies you would have to proved GFP.

I'v heard so many opinions including my own that I'm officially confused

I wanted to apply rule 240.92, but I was told that I can't add the ampacity of the separate feeds to do the calculations. Now thats a no-go option.

my only other option to provide GFP is to set the GFP to a high value and a slow response( accepting the risk of damage to the equipment, but not worrying about nuisance tripping).

 

[don_resqcapt19]

...
I wanted to apply rule 240.92, but I was told that I can't add the ampacity of the separate feeds to do the calculations. Now thats a no-go option.

I don't know what you are asking when you say you can't add the ampacities.

my only other option to provide GFP is to set the GFP to a high value and a slow response( accepting the risk of damage to the equipment, but not worrying about nuisance tripping).

You are permitted to set the GFP to 1200 amps with a 1 second delay.
Are there any line to neutral loads? If not you could look at a resistance grounded system in place of the solidly grounded system.

 

[jado85]

I don't know what you are asking when you say you can't add the ampacities.

You are permitted to set the GFP to 1200 amps with a 1 second delay.
Are there any line to neutral loads? If not you could look at a resistance grounded system in place of the solidly grounded system.

The calculation uses the Ampacity of of the conductor for that feed, I was mistaken and added the Ampacity for the multiple feeds on the secondary .

I dont have any L-N, how would the resistance grounding system benefit?

 

[jado85]

The calculation uses the Ampacity of of the conductor for that feed, I was mistaken and added the Ampacity for the multiple feeds on the secondary .

I dont have any L-N, how would the resistance grounding system benefit?

Correction, I have L-N loads

 

[jado85]

240.21.B.4

240.21.B.4

if my transformer(480 is located on a rooftop and we are in a high bay manufacturing facility, can I use 240.21(B)(4) for the transformer secondary since it is also defined as a Feed? or I'm i strictly bound to use 240.21(C)(3), the secondary is longer that 25 ft, what rules do I fall under?

Actual scenario, Xf on roof tip, run the secondary unlimited length outside the building, tap into the building to and OCPD that is not over 30ft horizontally and not over 100 ft in total length(measured to the entry point). is this acceptable?

 

[don_resqcapt19]

Correction, I have L-N loads

You can't use a resistance or impedance grounded system if you have line to neutral loads.