pzrapnbeast
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That's close to the infinite source at the transformer but after accounting for the cable you get about 35.6kA at the panel.
Overcurrent protection, and breaker rating
- Thread starter jado85
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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
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Use the tables located on page 9-3 (pdf page 253) to determine series ratings for your equipment.
pzrapnbeast
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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
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
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?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
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?
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
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
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.
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)
- Location
- Illinois
- Occupation
- retired electrician
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?
- Location
- Illinois
- Occupation
- retired electrician
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).
- Location
- Illinois
- Occupation
- retired electrician
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?
Correction, I have L-N loads
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?
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?
- Location
- Illinois
- Occupation
- retired electrician
You can't use a resistance or impedance grounded system if you have line to neutral loads.
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