Calc Help

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Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Have a design build scenario I’m trying to price.

Power source is 277480v switchgear. They state there is 1600A/1200A breaker that is to be used to bring power to our project. There are (4) 4” empty conduits for us to run our wiring through.

  1. What if we only needed say 200A to 400A service for our project power requirements? Could that still be done if they are saying to use the above stated breaker size? I would still have to pull 1200a?
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
You have a few choices. You could use less than 1200 amps worth of conductors but then you would need to apply a tap rule. If you only need a few hundred amps it might be better to set a fused disconnect switch next to the switchgear and use a tap rule so that you only need to run a 200-400 amp feeder that you've mentioned. Another choice is to run the full 1200 amps. Is changing the breaker an option?
 
A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
infinity said:
You have a few choices. You could use less than 1200 amps worth of conductors but then you would need to apply a tap rule. If you only need a few hundred amps it might be better to set a fused disconnect switch next to the switchgear and use a tap rule so that you only need to run a 200-400 amp feeder that you've mentioned. Another choice is to run the full 1200 amps. Is changing the breaker an option?
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Sorry calcs screw me up sometimes. So if I were to run the full 1200A to my service I would need a main switch or OCPD sized to 1200A?
 
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Why do you say 'to our project' in the first post and 'to my service' in your first reply? A service is the connection to the utility. Will you be installing that or does it exist?

And why do you ask how to size conductors to a breaker in the first post and how size a breaker to conductors in your first reply? Please clarify what is given for you to work with and what can be designed or negotiated. Also what was it about infinity's reply (which didn't contain calcs) that didn't answer the question?
 
Knightryder12

Knightryder12

Senior Member
Location
Clearwater, FL - USA
Occupation
Sr. Electrical Designer/Project Manager
Alwayslearningelec said:
Sorry calcs screw me up sometimes. So if I were to run the full 1200A to my service I would need a main switch or OCPD sized to 1200A?
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It is not required, but it would be a good design choice so that if something were to need servicing on your service you could turn it off at that point and bot have to go all the way back to the main switchboard.
 
A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
jaggedben said:
Why do you say 'to our project' in the first post and 'to my service' in your first reply? A service is the connection to the utility. Will you be installing that or does it exist?

And why do you ask how to size conductors to a breaker in the first post and how size a breaker to conductors in your first reply? Please clarify what is given for you to work with and what can be designed or negotiated. Also what was it about infinity's reply (which didn't contain calcs) that didn't answer the question?
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Your correct it is not a connection to the utility so thus not service conductors.

I only have the following info.

1. Existing 1600/1200A(277/480v) breaker in existing gear to feed distribution equipment for the project.
2. Distribution equipment size for the project is unknown. The only info I have right now is there are about 200 LED site lights that are about 30W each.
3. If those are the only loads the distribution equipment needs to feed then I guess( as Infinity says) I should tap down.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It seems to me that your best bet is to talk with the end user and see what he thinks makes sense. he probably has a better feel for what might be needed down the road than anyone else.

If I was only going to run 200 or 400 A worth of conductors I would tap off the 1200 A breaker and go to a 200 or 400 A enclosed breaker or fused disconnect than head off to where the equipment is located.

I will point out that if the tap conductors are outside as these seem to be, the length is not limited to the typical distances if you can follow the rules for such things.

(200) 30 W LED lights is only about 13 Amps of single phase 480 V. I would be looking at running probably single phase #4 AL if those were the only loads.
 
Last edited:
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Alwayslearningelec said:
This is the "service" that the 1600/1200A breaker is feeding. No other info given. The breaker is in a building about 900' away.
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It's fed by a feeder so it's not a service even though they're using that terminology. At 900' you probably don't want to run the full 1200 amps.
 
A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
infinity said:
It's fed by a feeder so it's not a service even though they're using that terminology. At 900' you probably don't want to run the full 1200 amps.
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Exactly. So I've confirmed that 200A(277/480v) pedestal distribtuion equipment is fine. So I need to run 200A from the 1600/1200A breaker that is 900' away.

1. I will install a 200A disconnect switch next to the gear with the 1600A/1200A breaker. I would need to run 1200A from breaker to disconnect and tap down???
2. I will then run (2) sets of 3/0 copper to the pedestal distribtion equipment which will be 200A. I will have to tap down there as well?
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Alwayslearningelec said:
1. I will install a 200A disconnect switch next to the gear with the 1600A/1200A breaker. I would need to run 1200A from breaker to disconnect and tap down???
Click to expand...
No you don't need 1200 amps for the tap condcutors. If you use the 10' tap rule you can run 200 amp conductors to the disconnect. The 25' tap rule would require 400 amp (1/3) conductors. You'll need to check the size of the terminals on the 1200 amp breaker that may tell you what the minimum size conductors can be used.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Alwayslearningelec said:
Exactly. So I've confirmed that 200A(277/480v) pedestal distribtuion equipment is fine. So I need to run 200A from the 1600/1200A breaker that is 900' away.

1. I will install a 200A disconnect switch next to the gear with the 1600A/1200A breaker. I would need to run 1200A from breaker to disconnect and tap down???
Click to expand...
These conductors can be rated for 200 A if you meet the tap requirements.

Alwayslearningelec said:
2. I will then run (2) sets of 500MCM Cooper to the pedestal distribtion equipment which will be 200A. I will have to tap down there as well?
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It won't be a tap since these conductors will be protected by the 200 A fused disconnect.

(2) sets of 500 MCM copper seems like a crazy number to me for 200 A even at 900 feet. have you run this past a voltage drop calculator? Have you considered AL?.
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The 500 kcmil does not seem necessary. You need to take the actual load into consideration. Even a full 200 amp load would only require a 350 for 3% VD>
 
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