# Thread: Branch Circuit Feeders

1. Junior Member
Join Date
Aug 2013
Location
new york
Posts
11

## Branch Circuit Feeders

Hey Forum

I am installing a 277/480 service and have a question on wire size.

Long Island

Coming off main service in shopping center and feeding a Trans S Meter Cabinet
Than installing 200 amp / 600 volt Safety switch. 6x6 x4 rain tight trough
Going to fuse switch down to 150 amp ??
I have too run 400 feet to supply and 480 step down transformer. Stepping down to 120/208.
This 120/208 panel are main breaker at 150 amps
I was looking up wire size and at 3% I could run 1/0

The question
Do I have to run full size 1/0. Which is good up to 427 feet at 480 volt delivering 150 amps at the end, or Can I run wire size that is 80 percent of the 150 main breaker in the 120/208 panel

2. Originally Posted by zodiack
....Can I run wire size that is 80 percent of the 150 main breaker in the 120/208 panel
I am confused at what you are asking.

80% of 150A = 120A and a 120A conductor would need to be protected with a 125A breaker.

3. Junior Member
Join Date
Aug 2013
Location
new york
Posts
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## Wire Size

I know I can run 1/0 wire

Can I size the wire 80 percent of the 150 amp breaker

In turn using a 125 amp fuses in disconnect switch down sizing the wire. to #1 wire

4. Originally Posted by zodiack
I know I can run 1/0 wire

Can I size the wire 80 percent of the 150 amp breaker

In turn using a 125 amp fuses in disconnect switch down sizing the wire. to #1 wire
Isn't that disco on the primary side of the transformer that is in turn feeding the 150A panel?

5. This whole idea of sizing a conductor to 80% of a breaker makes no sense to me.

This what a typical tranny set up looks like:

480V to 208/120V

Attachment 18169

6. Member
Join Date
Jan 2014
Location
ny,ny
Posts
49
Agreed jumper...where is this sizing the wire to 80% of the OCPD coming from? Is it a misunderstanding of not designing to use more than 80% of a circuits rating? Im confused but no you can't undersized the wire to 80% of the OCPD. In the transformer diagram, I'd prob parallel the secondary instead of 700kcmil but correct as it stands.

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7. Moderator
Join Date
Dec 2012
Location
Placerville, CA, USA
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You can sort of size the wire to 80% some of the time as follows:
If the load can only be 80% of the breaker rating, then you can size the wires to the load and then possibly use the larger breaker as the next size up from the load current.
Or, depending on the load, you might have to size the wire all the way up to the breaker size to satisfy the 80% rule for the breaker.

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8. Originally Posted by jumper
This whole idea of sizing a conductor to 80% of a breaker makes no sense to me.

This what a typical tranny set up looks like:

480V to 208/120V

Attachment 18169
80 % is for future expansion. i believe the code states that some where i will look at it when i get a chance.

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9. Originally Posted by Daniel malack
80 % is for future expansion. i believe the code states that some where i will look at it when i get a chance.

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You can look, but you ain't gonna find it.

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