Sizing Feeder

horsegoer

Senior Member
Location
NJ
When sizing wire/feeder from breaker to to a panel the wire should be sized to the breaker feeding the panel and not the panel ratings, correct?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Short example.. it is permissible to feed a 400 amp panel with a 100 amp feeder conductor fed from a 100 amp breaker.
Does that clarify it for you ?
 

horsegoer

Senior Member
Location
NJ
Short example.. it is permissible to feed a 400 amp panel with a 100 amp feeder conductor fed from a 100 amp breaker.
Does that clarify it for you ?
Yes, which wouldn't be smart. So obviously feeder ampacity should be according to breaker size,
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The conductors and the down stream panel must both be protected at or below their ratings. What the feeder rating is becomes a design issue.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You've been hanging out with "sting" :)
If that's the specs on your "load" side panel, a 175 amp MCB would require a minimum of 2/0cu but the actual size conductor required could well be determined by the "supply" OCP, tap rules, etc,
You need to present the full picture ie: what protectes the feeder at it's origin, etc.
 

horsegoer

Senior Member
Location
NJ
You've been hanging out with "sting" :)
If that's the specs on your "load" side panel, a 175 amp MCB would require a minimum of 2/0cu but the actual size conductor required could well be determined by the "supply" OCP, tap rules, etc,
You need to present the full picture ie: what protectes the feeder at it's origin, etc.
Understood

So here's two scenario:

1. 400a bus duct switch with 350A fuses feeding a panel with bus rating of 225A and MCB rated at 150A

So the protection(bus duct switch/fuses) is above the panel rating which is incorrect?? No feeder size info was given but as mentioned in post above the panel and conductors must be protected at or below their ratings which they are not.
 
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david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
1. 400a bus duct switch with 350A fuses feeding a panel with bus rating of 225A and MCB rated at 150A

So the protection(bus duct switch/fuses) is above the panel rating which is incorrect??
No, that's fine. The panel is protected by the 150A MCB.
No feeder size info was given but as mentioned in post above the panel and conductors must be protected at or below their ratings which they are not.
The feeder must be protected by the 350A fuses, so 400mcm minimum...unless the feeder length is short enough to comply with one of the tap rules in 240.21(B), then #1/0 minimum.
 

horsegoer

Senior Member
Location
NJ
No, that's fine. The panel is protected by the 150A MCB.

The feeder must be protected by the 350A fuses, so 400mcm minimum...unless the feeder length is short enough to comply with one of the tap rules in 240.21(B), then #1/0 minimum.
Thanks. So you says MUST. You mean IS, correct?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Understood

So here's two scenario:

1. 400a bus duct switch with 350A fuses feeding a panel with bus rating of 225A and MCB rated at 150A

So the protection(bus duct switch/fuses) is above the panel rating which is incorrect?? No feeder size info was given but as mentioned in post above the panel and conductors must be protected at or below their ratings which they are not.
This is a silly design. Just install 225 amp fuses and conductors and use a MLO panel.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
We don't know what "is" is (makes me feel so presidential)
The 150 amp MCB would required that the feeder be at least a 1/0 Cu
(Note you changed the size in your post from 175 to 150.. if it's 175 amp then 2/0)
BUT
Since the buss plug has 350 amp fuses the conductor must be sized accordingly (400 kcmil) UNLESS a tap rule can be used.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I don't see MLO panels often, mostly MCB. Must be a design reason MCB are used most often.
It often depends on where the OCPD for the feeder is located. If it's in the same room as the panel the MCB is a waste of money.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If the feeder originates on another floor or a long distance from the panel we typically see that panels with main circuit breakers so that it can be shut down locally. If the feeder originates right next to the panel the main CB isn't really needed.
 
I don't see MLO panels often, mostly MCB. Must be a design reason MCB are used most often.
Sometimes a MB may be required for series ratings. The breaker doesn't have to specifically be located in the panel , but due to a variety of factors, such as different manufacturers, use of feeder fuses that don't series rate, or taking advantage of the conductor impedance, a MB panel board may be the best option.
 

horsegoer

Senior Member
Location
NJ
No, that's fine. The panel is protected by the 150A MCB.

The feeder must be protected by the 350A fuses, so 400mcm minimum...unless the feeder length is short enough to comply with one of the tap rules in 240.21(B), then #1/0 minimum.
So wire ampacity has be equal to or greater than the OCPD protecting the wire?
 
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