Back fed main

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mka

Member
Hello all,

Can someone explain the concept of a back fed main? Aside from the basic concept, I will provide my situation.

I have a main/service panel that is a 100A and a sub panel that is a 125A main lug panel that is fed from the main with a 60A breaker. The sub panel now has 8 breakers and code says it has to have a main breaker because it is over the 6 limit. Also can someone give the article number for that? The sub can be converted to a main breaker but only with a 125A breaker.

Also, when using a back fed main, it requires a hold down kit...why is that? I noticed the service/main panel breaker has a hold down kit as well.

Thanks,

Mka
 

480sparky

Senior Member
Location
Iowegia
Most breakers will function as an overcurrent device no matter which direction they are fed from.

As for a hold-down, this prevents the breaker from being removed while the power is still on. Prevents arcing.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I have a main/service panel that is a 100A and a sub panel that is a 125A main lug panel that is fed from the main with a 60A breaker. The sub panel now has 8 breakers and code says it has to have a main breaker because it is over the 6 limit. Also can someone give the article number for that? The sub can be converted to a main breaker but only with a 125A breaker.
The code has only required a protective device, it has not required that device to be in the same panel. Unless the sub panel is in a different building, the upstream 60A breaker is the panel's 'main device'.
 

hurk27

Senior Member
Hello all,

Can someone explain the concept of a back fed main? Aside from the basic concept, I will provide my situation.

I have a main/service panel that is a 100A and a sub panel that is a 125A main lug panel that is fed from the main with a 60A breaker. The sub panel now has 8 breakers and code says it has to have a main breaker because it is over the 6 limit. Also can someone give the article number for that? The sub can be converted to a main breaker but only with a 125A breaker.

Also, when using a back fed main, it requires a hold down kit...why is that? I noticed the service/main panel breaker has a hold down kit as well.

Thanks,

Mka


Ok let me thread though this:

You have a service panel with a 100 main breaker in it, and it has a hold down.
you have a 60 amp breaker for the fedders to a 125 amp sub panel, it doesent have a hold down, and code does not require it.

and if this sub panel is in the same building, it is not required to have 6 breakers or less.

Am I close to your set up?
 

mka

Member
Yes Wayne that is my set up the main/service has a 100A with hold down, from the 100A main/service I have a 60A breaker that feeds my basement in the same building with a sub panel that has 8 breakers.

Mka
 

iMuse97

Senior Member
Location
Chicagoland
Yes Wayne that is my set up the main/service has a 100A with hold down, from the 100A main/service I have a 60A breaker that feeds my basement in the same building with a sub panel that has 8 breakers.

Mka
The requirement it to have six or less movements of the hand to cut power to the building. Your main breaker in your main panel does this. Threrefore, it does not matter how many breakers you put in the subpanel.
 

mka

Member
The requirement it to have six or less movements of the hand to cut power to the building. Your main breaker in your main panel does this. Threrefore, it does not matter how many breakers you put in the subpanel.
Ahh...I see - but if it were a separate building such as a garage, it would need it's own disco if more than 6 breakers - correct?

Mka
 

tom baker

First Chief Moderator
Staff member
If the back fed breaker came off the buss, its stabs would be hot if the breaker was ona. Imagine a 100 amp breaker with 2 AWG conductors and the weight of the wire and strain could pop the breaker off the buss.
 

480sparky

Senior Member
Location
Iowegia
If the back fed breaker came off the buss, its stabs would be hot if the breaker was ona. Imagine a 100 amp breaker with 2 AWG conductors and the weight of the wire and strain could pop the breaker off the buss.

How is that any different than feeding the breaker with the bus bars and the weight of the #2s are the same?
 

hurk27

Senior Member
Being an AC system, what difference does it make whether the source is the bus bar, or the stabs of the breaker?

Think about some of the breakers that have open stabs sticking out, like Wadsworth, FPE, Etc....

A back fed breaker that came off the bussbar would have these stabs sticking out hot if the breaker was on, a normal busbar fed breaker would not have hot stabs sticking out.
 

480sparky

Senior Member
Location
Iowegia
Think about some of the breakers that have open stabs sticking out, like Wadsworth, FPE, Etc....

A back fed breaker that came off the bussbar would have these stabs sticking out hot if the breaker was on, a normal busbar fed breaker would not have hot stabs sticking out.


I don't get it. Pulling a breaker out (that's energeized and has a load on it) will create the exact same arc, whether it's fed from the bus bars or back-fed through the terminals. It doesn't make a difference.... it'll arc just the same.

You are basically seperating two pieces of metal that are in contact with each other. Whether they 'stick out' or not... the end result is the same.
 

mivey

Senior Member
I don't get it. Pulling a breaker out (that's energeized and has a load on it) will create the exact same arc, whether it's fed from the bus bars or back-fed through the terminals. It doesn't make a difference.... it'll arc just the same.

You are basically seperating two pieces of metal that are in contact with each other. Whether they 'stick out' or not... the end result is the same.
With bus-fed you get one arc. With breaker fed you can have a hot wire and /or breaker flopping around and get many more subsequent arcs.
 

hurk27

Senior Member
I don't get it. Pulling a breaker out (that's energeized and has a load on it) will create the exact same arc, whether it's fed from the bus bars or back-fed through the terminals. It doesn't make a difference.... it'll arc just the same.

You are basically seperating two pieces of metal that are in contact with each other. Whether they 'stick out' or not... the end result is the same.

It has nothing to do with the arc the load makes at the buss.

It has to do with having the energized stabs flopping out at you when your taking the panel cover off.

If a back stab breaker have some twist tension on the wires connected to the breaker, the breaker can come off the buss and flip out and hit you before you know it, or hit the cover causing an arc flash, that could cause you sever burns. a normal fed breaker would not be energized, once it has come off the buss.;)
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
You are basically seperating two pieces of metal that are in contact with each other. Whether they 'stick out' or not... the end result is the same.
Look at the bus clips on a QO or CH breaker, and imagine power on the wires on the terminals and the breaker in the 'on' position.

In a lug-fed or MB-fed panel, you know where the hot parts are, and they're normally stationary; an unsecured breaker may not be.
 

mka

Member
The code has only required a protective device, it has not required that device to be in the same panel. Unless the sub panel is in a different building, the upstream 60A breaker is the panel's 'main device'.

Thanks Jim Dungar
 

mka

Member
It has nothing to do with the arc the load makes at the buss.

It has to do with having the energized stabs flopping out at you when your taking the panel cover off.

If a back stab breaker have some twist tension on the wires connected to the breaker, the breaker can come off the buss and flip out and hit you before you know it, or hit the cover causing an arc flash, that could cause you sever burns. a normal fed breaker would not be energized, once it has come off the buss.;)

Thanks all for your feedback. Ok I got it. Man, that makes me wonder if that cheesy piece of plastic is worthy of the serious job it has. That's kind of scary. I need to go back in the panel and make sure everything is properly torqued and tension off the feeders. It's a Murray panel with a cheesy piece of hard plastic that makes you wonder if it's in correctly when you pop the breaker in.

Cheers,

Mka
 

genesparky

Member
Location
Kingman Az.
I love this forum, I never knew the reason for the hold down clip. Makes total sense now. However one more question does the sub need a main breaker (in the panel) or not?
 

hurk27

Senior Member
I love this forum, I never knew the reason for the hold down clip. Makes total sense now. However one more question does the sub need a main breaker (in the panel) or not?

If it is in the same building, and has OCPD ahead of the feeders then it doesn't need a main, but can have one, as we can get a MB load center cheaper than a ML center, but if is in a different building, and you have more then 6 breaker handles, or more then "six movements of the hand" then a main breaker or a disconnect would be required.
See 225.33
 
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