sub panel

[ronlove]

if i have a 3 phase 400 amp service with only one 100 amp breaker free, and I need no feed a sub panel with a 60 amp main. does the wire feeding the 60 amp sub need to rated for 100 or 60?

 

[stickboy1375]

if i have a 3 phase 400 amp service with only one 100 amp breaker free, and I need no feed a sub panel with a 60 amp main. does the wire feeding the 60 amp sub need to rated for 100 or 60?

Which do you think? just asking.

 

[ronlove]

100 amp wire

 

[480sparky]

100 amp wire

And why do you think 100a?

 

[stickboy1375]

100 amp wire

Any particular reason?

 

[ronlove]

The breaker (100amp) is protecting the wire. eventhough the load will not excede 60 amp (secondary)

 

[stickboy1375]

The breaker (100amp) is protecting the wire. eventhough the load will not excede 60 amp (secondary)

Wouldn't the 60a main do the same thing? In other words, how could you possible draw more than 60 amps?

 

[ronlove]

you could not

 

[ronlove]

so you would size the wire for the load, not the breaker feeding it?

 

[stickboy1375]

so you would size the wire for the load, not the breaker feeding it?

I would size it to the smallest breaker protecting the wire, in your case the 60 amp main in the sub-panel.

 

[Dennis Alwon]

so you would size the wire for the load, not the breaker feeding it?

Correct as long as there is a 60 amp breaker protecting the wire in the sub panel.

 

[ronlove]

the 100 amp breaker is protecting the wire to the top of the 60 amp breaker. the 60 amp breaker is protecting the hole sub?

 

[ronlove]

so it would make a differnce if I did not have the 60 main in the sub. if I had no main at all in the sub?

 

[stickboy1375]

the 100 amp breaker is protecting the wire to the top of the 60 amp breaker. the 60 amp breaker is protecting the hole sub?

How can it? No more than 60 amps can be drawn, so the 60 amp breaker is protecting the wire.

 

[stickboy1375]

so it would make a differnce if I did not have the 60 main in the sub. if I had no main at all in the sub?

Correct, then the only OCPD would be the 100 amp breaker.

 

[gar]

081105-2144 EST

If you have a cable rated for 60 A and at the input end of this cable you use a 100 A breaker, then that cable is not protected from failure. Put a 100 A load on the cable and the breaker may never trip, but after a period of time the cable will probably overheat melting the insulation and fully short.

A 60 A breaker at the load end of the cable does not protect the cable. This 60 A breaker only protects the wiring or load following the 60 A breaker.

There is nothing to prevent an overload across the cable preceeding the 60 A breaker.

The entire theory of circuit protection is that the protective device is to protect that circuitry that follows the protective device. As you progress to the final load you will probably have protective devices that have lower ratings the closer to the load.

.

 

[kb8713]

How about 240.21. Overc urrent protaction shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductor recieves thier supply. Also see 210.3

 

[mistermudd]

IMO, since the upstream OCPD is protecting the wire at more than its ampacity, unless you are following a tap rule this is a violation.

 

[LarryFine]

IMO, since the upstream OCPD is protecting the wire at more than its ampacity, unless you are following a tap rule this is a violation.

And that opinion is correct.

 

[Doug S.]

I would size it to the smallest breaker protecting the wire, in your case the 60 amp main in the sub-panel.

The way I read it is the 60 breaker is protecting the sub-panel, and the 100a is protecting the wire.
So although the 60a breaker would limit the current on the feeders, 240.21 leads me to believe that "doesn't count" and the wire should be sized for the 100a breaker.

Doug S.


EDIT: OOPS, sorry somehow I totally missed page two.... Just ignore the blabbering buffoon.