Using a 400A breaker to protect 500MCM cable

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mull982

Senior Member
Lets say I've done a load calculation and determine that the load is under 380A and thus I can use a 500MCM cable. Can I use this 500MCM cable on a 400A breaker since I can use the "next size up rule"?

The way I understand it though I would not be able to use the 500MCM cable on a 450A breaker since this 450A brekaer is not the next size up. Is this correct?

So if I had a 450A breaker I would need to use a 600MCM cable at minimum. Is this correct?

When a breaker is upsized for transformer inrush then must you still also increase the cable size so that the cable size is protected by the "next size up" rule as well?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Lets say I've done a load calculation and determine that the load is under 380A and thus I can use a 500MCM cable. Can I use this 500MCM cable on a 400A breaker since I can use the "next size up rule"?
Yes, but you can only do this up to 800A. If the breaker is over 800A, you have to have wire rated for at least the breaker rating or setting.

The way I understand it though I would not be able to use the 500MCM cable on a 450A breaker since this 450A brekaer is not the next size up. Is this correct?
Correct.

When a breaker is upsized for transformer inrush then must you still also increase the cable size so that the cable size is protected by the "next size up" rule as well?
A little harder to answer. If you still fall within the tapp rules, you don't have to upsize the wire. Otherwise, yes, you are correct.
 

Smart $

Esteemed Member
Location
Ohio
Sorta hate to muddy the water here, but as long as the adjusted ampacity of your 500's is greater than 400A, you can protect them with a 450A OCPD.
 

wiigelec

Member
Location
Red Desert
In order to answer your question fully you will need to provide much more detail as there are several considerations required when sizing transformer feeders and overcurrent protection. The NEC has several variations depending on your situation; voltage, installation, primary/secondary protection etc. Start by looking at 450.3.

Generally speaking your protective device settings must be higher than the inrush and operating load characteristics and lower than the equipment damage characteristics...
 

mull982

Senior Member
Sorta hate to muddy the water here, but as long as the adjusted ampacity of your 500's is greater than 400A, you can protect them with a 450A OCPD.

I dont follow you here. Even if you did use 90deg cable and adjusted the ampacity which I assume you are reffering to, the final adjusted ampacity cannot be larger thatn the value in the 75 deg column which is 380A
 

Smart $

Esteemed Member
Location
Ohio
Did you mean 500's or 600's?
I meant 500's.

I dont follow you here. Even if you did use 90deg cable and adjusted the ampacity which I assume you are reffering to, the final adjusted ampacity cannot be larger thatn the value in the 75 deg column which is 380A
Refer to 240.4. It says conductors are to be protected against overcurrent in accordance with their ampacities specified in 310.15. Read 310.15(A)(2)Exception. The ampacity of the 500's other than at the termination is what? If 401A, would that not constitute two adjacent ampacities? ...and the larger is?

As I said, muddying the water... :roll::cool:;)

...and btw, not a recommendation

...not trying to start a debate either

...just food for thought!
 
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Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
It can get tricky on 240.21(B) vs 240.21(C) transfromer rules....
 

mull982

Senior Member
What if your conductor rating falls on the rating of an OCPD. For example I the cable size if sized to 3/0 then you cannot use a 225 breaker and therefore must use a 200A breaker?

I just saw a project where #4 was used on several 100A breakers. Isn't this a violation since #4's are rated at 85A and the next size up would be a 90A breaker?
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
I just saw a project where #4 was used on several 100A breakers. Isn't this a violation since #4's are rated at 85A and the next size up would be a 90A breaker?
What were the 100 amp breakers feeding? Possibly motor loads?
 

mull982

Senior Member
What if your conductor rating falls on the rating of an OCPD. For example I the cable size if sized to 3/0 then you cannot use a 225 breaker and therefore must use a 200A breaker?

Can anyone chime in on this one? I cant seem to find the answer.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
3/0 copper is only good for 200 amps and 200 is a standard size breaker. In general, you cannot go up to 225 unless we are talking motors or a/c where the breaker can be larger than the conductor size when overload protection is available.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I'm sure it was a slip of the tongue, but you can't know that, without knowing what they're feeding...

True and you could be right, but I was guessing that a 85 amp conductor serving a motor would have a larger than 100 amp OCPD and that it was more likely that someone misapplied the ampacity from Table 310.16.
 
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