Fuse vs Circuit Breakers - design Q

[ChrisCentral]

I have a 243A peak load which is normally protected by a 300A fuse; by code I have to consider the load continuous for this design by the OESC.

Can I want to replace the fuse with a circuit breaker?
Since the breaker is 80% rated I have to apply 1.25 to the ampacity of my load and I get 303.75A which exceeds the 300A breaker rating meaning the breaker will likely run hot and eventually fail so I should I go up to a 325A breaker?

The above are questions as I clearly am very confused.

Shouldn't the fuse be increase to 325A as well?

Or since the load is defined at 243A is there an oppourtunity or allowance to bring the fuse size down and decrease my conductor?

Are fuses 100% duty rated?

 

[jim dungar]

Are fuses 100% duty rated?

Yes. But fuse holders in enclosures are not.

 

[ChrisCentral]

Does that mean that the 300A fuse design is no good also? According to my supervisor the 300A fuse design has been approved for installations in Ontario?

 

[G._S._Ohm]

meaning the breaker will likely run hot and eventually fail so I should I go up to a 325A breaker?

(304A/300A)^2 = 1.03 = 3% hotter.

IMO I can't imagine this being a problem. For one thing it will be swamped out by ambient temp. changes. Also the variation in trip curves or clearing time curves will make this difference academic.

 

[templdl]

I have a 243A peak load which is normally protected by a 300A fuse; by code I have to consider the load continuous for this design by the OESC.

Can I want to replace the fuse with a circuit breaker?
Since the breaker is 80% rated I have to apply 1.25 to the ampacity of my load and I get 303.75A which exceeds the 300A breaker rating meaning the breaker will likely run hot and eventually fail so I should I go up to a 325A breaker?

The above are questions as I clearly am very confused.

Shouldn't the fuse be increase to 325A as well?

Or since the load is defined at 243A is there an oppourtunity or allowance to bring the fuse size down and decrease my conductor?

Are fuses 100% duty rated?

What is the fuse protecting? I reread the OP and you have not included the size/rating of the wire that is feeding the load.
If the cable has been sized correctly to carry the load, the OCPD fuse or breaker is sized to protect the cable. (Where does the 80% rating or 1.25% of the breaker ampacity come in?) If I'm to understand you correctly your load is 243a continuous. That x 1.25% is 303.75a. Your cable must be sized to carry the 303.75a. That means your cable must be 350MCM @ 75degC. The rating of the cable is actualy 310A which is used to size the OCPD and not the laod. You are allowed to use the next highed ampacity OCPD which would be a 350a breaker since 310a is not a standard breaker rating. As such use a breaker with a 350AT.

 

[ChrisCentral]

Now I am even more confused?? I feel really dumb asking this but here goes...

243A x 1.25 = 304A can this not be the OCPD and conductor rating?

300A required OCPD (300A fuse or a 325A cct brkr)
300A allowable current carrying ampacity (350MCM Cu @ 75deg C = 310A)

 

[petersonra]

Now I am even more confused?? I feel really dumb asking this but here goes...

243A x 1.25 = 304A can this not be the OCPD and conductor rating?

300A required OCPD (300A fuse or a 325A cct brkr)
300A allowable current carrying ampacity (350MCM Cu @ 75deg C = 310A)

In a highly legalistic sense, if your required ampacity is 304A, you cannot use conductors rated at 300A, not that it will matter any in real life. that is just what the code says you have to do.

Since 310A is not standard size fuse or CB, you are allowed to go up to the next standard size.

 

[iceworm]

Now I am even more confused?? I feel really dumb asking this but here goes...

243A x 1.25 = 304A can this not be the OCPD and conductor rating?

300A required OCPD (300A fuse or a 325A cct brkr)
300A allowable current carrying ampacity (350MCM Cu @ 75deg C = 310A)

CC -
The guiding code sections are
210.19 branch circuit ampacity (min wire size)
210.20 Branch circuit OCP (min rating)
215.2 Feeder ampacity
215.3 Feeder OCP (Min rating)
240.4 OCP of conductors (if ampacity is between sizes, allows next size up if <= 800A)
240.6 standard OCP ratings

For your example:
215.3
ampacity = non-continuous + 125% x continuous = 304A
Ampacity of 350kcmil CU = 310A so that is okay.

OCP
Standard sizes are 300A and 350A. Load is 304A, per 240.4, 350A CB or fuses is okay.

ice

 

[kenaslan]

The OP is not in the US and the NEC is not the code used. He is in Canada and uses the CEC.

 

[iceworm]

The OP is not in the US and the NEC is not the code used. He is in Canada and uses the CEC.

Ken -
Thanks, I missed that.

CC -
Whoops. Sorry, I can't help.

ice