Current Limiting Fuses

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Luis De Castro

Member
Location
Miami Fl.
Occupation
Electrical Designer
I have aa job with a 2p-600A fusible switch with Class T current limiting fuses. My problem is that the main Fused disconnect feeds a meter bank with 10 meters each serving residential panels with al 100a breakers. MY available current at the disconnect is 16,606 a , already calculated. What will be my next step. coordination with the fuse and the breakers?
 
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When are you concerned about available fault current at the downstream device or selective coordination? Just a guess but I wouldn't think coordination would be an issue with that pairing. What is the AIC of the 100 amp breakers? I think you are unlikely to have a series rating for that pairing using standard 10kaic breakers, probably need a 22k
 
I think your next step would be to check the breaker manufacturer's data sheet and see what available current that breaker with Class T fuses is rated.
It may be series rated well above your available.
 
augie47 said:
I think your next step would be to check the breaker manufacturer's data sheet and see what available current that breaker with Class T fuses is rated.
It may be series rated well above your available.
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Yes, thanks. I need to contact the electrical contractor that did the job , I already ask for the aic of those 100a brekers
 
Luis De Castro said:
Yes, thanks. I need to contact the electrical contractor that did the job , I already ask for the aic of those 100a brekers
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You need to know the series rating of the breakers and fuses. This means knowing the part numbers of the breakers and the voltage rating of the fuse (sometimes people use 600V fuses instead of 300V ones.

Do you have multiple breakers in series like 600A fuse, 100A breaker at the meter, 100 main in the apartment, and then branch breakers?
 
jim dungar said:
You need to know the series rating of the breakers and fuses. This means knowing the part numbers of the breakers and the voltage rating of the fuse (sometimes people use 600V fuses instead of 300V ones.

Do you have multiple breakers in series like 600A fuse, 100A breaker at the meter, 100 main in the apartment, and then branchThe breKERS AT THE METER BANK ARE ALL
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What manufacture and type of breakers are the 100 amp feeder breakers? Usually the series ratings are on the manufactures web site.
 
All the brakers at the meter center are 22,000aic
jim dungar said:
You need to know the series rating of the breakers and fuses. This means knowing the part numbers of the breakers and the voltage rating of the fuse (sometimes people use 600V fuses instead of 300V ones.

Do you have multiple breakers in series like 600A fuse, 100A breaker at the meter, 100 main in the apartment, and then branch breakers?
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the fuses are 300vac 200ka.You mean I need to get the circuit breaker b catalog number and check the series rating, I dont see any problem but the bldg dpt wants the short circuit calculation and show those figures in the riser. A pain in the neck. I already have 16,606 at the disconnect, now I need to show the rest. Do i also need the let thru current of the fuse? Im retired doing some part time job, in my time we use to get the engineer for Bussman or Square D and they took care. They all retired too
 
electrofelon said:
When are you concerned about available fault current at the downstream device or selective coordination? Just a guess but I wouldn't think coordination would be an issue with that pairing. What is the AIC of the 100 amp breakers? I think you are unlikely to have a series rating for that pairing using standard 10kaic breakers, probably need a 22k
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All10 circuit breakers at the meter bank are are 22,000 aic
 
If your utility available is 16600 and your breakers are 22000 there should not be a question
 
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Luis De Castro said:
Do i also need the let thru current of the fuse?
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No.
You have 16kA at the meter center. You now need to calculate the available short circuit at each panel. This is the only way to know if 10kA breakers are suitable or if a series rating is required.
 
jim dungar said:
No.
You have 16kA at the meter center. You now need to calculate the available short circuit at each panel. This is the only way to know if 10kA breakers are suitable or if a series rating is required.
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Well , the problem is that the main fused disconnect is a Class T current limiting fuse so I cannot trated like it is Main breaker it gets tricky
 
If your feeder breakers are 22 and your available is 16 they aren't in question. As Jim says if your branch breakers are 10k you might see if they series rate with your feeder breakers. If you actually do the conductor calculations I would bet you don't have 10 available at the branch. Use bussmann fc2 app to confirm
 
Luis De Castro said:
Well , the problem is that the main fused disconnect is a Class T current limiting fuse so I cannot trated like it is Main breaker it gets tricky
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Series ratings are similar for Fuse-Breaker as they are for Breaker-Breaker especially if they are 300V Class T fuses.
 
Luis De Castro said:
Well , the problem is that the main fused disconnect is a Class T current limiting fuse so I cannot trated like it is Main breaker it gets trickyyO
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That does not change anything. with 16,600 at the disconnect, and the meter breakers rated at 22 KIA, 20' of 3 conductor 4 AWG copper cable will take you below 10KIA at the panel. If the runs are shorter you may have look to a series rating between the 100 amp breakers and the breakers in the panels.
 
don_resqcapt19 said:
That does not change anything. with 16,600 at the disconnect, and the meter breakers rated at 22 KIA, 20' of 3 conductor 4 AWG copper cable will take you below 10KIA at the panel. If the runs are shorter you may have look to a series rating between the 100 amp breakers and the breakers in the panels.
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The main fuse disconnect is part of the meter bank, no cables just bars
 
augie47 said:
If you actually do the conductor calculations I would bet you don't have 10 available at the branch.
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(y)

I ran numbers through fault current calculator (assumed 208/120 volts) If there is 3 AWG copper feeder conductors the AFC at the end of 23 foot feeder run will be below 10kA. If a dwelling and you go with the allowed 4 AWG the feeder can be 19 feet long and still be under 10kA. 2 AWG aluminum could be as little as 15 feet before going over 10 kA.
 
Luis De Castro said:
The main fuse disconnect is part of the meter bank, no cables just bars
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It doesn't matter the use of the current limiting fuses cannot increase the fault current. The panel with the 10 KAIC breakers is at the load end of the conductors connected to the 100 amp breaker that is listed for 22 KAIC. The impedance of the conductors between the 100 amp breaker and the panel will limit the current at the panel. My calcs show 20' of 4AWG copper on the load side of the 100 amp breaker will take the available fault current at the panel below 10 kA.
 
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