AIC rating

[blueheels2]

I have 15900 A of available fault current at my service main. I’m installing a 600 amp disconnect with Class J fuses as the service disconnect. Leaving there it hits a transfer switch that has a withstand rating that exceeds the rating needed. From there I hit a main lug ILine panel that will have 200 amp breakers that will feed 200 amp mlo subs. It appears to me that in this configuration the series rating will be 10k at the branch circuit breakers. Looking at the Swuare D charts it appears the J fuse would series rate with a qo breaker for 10k regardless. Anyone find fault with my logic?

 

[mayanees]

cable lengths

cable lengths

I'll take your word that the J fuse is listed with the breakers at 10k, but you started with 15,900 amps. what's the length of your feeder? I'll do the calc if you give me the lengths.

 

[jim dungar]

I have 15900 A of available fault current at my service main. I’m installing a 600 amp disconnect with Class J fuses as the service disconnect. Leaving there it hits a transfer switch that has a withstand rating that exceeds the rating needed. From there I hit a main lug ILine panel that will have 200 amp breakers that will feed 200 amp mlo subs. It appears to me that in this configuration the series rating will be 10k at the branch circuit breakers. Looking at the Swuare D charts it appears the J fuse would series rate with a qo breaker for 10k regardless. Anyone find fault with my logic?

You can not use usually use series ratings for 3 devices in series.

The fuses may series rate with the I-Line and the I-Line may series rate with the QO, but unless the 3 devices have been tested, you cannot use fuse-breaker-breaker series ratings. Although a typical exception is multi-metering equipment and apartment loadcenters.

 

[blueheels2]

Used the Bussman app. Transformer is 4160 volt 100kva 240v secondary. Impedance is 1.5. 240 fault is 27800 120 fault is 41700. Transformer is around 100’ from the house. 100' of parallel 350MCM aluminum in PVC results in 15,908A L-L and 13,343 L-N


If series ratings will not work I will need 22k on the iline and the sub panels. I understand I can extendthe fault calculation but from disconnect to sub panel main lug you’re talking 25’ of bus and wire. Doesn’t look like their is much chance of it being below 10k.

 

[wbdvt]

It looks like you used infinite bus to determine the starting bolted fault current. If this is a new install, that is the most conservative method. But if it is an existing installation, you may want to get the available fault current from the utility at the transformer primary terminals, and run the calculations using that. You will probably find that the fault currents will be lower than the infinite bus calculations.

 

[electrofelon]

You can not use usually use series ratings for 3 devices in series.

The fuses may series rate with the I-Line and the I-Line may series rate with the QO, but unless the 3 devices have been tested, you cannot use fuse-breaker-breaker series ratings. Although a typical exception is multi-metering equipment and apartment loadcenters.

Jim,

Want to make sure I understand this. Plus it seems what the OP is doing a different than what you are saying. Say I have OCPD's A,B, and C. Say A & B series rate together, and B & C series rate together. So I cant use the series rating of B, and apply it as the starting point to the B,C combination correct? But if A series rates directly with C, is there any issue with having B in the middle? I believe the latter is what the OP was doing.

 

[jim dungar]

Jim,

Want to make sure I understand this. Plus it seems what the OP is doing a different than what you are saying. Say I have OCPD's A,B, and C. Say A & B series rate together, and B & C series rate together. So I cant use the series rating of B, and apply it as the starting point to the B,C combination correct? But if A series rates directly with C, is there any issue with having B in the middle? I believe the latter is what the OP was doing.

You can use the series ratings of A&B or B&C you cannot use the rating of A&C, unless the triple combination of A&B&C has been tested.

When looking at A&C it is possible that the dynamic impedance of B operating could negatively impact the opening times of A and C.

 

[blueheels2]

Not sure if it’s infinite bus but it is what the engineer gave me when Iaske him for the available fault current. Would love for it to be lower though.

It looks like you used infinite bus to determine the starting bolted fault current. If this is a new install, that is the most conservative method. But if it is an existing installation, you may want to get the available fault current from the utility at the transformer primary terminals, and run the calculations using that. You will probably find that the fault currents will be lower than the infinite bus calculations.

 

[kwired]

If you started with 15900 at service disconnect, by the time you get to the QO panel there seems to be a good chance you will have dropped to below 10kA, unless all is in a very close vicinity.

 

[blueheels2]

If you started with 15900 at service disconnect, by the time you get to the QO panel there seems to be a good chance you will have dropped to below 10kA, unless all is in a very close vicinity.

No such luck. 159300 is after 100’ of aluminum parallel 350’s

 

[kwired]

No such luck. 159300 is after 100’ of aluminum parallel 350’s

Presuming you meant 15900 and not 159300, or you had wrong figure in OP.

If you actually have 15900, it won't take very much circuit length to drop to 10k or less by the time you get to the QO panel that probably has 10k rated breakers, but if you have 159300, that will most definitely need some more circuit length to drop it to 10k.

 

[blueheels2]

Typo