Fault Current Calculation

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Captainjon0107

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Littleton, Colorado, USA
I'm trying to complete some fault current calculations and I need a little assistance.

We have a 150KVA Transformer with a 5.5% Impedance. 13.8KV Primary to 120/208 Secondary.
We have parallel 400s running to a gutter, where they are tapped to feed 3 disconnects and a house panel. Wire size to the disconnects are 300MCM, 3/0, 3/0, and #6.

When calculating the fault current, do I treat each disconnecting means as a separate system, or should I treat the entire system as one? Is the fault current limited by the lowest calculated section of the system? This is the first time I've ever had to do one of these, as normally all of these things are included in the engineered plans for the installation. Any help or insight would be greatly appreciated!
 

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  • 60A Disconnect Fault Current Calculation.pdf
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  • 200A Disconnect Fault Current Calculation.pdf
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  • 400A Disconnect Fault Current Calculation.pdf
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I would calculate the SCA at the transformer, then the SCA at then end of your 400s, then the SDA at each disconnect based on that individual run using the number at the end of the 400s.
 
Yes you treat the system as one, which means any motor contribution (which you didn't mention but need to be accounted for) will apply at each fault current node. The fault current will generally decrease from source to load as you increase the overall impedance from the source (again, this depends on motor contribution). Your 200 A and 400 A analyses do not look correct, you should not increase fault current from node 2 to node 3 based on the inputs you've provided there.
 
EC Dan said:
Your 200 A and 400 A analyses do not look correct, you should not increase fault current from node 2 to node 3 based on the inputs you've provided there.
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The 60A is incorrect as well. It appears that the program figures each cable as coming from the transformer.
 
EC Dan said:
Yes you treat the system as one, which means any motor contribution (which you didn't mention but need to be accounted for) will apply at each fault current node. The fault current will generally decrease from source to load as you increase the overall impedance from the source (again, this depends on motor contribution). Your 200 A and 400 A analyses do not look correct, you should not increase fault current from node 2 to node 3 based on the inputs you've provided there.
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The only motors are HVAC and an exhaust fan. Do those contribute enough to the fault current to impact the calculation? HVAC motors are 6HP and 2 HP, and the exhaust fan is like a 120V, 1/8HP or less motor.
 
Captainjon0107 said:
The only motors are HVAC and an exhaust fan. Do those contribute enough to the fault current to impact the calculation? HVAC motors are 6HP and 2 HP, and the exhaust fan is like a 120V, 1/8HP or less motor.
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The 1/8 HP will be negligible, the HVAC motor will contribute between 20-40 A, depending on if they are single-phase or three-phase 208 (typically 4 x FLA of motor, but ultimately depends on X/R ratio). I'd say it's good practice to include it. Keep in mind this is a simplistic version of a fault current analysis, there are other factors that would be worth considering especially if you are near the SCCR of equipment. On the flip side, with the infinite bus method you are using you will likely be overestimating the fault current which would be problematic for arc flash analysis.
 
Captainjon0107 said:
I'm trying to complete some fault current calculations and I need a little assistance.

We have a 150KVA Transformer with a 5.5% Impedance. 13.8KV Primary to 120/208 Secondary.
We have parallel 400s running to a gutter, where they are tapped to feed 3 disconnects and a house panel. Wire size to the disconnects are 300MCM, 3/0, 3/0, and #6.

When calculating the fault current, do I treat each disconnecting means as a separate system, or should I treat the entire system as one? Is the fault current limited by the lowest calculated section of the system? This is the first time I've ever had to do one of these, as normally all of these things are included in the engineered plans for the installation. Any help or insight would be greatly appreciated!
Click to expand...
Unless the 400s have overcurrent protection you can't do that.
 
Captainjon0107 said:
Sure you can. The 400s are fed off the transformer, and then tapped to the individual disconnects. 240.21 (C), I believe.
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The secondary conductors do not have overcurrent protection and can't be "tapped".
240.21 Location in Circuit.
Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). Conductors supplied under 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4.
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The parallel 400s connected to the transformer are connected per 240.21(C) and you cannot connect another conductor to them without going through an OCPD first.
 
don_resqcapt19 said:
The secondary conductors do not have overcurrent protection and can't be "tapped".

The parallel 400s connected to the transformer are connected per 240.21(C) and you cannot connect another conductor to them without going through an OCPD first.
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240.21(C)
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6).

I know you're trying to win a pointless argument that's not even germane to the original question, but it doesn't really matter. It's an engineered specification that's already been inspected and approved by the local AHJ and the poco. Thanks for your help!
 
Captainjon0107 said:
240.21(C)
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6).

I know you're trying to win a pointless argument that's not even germane to the original question, but it doesn't really matter. It's an engineered specification that's already been inspected and approved by the local AHJ and the poco. Thanks for your help!
Click to expand...
I think Don's point is that the layout doesn't meet any of the "as specified in 240.21(C)(1) through (C)(6)" requirements.

But sounds like the conductors are service conductors, not feeder conductors, so 240.21(C) wouldn't apply.
 
If this is a utility transformer and these are service entrance conductors and service disconnects (which i was assuming they were when I read the OP) than 240.21 does not apply and it's ok, if the disconnects are grouped.
 
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