is it necessary to do a short circuit for this?

Isaiah said:
I went out and checked nameplate myself this time - it’s actually 225kVA, not 45kVA. And the impedance is 5.3%
The kid that sent me the info is an apprentice with 6 months experience


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Now you also need any motor loads that will add to AFV
 
Fred B said:
Now you also need any motor loads that will add to AFV
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Generally only if they are 50HP and larger. But they are all usually ignored when an infinite bus value is used for the source.

Rule of thumb is a standard motor only contributes about 6x FLA as short circuit current.
 
Unless I made a mistake;

225 kva @ 5.3% shows 5,673
adding 100 ft of #4 brings it down to 3,689
 
Isaiah said:
So the cable size should be increased to meet the SCCR at the equipment terminals?


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You don't want to know how large the conductors likely need to be to get it up to 5kA at the equipment when the source is 5673.

What you don't want is for the available current to be over 5 kA at the equipment, the equipment (supposedly) has not been evaluated to be able to safely withstand over 5kA of current.
 
kwired said:
You don't want to know how large the conductors likely need to be to get it up to 5kA at the equipment when the source is 5673.

What you don't want is for the available current to be over 5 kA at the equipment, the equipment (supposedly) has not been evaluated to be able to safely withstand over 5kA of current.
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Thanks Kwired. The number 4 AWG has enough ampacity at 85A and the VD is low.
at the HVAC incoming lugs, the SCCR has been reduced, which is ok since it doesn’t exceed the 5kA rating of the equipment therefore the 4Awg is sufficiently sized, correct?


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Just to see results I did plug 100 feet of 4/0 copper into the calculator and came up with 4995 available at load end. I was thinking it may need to be even larger than that when I said you don't know how large of conductor you would need. Still a lot more copper than the #4 though.
 
kwired said:
Just to see results I did plug 100 feet of 4/0 copper into the calculator and came up with 4995 available at load end. I was thinking it may need to be even larger than that when I said you don't know how large of conductor you would need. Still a lot more copper than the #4 though.
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My only concern is management will go thru the roof if we hace to route a #4/0 cable to a 70A load. But understood, as well this is what the NEC states right? (110.9 and 110.10)


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Isaiah said:
My only concern is management will go thru the roof if we hace to route a #4/0 cable to a 70A load. But understood, as well this is what the NEC states right? (110.9 and 110.10)
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The SCCR of equipment is a maximum. You actually want the available fault current to be low, so keep your conductors as small as possible.
 
Isaiah said:
My only concern is management will go thru the roof if we hace to route a #4/0 cable to a 70A load. But understood, as well this is what the NEC states right? (110.9 and 110.10)


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Those sections are not requiring there to be 5kA available at your equipment. Yes you want a low impedance fault return path but your 70 amp breaker with 4 AWG should trip before extensive damage occurs. It may trip slower than if you had 5kA available but be still within acceptable limitations and that time difference is still probably in the tenths of a second range maybe even hundredths.
 
Isaiah said:
My only concern is management will go thru the roof if we hace to route a #4/0 cable to a 70A load. But understood, as well this is what the NEC states right? (110.9 and 110.10)


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You never have to upsize a cable on the load side of the OCPD for the available short circuit current. The only rule is that the cable be properly protected by the OCPD.
 
don_resqcapt19 said:
You never have to upsize a cable on the load side of the OCPD for the available short circuit current. The only rule is that the cable be properly protected by the OCPD.
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….love the Forum…lots of smart people here!
Thanks
Don


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don_resqcapt19 said:
You never have to upsize a cable on the load side of the OCPD for the available short circuit current. The only rule is that the cable be properly protected by the OCPD.
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Super long runs do increase trip time as they will essentially be current limiting to some extent, but those trip times are normally still within the accepted standards for conductor protection. With some long runs I see on farms I've seen ~1500 foot run of conductor with a fault at the load end that clears itself by burning through a component at the load end before it trips the overcurrent device ~1500 feet away, where if same fault condition were to occur very near the supply end it would have taken out the OCPD almost instantly. This is usually even more typical with 480/277 than with 120/240 volts.
 
kwired said:
Super long runs do increase trip time as they will essentially be current limiting to some extent, but those trip times are normally still within the accepted standards for conductor protection. With some long runs I see on farms I've seen ~1500 foot run of conductor with a fault at the load end that clears itself by burning through a component at the load end before it trips the overcurrent device ~1500 feet away, where if same fault condition were to occur very near the supply end it would have taken out the OCPD almost instantly. This is usually even more typical with 480/277 than with 120/240 volts.
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Yes, we had member that was stressing that point at one time. I did a few fault calculations for pivots just to see. Lucky the fuses ever blow when the fault is at the end tower.
 
ptonsparky said:
Yes, we had member that was stressing that point at one time. I did a few fault calculations for pivots just to see. Lucky the fuses ever blow when the fault is at the end tower.
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Does make troubleshooting easier, your problem is usually obvious once you remove the right cover.
 
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