Location used for calculating utility available fault current

[Natfuelbilll]

After some discussions with the account Representative he went back to his engineering and they recalculated at the point of termination. It turned out to be lower when they removed their assumptions and calculated based on what is installed.

It went from 90.000 to 74,000.

 

[lielec11]

Without you sharing the number you received I cannot comment accurately. But in my experience they almost always list a number upwards of 200kAIC because of their underground network. This is normal in NYC.

 

[JDBrown]

It never ceases to amaze me what some utilities are able to get away with. As an Engineer for a CA POCO which shall remain nameless, I provide available fault currents to customers regularly.

I don't know how things work in NY, but here the Service Planner typically provides the customer/contractor a "book value", which is the maximum short circuit current for a given service voltage and size anywhere in our service territory. If the customer/contractor wants something more specific to their particular location, they can request it (fees may or may not apply, depending on what the customer is doing).

When I get a specific/actual short circuit current request, i have to verify the lines between the substation and the customer's meter panel, as well as the customer's service panel information. The results of my calculations go in a written report (form letter) with two scenarios: (1) everything as it is today, and (2) near-infinite bus with the transformer and service conductors increased to what they would be if the customer loaded up their panel to full capacity. I think most Contractors use the first calculation for Arc Flash, and the second one to specify AIC ratings for equipment. I always thought this was standard across the industry, but apparently it's not.

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[dionysius]

I routinely see utilities give, what I consider to be, unreasonably high figures for fault current. Last time I did a new medium sized 480 service, the utility gave me a figure of 39K, and when I calculated it from the transformer assuming infinite buss, it was a third of that. Some people say, "you have to use what the utility gives you" but now I would pretty much always go with the actual transformer data if it is available or I can get it. I can see providing for a larger transformer due to customer expansion, if that is of significant likelihood, but as far as the theory that utility may replace it with a a larger unit "they have on hand" if it fails, or with one with an impedance drastically lower (which I have never seen in 20 years), those are too far in the "what if" field for me.

I agree with this. I got a 30k number for a 1000 kVA 480 XFR. When I questioned it I then got a 21k number which was much more reasonable. One should also ask for how it was calculated which I have not done but always will from now on.

 

[Natfuelbilll]

It never ceases to amaze me what some utilities are able to get away with. As an Engineer for a CA POCO which shall remain nameless, I provide available fault currents to customers regularly.

I don't know how things work in NY, but here the Service Planner typically provides the customer/contractor a "book value", which is the maximum short circuit current for a given service voltage and size anywhere in our service territory. If the customer/contractor wants something more specific to their particular location, they can request it (fees may or may not apply, depending on what the customer is doing).

When I get a specific/actual short circuit current request, i have to verify the lines between the substation and the customer's meter panel, as well as the customer's service panel information. The results of my calculations go in a written report (form letter) with two scenarios: (1) everything as it is today, and (2) near-infinite bus with the transformer and service conductors increased to what they would be if the customer loaded up their panel to full capacity. I think most Contractors use the first calculation for Arc Flash, and the second one to specify AIC ratings for equipment. I always thought this was standard across the industry, but apparently it's not.

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Live and learn. I've learned a project needs both maximum fault current and fault current of existing conditions. The first for equipment ratings and the second for arc flash calculations.

Where is this spelled out any clearer?

 

[electrofelon]

Live and learn. I've learned a project needs both maximum fault current and fault current of existing conditions. The first for equipment ratings and the second for arc flash calculations.

Where is this spelled out any clearer?

I dont know what "maximum" and " existing" fault conditions are. As far as the nec is concerned, there is no requirement to provide for future conditions. If a client/job specs wants to provide for some future potential condition, they can certainly require such.

 

[Ingenieur]

Can the available utility supplied fault current at a xfmrs sec terminals ever be greater than that determined using an infinite bus and the xfmr kva and pu Z?
I know load side motors, etc can contribute

 

[Natfuelbilll]

NEC 110.24 For maximum fault current (equipment sizing)

NFPA 70E 130.5 for available (arc flash)

Right?

 

[rian0201]

NEC 110.24 For maximum fault current (equipment sizing)

NFPA 70E 130.5 for available (arc flash)

Right?

i agree with this. there are even some that requires minimum fault values to check the functionality of the protective devices.

maybe as far as nec is concerned, it is different. but at higher voltages, this is a different story.. and this values are a must.


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[mivey]

Can the available utility supplied fault current at a xfmrs sec terminals ever be greater than that determined using an infinite bus and the xfmr kva and pu Z?
I know load side motors, etc can contribute

Most certainly, like if the transformer is changed. Could be larger kVA or smaller pu Z or both. It is reasonable to give the larger fault value if it is reasonable to expect a change during the life of the site. See my posts #10 & #13.