MAXIMUM FAULT CURRENT CALCULATIONS

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PE (always learning)

PE (always learning)

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Saint Louis
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Professional Engineer
Hey Everyone,

Hope you are all doing well. I have been doing power systems studies on SKM power tools for a couple years now and when I do my studies I always try and do my reports starting with the utility provided transformer size / impedance and fault current information at the utility transformer primary bushings. The utility will provide this information after I request it. Usually I consider what the utility provides me as the source for my maximum fault current information, but sometimes I have seen people calculate the maximum fault current based on what could possibly be provided in the future. For instance, I am doing a study in California and the utility provided me with what is actually at the service entrance, a 750 KVA transformer and what is capable of being provided in the future, a 2500 KVA transformer. Obviously the 2500 KVA transformer would provide way more fault current and this would change the ratings for a lot of my equipment. For anyone who does power systems studies, when you calculate your maximum available fault currents do you use what is actually provided or do you look at would could be provided and use that as the basis for maximum available fault current?

Best Regards
 
I usually don't bother asking the utility for their available fault current, unless things prove to be too tight for my comfort level. I insert the "utility" symbol in SKM, include a short run of primary conductors (regardless of the actual distance), and assign the utility a fault value of 300,000 MVA (three phase) and 100,000 MVA (single phase). That is my way of modelling the utility as an "infinite bus." When SKM runs that through the transformer, it gives a maximum possible fault current on the secondary side. This is where the "too tight for comfort" might come into play. If for example the results tell me the secondary fault is 61KA, a value too close to a standard switchboard rating of 65KA, then I will try to obtain a smaller value by using actual utility primary fault information.
 
How likely is it that a major remodel is needed for them to replace the 750kVA with the 2500kVA? The answer for which to use is that it depends.
If you don't use the 2500kVA and it shows up pretty soon, and the equipment recently installed then becomes overdutied, the client will be sad.
Be sure you are using the 750 for arc flash calcs with real primary fault current available so you get more accurate ppe results.
 
I try to use the utility 'design' transformer size when doing my equipment evaluation. In your case the utility has the option to drop in a much larger transformer any time they want to, say the original was damaged and failed and the only one they had in the yard was larger.

The Arc Flash can be rerun and new labels put on with out significantly impacting the facility, but having equipment with inadequate SCCR and AIC ratings can be very costly and time consuming to remedy.
 
Ask your customer if they you would like you to do the calculations for a 750KVA transformer or a 2500KVA transformer.
Discuss with them how the short circuit ratings of their equipment could effected and how the arc flash ratings will change as well.
After the customer has been well informed, allow them to make the decision for you. You can't go wrong if your customer sends their decision to you in an email or some other written form.
 
engineerintraining12 said:
what is capable of being provided in the future,
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I would want some more information about this. Was this just thrown out as the maximum that could easily be installed, say, based on vault or pad size or other equipment limitations? What is the likelihood that that customer will need more juice later? I usually make a decision based on a number of factors: actual FC value based on actual transformer specs, utility provided FC value (often is 2-3 times actual), likelihood of client adding substantial equipment, and cost. Often I like to provide allowance for the next size up transformer.
 
charlie b said:
I usually don't bother asking the utility for their available fault current, unless things prove to be too tight for my comfort level. I insert the "utility" symbol in SKM, include a short run of primary conductors (regardless of the actual distance), and assign the utility a fault value of 300,000 MVA (three phase) and 100,000 MVA (single phase). That is my way of modelling the utility as an "infinite bus." When SKM runs that through the transformer, it gives a maximum possible fault current on the secondary side. This is where the "too tight for comfort" might come into play. If for example the results tell me the secondary fault is 61KA, a value too close to a standard switchboard rating of 65KA, then I will try to obtain a smaller value by using actual utility primary fault information.
Click to expand...
Maybe, just maybe, there are typos in your post! Can't imagine an available fault current from a 300,000 MVA at 13.2 kV! Is it 300MVA/100MVA?
 
electrofelon said:
I would want some more information about this. Was this just thrown out as the maximum that could easily be installed, say, based on vault or pad size or other equipment limitations? What is the likelihood that that customer will need more juice later? I usually make a decision based on a number of factors: actual FC value based on actual transformer specs, utility provided FC value (often is 2-3 times actual), likelihood of client adding substantial equipment, and cost. Often I like to provide allowance for the next size up transformer.
Click to expand...

Here's the scenario. I have a 3000 amp switchboard and the utility has sized their transformer based on the actual demand load, which is significantly less then what you would use for the full 3000 amps. They ended up going with a 750 kVA transformer, but provided me with a letter that gives me available fault current information for the 750 kVA transformer and also the available fault current information based on their largest transformer (2500 kVA) capable of serving a 3000 ampere main service switchboard. It's like they are saying here's what we have at that location, but be prepared for a possible upgrade I guess. I'm just going to size the short circuit current ratings for the panels as if it were being fed by a 2500 kVA transformer and when I go back and do the arc flash I'll obviously just use what's actually there.
 
engineerintraining12 said:
Here's the scenario. I have a 3000 amp switchboard and the utility has sized their transformer based on the actual demand load, which is significantly less then what you would use for the full 3000 amps. They ended up going with a 750 kVA transformer, but provided me with a letter that gives me available fault current information for the 750 kVA transformer and also the available fault current information based on their largest transformer (2500 kVA) capable of serving a 3000 ampere main service switchboard. It's like they are saying here's what we have at that location, but be prepared for a possible upgrade I guess. I'm just going to size the short circuit current ratings for the panels as if it were being fed by a 2500 kVA transformer and when I go back and do the arc flash I'll obviously just use what's actually there.
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What is the voltage, 208 or 480?
 
engineerintraining12 said:
Here's the scenario. I have a 3000 amp switchboard and the utility has sized their transformer based on the actual demand load, which is significantly less then what you would use for the full 3000 amps. They ended up going with a 750 kVA transformer, but provided me with a letter that gives me available fault current information for the 750 kVA transformer and also the available fault current information based on their largest transformer (2500 kVA) capable of serving a 3000 ampere main service switchboard. It's like they are saying here's what we have at that location, but be prepared for a possible upgrade I guess. I'm just going to size the short circuit current ratings for the panels as if it were being fed by a 2500 kVA transformer and when I go back and do the arc flash I'll obviously just use what's actually there.
Click to expand...
Ok this sheds more light. 480V, got it. Obviously the utility doesn't care about the size of your main switchboard, they just want to know the KVA you will be pulling on average or demand load.

If your switchgear and panels are EXISTING and you are just doing a study, then use the 750 KVA transformer model and use the short circuit ratings and arc flash analysis for that size transformer.

If you are engineering new switchgear and installing new panels, then you could consider using a larger size transformer size to calculate the short circuit ratings for the electrical gear that you are going to purchase. Again, consult your customer for the future plans. Perhaps somewhere in-between 750KVA and 2500 KVA would be more realistic if they have plans for expansion. 2500 KVA seems too big as that would be 3000 amps from the secondary which doesn't quite fit the 80% design fudge factor (2400 A) they build in.

That being said, for arc flash analysis you need to use the 750 KVA transformer rating per my opinion since that is the size the utility company is installing. You are supposed to install new arc flash labels every 5 years anyway. So the chances of them installing a larger transformer in the next 5 years is not likely. And if they do install a larger transformer in the next 5 years, you and your company will have to be hired for more arc flash analysis.
 
powerpete69 said:
Ok this sheds more light. 480V, got it. Obviously the utility doesn't care about the size of your main switchboard, they just want to know the KVA you will be pulling on average or demand load.

If your switchgear and panels are EXISTING and you are just doing a study, then use the 750 KVA transformer model and use the short circuit ratings and arc flash analysis for that size transformer.

If you are engineering new switchgear and installing new panels, then you could consider using a larger size transformer size to calculate the short circuit ratings for the electrical gear that you are going to purchase. Again, consult your customer for the future plans. Perhaps somewhere in-between 750KVA and 2500 KVA would be more realistic if they have plans for expansion. 2500 KVA seems too big as that would be 3000 amps from the secondary which doesn't quite fit the 80% design fudge factor (2400 A) they build in.

That being said, for arc flash analysis you need to use the 750 KVA transformer rating per my opinion since that is the size the utility company is installing. You are supposed to install new arc flash labels every 5 years anyway. So the chances of them installing a larger transformer in the next 5 years is not likely. And if they do install a larger transformer in the next 5 years, you and your company will have to be hired for more arc flash analysis.
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Agree 100% with you on this. This is a new design, so I will use the 2500 KVA transformer for short circuit analysis and the 750 KVA transformer for arc flash analysis.
 
electrofelon said:
I dont see that as being what powerpete said. If I were the one writing the checks, I would want to know the upcharge for calculating AFC with a transformer 3 times lager than currently needed.
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Yea, I see what you are saying. I missed that last part of his post. The cost associated with a higher short circuit rating for something that might not be very likely is something I need to have a discussion with the owner / contractor / utility. I agree that it probably needs to be discussed first and it would most likely make more sense to just go with the 750 KVA for both short circuit and arc flash analysis.
 
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