ECN Forum Forums General Topics of the Trade General Discussion Area available fault current

Make every effort to get the numbers from the POCO. They do have them.

We got scr--ed bad once. We made calculations for a 600A 208Y/120 3PH system for an office building fed from a dedicated pad mount transformer. Not taking many chances we installed 42K equipment (225KVA, 625A, 1.5%Z). When the inspector called the POCO for their fault level they said 65K because, during an emergency, they might choose to install a larger transformer.

JBD, That sounds like a number that they pulled out of their rump based on the max withstand found on equipment suppliers catalogs. I really don't see how you could be held responsible for a change made by the utility at a later date. They seem to forget that the equipment goes up in price as the short circuit withstand increases.

thanks again guys.

JBD, I'm going to do that, (try to) get the numbers from the poco. Like I told them, I've gotten them from them before. I think he was just being lazy, or worse, purposefully trying to make things difficult. Oh well.

Just because one engineer did not know what the available fault current was does not mean that another doesn't know. In many cases, you could get an inexperienced engineer (or a lazy one) that doesn't know and doesn't want to ask someone else.

We will give the worst case like 77,000 amperes at the secondary bushings, assuming an infinite bus and a bolted fault. We will not give the available primary fault current because we have a dynamic system. The bank may be fed from one substation today and another one tomorrow.

We will size our transformer bank at around 50% of the actual service size. For instance a 1200 ampere service at 480 volts will have a transformer sized for about 500 kVA of demand.

The reason for the 200 kA in NYC is that several different transformers will feed into the fault at the same time since they use a networked system.

Utility guy from Indianapolis

CharlieE,
Does your utility have access to demand data from metered sites? For instance, if you have a new school of a square footage and certain types of equipment, that you could pull archived data from similar sites and size your transformer based on that information. I know of one in our area who looked at such data to make those decisions (when necessary). Their concern was the initial costs of supplying an oversized transformer and the continuing costs of supplying the magnetizing power to that unit.
I guess where I'm going with this is that if you can nail down the transformer to be used more precisely, it can allow for more
flexibility in what can be done on the customers side of the project.

[This message has been edited by GamecockEE (edited 07-31-2004).]

Yes, we have demand data from the larger, existing customers. However, we do not have that data available for smaller customers. Along with the demand data, we have the power factor data. I teach our engineers to do just that for their load calculations. This works well for chains of all kinds from restaurants to hardware stores, from department stores to sporting goods stores. We will share this data with electrical contractors after we sanitize it (we have to remove the customer's name, account number, address, etc. and then we have to provide the approximated size, type of usage, etc. so the EC will have useful data.

What is interesting about power factor is that we charge for poor power factor (over 85%) and we give a credit for improving power factor over 85%. Most customers do not want to be charged for the poor power factor but are not willing to correct past 85% to get the credit. I fail to see the difference, they are both cash.