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	Subject - Case Study : Sub-panel Addition Scott Vickrey Here's a new twist to the forum. In this topic using the information given try to step through the thought process of completing this hypothetical installation. Try to concider everything and list as many code references as you can. Don't feel you have to do the whole thing. Partial answers are expected. Known Information: This job is located at a residence. They need power to a new shop located behind the home 250' from the main panel. We have determined that we need a 240v single-phase 100A 12 space sub-panel for a 18kw load and futures. After checking we are sure the main panel has the capacity and the breaker space. Both panels are located on the outside. It is agreed we need to run an underground feeder between the panels. This case study ends with the installation of the sub-panel. frenchelectrician Hi Scott: I will try this one but good challenge. I will try to list all the info much as I can here. 1] 18 Kw at 240 volts single phase. will end up 75 amp 2] 250 feet of run, OK I will choose #1 copper thwn wire (75 degrees termination type) or #1 also on 60 degrees conductors but, I will use 60 degrees conductor rating due that being buried in conduct that will be buried in dirt. 3] For this distance the voltage drop is 5.9 volts or 2.4% volt drop. 4] Conduit size, I will choose 1&1/2 inch size because it is a little easier to pull through for longer runs than 1&1/4 inch. BTW 1&1/4 is the minimum size that can be used. I would choose PVC schedule 40 here because it is for a residential area. I would bury the conduit at 24 inch deep per NEC codes. I figured the original kw rating was at 18 that is 75 percent of full load rating so, I tried 80 percent rating there and still have same answer there. 5] The wires, the hot and neutral conductors will be sized at # 1 but grounding conductor I can use #6 but being that kind of distance I will increase to #4 bare or green conductor . 6] This one I'm wondering about, 12 space sub feed box. I'm not sure where it stands in some areas but, It should have a main breaker in there being more than 6 disconnects there (correct me if I am wrong here). This is the best I can do for now I'm going to wait and see what other say about this. merci , marc David Hyatt Marc, without doing the calculations for VD, I assume you are correct. As to burring the PVC you can dig 18" deep instead of 24" deep. I almost always dig 24" and use direct burial cable. Very seldom use conduit underground. But thats just me. Also it is my interpretation of the NEC that the six switch rule would not be in affect for this panel because it has a main from the first panel. Let me know about this. But since this is a separate building 225.32 will require a main. cal_sparky Great topic! Don't forget the ground rods (or other electrode) at the outbuilding. A grounding electrode system is required for any separate structure that has a subpanel. I always run an EGC too, because even if there isn't a metallic path now, there may be in the future. Cliff cs409 #4 for the egc per 250.122(b)....as cal stated, I always install as though the metal bonding between the 2 buildings is there, because sooner or later, it probably will.... Scott Vickrey Thanks to all who have responded this far but, we still have much to consider and we still need code references. I was not clear on what I hope to accomplish here. Try to walk through the code book mentioning as much as you deem important. Ideally we would go through ever applicable section of the code. Realistically, I know this is very hard to do so use your judgment as to how far you are willing to go. You don't have to do it all. Stop where ever you want and some one else (Hopefully) will continue where you left off and take it a little further. Let me start (I know... I should have done this to begin with.) One of the first consideration would be: Wiring Method How will we get the wires out to the sub-panel? Marc has suggested PVC, David mentioned running direct burial cable. I suppose this a matter of personal preference and should be a topic of it's own. I created a new topic specifically for this discussion. If you care to voice your opinion on this subject do it here. For this case we will be using PVC or rigid non-metallic conduit (RNC). Covered by article 352. Since this is to be an underground installation, we also have to follow section 300.5 and 300.50 both named Underground Installations (go figure). We get our burial depth from T300.5 which in our case is found to be 18 inches. when it comes time for backfill section 300.5(F) lays out our backfill requirements. note: a good place to start from here would be to see if I missed anything you believe to be important and mention it or, move on to the next consideration. Which could be wire sizing. frenchelectrician hello there: david thanks for asking this one for voltage drop caluation i did limit the vd to 5% max and i will admit here i have a electricalc aka a specal caluatior for electrican so i use it and i use the defalut setting for 5% volt drop i can increase the vd with smaller wire but myself i am not too keen with getting too high a volt drop espcally with this kind of distance . thanks david for pointing it out :) david let me add one more thing here , about burial depth i like to keep it under the frost line in northeast wisconsin the frost do go much as 22 inch deep. i dont know about your area it might be less. yes i was well aware about the 18 inch depth but i just got a habit to run at 24 so i can advoid troble with broken pvc pipes. yes the sec 250.122[b] did mention that clear about the grounding conductor. myself i dont rember where i did see that Ryan did bring up the disscussion for sizing there. [ Ryan you are welcome to point it out to me anytime ] ok let get back to the orginal subject here about what scott want us to do now ,,,, as far i did have older chart from the nec code book [96 verison] and look up at table C10 and i look up the THWN-2 wire and the listing for 1 1/4 inch pvc conduct show with #1 wire it show 3 wires but go with 1 1/2 inch pvc it show 5 wires on the listing. i did make a small error so let me correct my mistake on the listing before i did overlook the grouning wire so i will stay with 1 1/2 inch conduct [ the cost between the two is small] sorry about that i was typing very late lastnite and my brain was not running on full three phase the reason i did chose the THWN-2 wires in the conduct because this is very common type of wire to be used as far for other type of wires you are welcome to speak up here too. and i did choose the copper wire sure i know it is far more expenisve than alum wire are but in long run have less troble with it merci, marc p.s. if i overlook anything please let me know ,,, we can help each other,, thank again