# Can tails



## Joe Robert (Dec 21, 2017)

So my crew was tasked with replacing a 50kva xfmr with a 100kva. My leader tells me to cut some get some can tails and I return with 4/0 copper can tails. He flips out on me and says 100kva can tails need to be 500 copper, okay no big deal bigger can=more load=bigger wire I get it. But when we get to the job the jumpers and secondaries are all #2 open wire. What’s the point of using 500 tails if you’re just tying them to #2? Or should the planners have had us replace the secondaries as well?


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## EB Electric (Feb 8, 2013)

The tails you are making up are like an extension of the transformers secondary bushings. Basically you are making (physically speaking) a small secondary bus with your 500's. The reason they are so large is because they need to be rated to carry the transformers full load current on the secondary side. A 100kVA can is capable of supplying upwards of 400A on each hot leg, so you need to size your tails to have the ability to carry the full load rating. If you were to install smaller tails say 4/0 you are making a smaller (electrically speaking) secondary bus, which would not be sufficient to carry the full load current on the secondary side. In short, it might seem weird crimping a few #2's for little services onto big 500's, but remember in the future there can be additional services/loads hooked onto the tails/bus. If you use a smaller wire as the tails not capable of carrying rated full load of the can think of it like choking it down for lack of proper terms. Tails not sized large enough to carry rated full load on the secondary side you are creating the opportunity to still connect full load through undersized conductor, possibly burning up. Sizing the tails larger for full load secondary current you do not have that risk.

The secondaries do not need replaced, they are sized individually to carry the current for the service. If you clamped one of the #2's and read 40 amps it can carry that load all day, whether it is connected to another #2 or 1000mcm tails makes no difference electrically, the #2 is capable of supporting whatever load/service it is connected to, that does not change. Now saw you have 500's for a big service clipped onto #2 tails on the transformer, now there is an issue. In the later case you would have ability to draw a large lets say 400amp load current but now that current is passing through a little #2 on it's way, that #2 is going to burn up. 

Hopefully that made some sense and can help you understand the reasoning behind the size of the tails. I am not a lineman by trade, I am substation electrician, so my terms and the linemans terms may not be the same. Clear as mud?


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## Joe Robert (Dec 21, 2017)

I understood all your terms, but there were not multiple crimps we used bugs) on the 500 tails. We tied the tails directly to the #2 jumpers, only one connection per tail. Even if the load was balanced between each side of the jumpers (I don’t remember) that still allows 200A on each side of the #2 jumpers. Seems wrong but I’m not the boss


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## micromind (Aug 11, 2007)

If you're talking about a 3Ø bank and the jumpers are connecting one side of each transformer to the others (thus forming the Y), they will carry the same current as the load conductors. 

If it's a 120/208 using 3 - 100KVA pots, the full-load current at each pot bushing will be 832 amps. I don't think a #2 will last very long........

If it's 277/480, the current will be 360 amps. Again, #2s won't hold. 

In short, the wire needs to be a the same size at each pot bushing because it carries the same current.


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## Joe Robert (Dec 21, 2017)

Single phase 120/240


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## paulengr (Oct 8, 2017)

Other than future proofing or not knowing, sounds grossly oversized.

There is a difference in that going from copper to aluminum ampacity drops and going from a tested assembly to field work sometimes it drops, and that if you go into molding, troughs, trays, buried lines, or anything else where you don't get as good of cooling as you do mounted in free air on cross arms, or based on the temperature limits of the connectors, bushings, and insulators sometimes you go up but not that much. Overhead lines can easily handle over 300 degrees if they are spaced and sagged right but your bugs would melt. Insulated cable by itself is only rated up to 90 C for most common insulation types. But there is much better insulation available but it's expensive. They use it in motors and transformers because cramming more copper in a smaller space makes it cheaper and more efficient. Starting to see Class N (200 C rating) out there. The fittings haven't caught up yet though never mind the UL/NEC crowd that still wants to derate control panels (UL 809A) for 60 C, insulation that hasn't even been on the market in over 40 years! I'm sorry but Thomas Edison died long ago and GE is on life support.

Crimps are the way to go. Crimps by the way are the way to go anyway. Sooner or later the screw will loosen and the connector fails or in an enclosure it rubs from 60 Hz vibration and shorts out. They're junk after 10 years or so. Also crimped connections are available for 200 C operation if you ever have an emergency or summer load. We're banned from using bugs because they fail so much.

Sent from my SM-T350 using Tapatalk


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## Joe Robert (Dec 21, 2017)

I know my POCO’s policy is that whenever a xfmr needs replaced they they replace it with a higher kva xfmr. Perhaps it was future proofing as you say and that’s why the leader used bugs instead of crimps, planning to replace the secondaries relatively soon.


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