# residential xfmr question



## Shockdoc (Mar 4, 2010)

Ours out here have internal circuit breakers, don't ask me how I know:whistling2:

But a dead short between poles on a 30 amp breaker is enough to trip them out and leave some angry neighbors.:laughing:


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## MTW (Aug 28, 2013)

The American Electricians Handbook has a very useful table with the available fault current of the commonly used distribution transformers (25, 37.5, 50 kVa, etc). Is that what you're interested in?


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## Pete m. (Nov 19, 2011)

In my area the POCO tells us that for a resi service 200 amps or less 10k.

Pete


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## BuzzKill (Oct 27, 2008)

Just interesting that a home service line can break and start arc-faulting on the ground and the xfmr's fuse/breaker doesn't trip. How many times would this need to happen in order to trip it? I recall a thread of BBQ telling of a conversation with a poco guy saying that their code is to keep the power on while ours is to shut the stuff off.


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## Pete m. (Nov 19, 2011)

FWIW... I have spoke with a POCO engineer and they fuse the primary at 200% of the primary FLI to avoid outages.

Don't know about your area but it kinda seems to line up.

Pete


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## BuzzKill (Oct 27, 2008)

FLI=full load amps???


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## Pete m. (Nov 19, 2011)

BuzzKill said:


> FLI=full load amps???


Exactly. The engineer I spoke with basically indicated that they would rather have a meltdown on the xfmr than to size it for intermittent loading.

Pete


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## sbrn33 (Mar 15, 2007)

Call your poco and ask about specs then do the calculations.


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## MTW (Aug 28, 2013)

Fault current is also limited by the conductor size as well. Since overhead conductors under poco control are usually much smaller than NEC sized conductors, that has a big limiting factor on available fault current.


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## e909 (Sep 4, 2013)

BuzzKill said:


> So the typical residential neighborhood xfmr serves 6-8 houses around here; anybody know the arc fault rating of the fuses of the average xfmr?


The common impedance of a single phase transformer is around 2.5%.

So on a 50kVA transformer, you're looking at 50 000VA / (2.5% * 240V) = 8,300A for a bolted fault. This is why utilities generally require 10kAIC.

The size of the transformer and impedance may vary. I don't see a lot of protection on the secondary side of single phase transformers though.


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## Big John (May 23, 2010)

e909 said:


> ...I don't see a lot of protection on the secondary side of single phase transformers though.


 I've talked to guys who say they've installed tons of CSP transformers in their regions, but as far as I remember I've never seen one. Seems like it'd be overkill for most customers.


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## MTW (Aug 28, 2013)

Big John said:


> I've talked to guys who say they've installed tons of CSP transformers in their regions, but as far as I remember I've never seen one. Seems like it'd be overkill for most customers.



If I'm not mistaken, the CSP transformers are standard practice for the REC's.

Yeah, I know. :nerd:


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## circuitman1 (Mar 14, 2013)

were on REA here, & all of there's have a secondary breaker. just recently they have installed fuse cutouts on the primarys also.:thumbup:


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## Big John (May 23, 2010)

circuitman1 said:


> were on REA here, & all of there's have a secondary breaker. just recently they have installed fuse cutouts on the primarys also.:thumbup:


 I'll be. That's entirely the opposite of how I'm used to seeing that installed. Why is it more important to protect the customer's drop than protecting the primary from a transformer fault?


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## circuitman1 (Mar 14, 2013)

Big John said:


> I'll be. That's entirely the opposite of how I'm used to seeing that installed. Why is it more important to protect the customer's drop than protecting the primary from a transformer fault?


most are the CSP type, with internal fuse,they just put an external fuse on them about three yrs ago, i belive because of small animals.i know a squirrel got on mine, come home no power, went out in back yard ,saw tail hanging off of top. called power company, came out knocked squirrel off with hot stick , replaced fuse , power back on. free squirrel dinner.:laughing:


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## Meadow (Jan 14, 2011)

Big John said:


> I've talked to guys who say they've installed tons of CSP transformers in their regions, but as far as I remember I've never seen one. Seems like it'd be overkill for most customers.


CSPs are fairly common around here, usually you can spot one in service by the "glowing dot":laughing::no: Its an overload light, technically. CSPs have basically all the protection without the external hardware. Lighting arrestor is mounted directly to the can (with cross arm mounted fuses you have the option of nuisance fuse blowing by having one on the can or reduced lightning protection by having it on the cross arm before the fuse), an internal fuse (set somewhere I think 4-6 times the load current), a resettable secondary overload and an overload signal light. The resettable over load can even be partially overridden on some units for emergencies. 


See page 2 for details:

http://www.cooperindustries.com/con...sources/library/201_1phTransformers/20110.PDF


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