Transformers...

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Dsg319

Senior Member
Location
West Virginia
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Wv Master “lectrician”
I know to size a transformers it is from the KVA of the load.

But just a wondering question in my mind.

I know it’s crazy and random but just came to my mind while setting around...

say I have a 30KVA transformer,I know from mikeholts electrical toolbox and it’s calculations I would end up with secondary conductors sized at #3awg and a 100 secondary OCPD.

BUT say I didn’t need secondary conductors or protection and panel near that size and just had that size transformer hanging around. Would it be permitted to do such. if I only needed a 50amp panel(based on load) and conductors sized 125% of that. Would that be allowed for the secondary conductors or no?

So in a nutshell, if you were to Lower secondary OCPD can you use smaller secondary conductors based on that?
 
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infinity

Moderator
Staff member
Location
New Jersey
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Journeyman Electrician
Yes you can use smaller conductors and a matching OCPD. The transformer kva rating is it's maximum but you can use "less" of its rating if you like.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
What size OC is on the primary side?
Are you protecting the transformer or the conductors?
I’m not actually doing this, just was thought of as an example. But if was going to what would you do with the primary OCPD. Size for maximum transformer kva since secondary side is already protected.
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Calculate your load. Size your secondary conductor to carry that load or lager. Protect you secondary overcurrent device to protect that conductor. Size your transformer for the load OR larger,
Using Art 450.3 make sure your transformer is adequately protected (primary or primary and secondary)
Select your primary conductors and protect them according to their ampacity.
(You can protect the primary at less than the kva rating but don't go so low as to cause nusience tripping)
 

paulengr

Senior Member
You CAN undersize the primary side if you could but the magnetizing current inrush is going to get you.

If you were working in switchgear you can however do this. When the breaker first closes or voltage first appears set up the normal trip curve for a fully rated system. As soon as current drops below your desired ampacity switch to your “normal” curve since inrush has passed.

It is far simpler though to simply set up secondary protection. The exception is when transformers get so large that breaker prices get excessive. That’s when it makes more sense to install bushing CTs connected to a protection relay that shunt trips a primary side breaker. The breaker protects “both sides” but since current is much lower on the primary side and you need primary side protection anyway it can do double duty.
 
. But if was going to what would you do with the primary OCPD. Size for maximum transformer kva since secondary side is already protected.

You CAN undersize the primary side if you could but the magnetizing current inrush is going to get you.

Perhaps Its worth noting that for transformers over 600 volts, the primary conductors must be sized to at least the transformer nameplate (215.2(B)(1)), but I am not seeing anything that says the OCPD supplying those conductors cant be less.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Would the conductors from the transformer to the first OCPD need to be full sized or subject to tap rules?

Except in some cases transformer secondary conductors are considered taps and already installed under the tap rules.

Transformer primary OCPD is 'oversized' for the transformer rating and connected load, because of magnetizing inrush. On top of this, the full output of the transformer could go though only a portion of the secondary, so that what looks like normal loading on the primary side could be an overload on a portion of the secondary.

All this means is that even full size secondary conductors are not protected at their ampacity on their supply side, but rather by the OCPD that they feed. Thus the secondary conductors are taps.

As long as you follow the appropriate tap rules you can reduce the size of these conductors and OCPD to match a lower than maximum transformer load.

-Jon
 

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
....Thus the secondary conductors are taps.
....
-Jon
While that is a common field use of the terms, transformer secondary conductors are just that...transformer secondary conductors. These conductors are not protected by the tap rules in 240.21(B), they are protected by the transformer secondary conductor rules in 240.21(C). The use of the term tap for transformer secondary conductors leads some people to use the wrong set of rules for the protection of the transformer secondary conductors.
.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
While that is a common field use of the terms, transformer secondary conductors are just that...transformer secondary conductors. These conductors are not protected by the tap rules in 240.21(B), they are protected by the transformer secondary conductor rules in 240.21(C). The use of the term tap for transformer secondary conductors leads some people to use the wrong set of rules for the protection of the transformer secondary conductors.
.

I agree, transformer secondary conductors are one thing, and taps are another thing.

240.21 is all about the _location_ of OCPD. The general rule is that OCPD needs to be at the source side of the conductor.

240.21(B) for tap conductors describes the situation and limitations for placing the OCPD at the load side of a set of conductors.

240.21(C) describes the situations in which secondary conductors may be connected to the transformer secondary without OCPD _at_ the secondary.

The rules for tap conductors and secondary conductors seem to follow very similar logic, presumably for very similar reasons.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
You CAN undersize the primary side if you could but the magnetizing current inrush is going to get you.
Available current at the load end of the supply circuit is the determining factor here. Say OP's transformer is supplied by a somewhat small transformer or has some length to feeders along the way, total impedance may limit the inrush where it would otherwise trip if installed close to source and/or on a lower impedance source.

I see this kind thing frequently with motors more so than with transformers, but typically I see many more motors than transformers.

Had a situation where two motors had been there for several years and no trouble starting them. This past year we added some load, POCO increased transformer size, new load was on separate feeder but from the upsized source. Now those two motors occasionally trip their feeder breaker when starting, nothing changed on that feeder. Ended up having to put in time delay so they don't start at same time. What changed, available current during starting because of lower impedance source.

Had similar happen many times, or even same source but installed larger feeders allowed more current at startup and maybe branch circuit OCPD that used to hold starts tripping on startup.
 
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