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Transformer primary conductor sizing

C

cppoly

Senior Member
Location
New York
For a 225 kVA transformer 480V delta primary, what size conductors are required with a primary OCPD at 350A (125% of FLA = 338 amps) fed from a distribution panel upstream.

Assuming noncontinuous loads, the transformer feeder minimum size would need to be sized at 100% of the transformer FLA = 270A = 300 kcmils (285 ampacity @ 75 degrees).

However, since the transformer OCPD sized is 350A, the conductors need to be protected in accordance with their ampacity.

But, the next size up rule applies, still applies.

So would 350 kcmils (310 ampacity @ 75 degree), 400 kcmils (335 ampacity @ 75 degree), or 500 kcmils (380 ampacity @ 75 degree) be required?
 
cppoly said:
For a 225 kVA transformer 480V delta primary, what size conductors are required with a primary OCPD at 350A (125% of FLA = 338 amps) fed from a distribution panel upstream.

Assuming noncontinuous loads, the transformer feeder minimum size would need to be sized at 100% of the transformer FLA = 270A = 300 kcmils (285 ampacity @ 75 degrees).

However, since the transformer OCPD sized is 350A, the conductors need to be protected in accordance with their ampacity.

But, the next size up rule applies, still applies.

So would 350 kcmils (310 ampacity @ 75 degree), 400 kcmils (335 ampacity @ 75 degree), or 500 kcmils (380 ampacity @ 75 degree) be required?
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350kcmil would be acceptable
 
cppoly said:
For a 225 kVA transformer 480V delta primary, what size conductors are required with a primary OCPD at 350A (125% of FLA = 338 amps) fed from a distribution panel upstream.

Assuming noncontinuous loads, the transformer feeder minimum size would need to be sized at 100% of the transformer FLA = 270A = 300 kcmils (285 ampacity @ 75 degrees).

However, since the transformer OCPD sized is 350A, the conductors need to be protected in accordance with their ampacity.

But, the next size up rule applies, still applies.

So would 350 kcmils (310 ampacity @ 75 degree), 400 kcmils (335 ampacity @ 75 degree), or 500 kcmils (380 ampacity @ 75 degree) be required?
Click to expand...
IMHO... Any of those would work.
 
What's the calculated load on the transformer?

215.2 still applies, and the transformer's 270A FLA is a continuous rating. So if the transformer is loaded continuously near its rating (unlikely), then 125% of the continuous load plus the non-continuous load could exceed 310A. That would make 350 kcmil Cu too small and require a larger size.

Cheers, Wayne
 
tortuga said:
Unless I am mistaken the NEC it does not really require a load calculation to size a transformer per se, just the feeder and the loadcenter etc.
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Well, if you prefer, then what's the load on the transformer primary conductors?

Since I don't believe the NEC assigns a load to the transformer itself, the load (in VA) on the primary conductors and secondary conductors (as a whole) will be the same, which I was calling the load on the transformer.

Cheers, Wayne
 
There is no load assigned yet. But, the primary OCPD is required to be sized at 125%. Wouldn't this dictate your feeder size?
 
cppoly said:
There is no load assigned yet. But, the primary OCPD is required to be sized at 125%. Wouldn't this dictate your feeder size?
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The relevant rules on conductor sizing are 250.2, 240.4, and 240.21.

250.2 says the conductors have to be large enough for the load (including a 125% continuous use factor). If the load were only 120A, then 120A conductors would comply with that requirement (but perhaps not the other requirements).

240.4 says the conductors have to be properly protected. So if the 350A primary OCPD is fixed, and you want that OCPD to protect the conductors, then per 240.4(B) the primary conductors need to be at least 301A.

240.21(B)(5) provides an option for protecting the primary conductors by the secondary OCPD if you comply with all the requirements. With a 350A primary OCPD, those requirements include a minimum primary conductor ampacity of 117A.

So with the load unspecified (but then how did you decide on a 225 kVA transformer?), if you want to be able to supply a non-continuous load up to 100% of the transformer rating, or a continuous load up to 80% of the transformer rating, use conductors with an ampacity of at least 301A. If you want to be able to supply a continuous load at 100% of the transformer rating, use conductors with an ampacity of at least 338A.

Cheers, Wayne
 
Wayne, thanks for the detailed explanation that was really good.

The load is 180 kVA, so a 225 kVA transformer is the next higher size.

But this has me thinking....

How does the NEC NOT require you to size the primary conductors based upon the transformer FLA?

If the NEC only requires you to size primary conductors based upon the current load and not the nameplate, then couldn't this be a major problem in the future when another load is added 10 years down the road and everyone is assuming that the full transformer nameplate kVA is available to use? If I see an existing 225 kVA transformer, I’m assuming the secondary is good for full the full 225 kVA and not limited to 180 kVA feeder on the primary.

Am I wrong in this type of thinking?

Thinking further into this, how is this not a code issue? Because the NEC already anticipates something similar with motors. We are required to size conductors based upon motor FLC not FLA because when a motor is replaced FLAs vary. So instead of having to replace the conduit and wire every time there's a motor replacement, we size the circuit based upon the worst case current using FLC.

But in the case of transformers, if we're only required to size based upon load and not the nameplate, then replacing the primary feeder every time load is added doesn't make sense to me.

Thanks for your input!!
 
wwhitney said:
Well, if you prefer, then what's the load on the transformer primary conductors?

Since I don't believe the NEC assigns a load to the transformer itself, the load (in VA) on the primary conductors and secondary conductors (as a whole) will be the same, which I was calling the load on the transformer.

Cheers, Wayne
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The transformer is not a load... It just transforms the power. Hence the name transformer. :)
 
The NEC is not a design manual (at least it isn't supposed to be). There is nothing stopping you from designing the conductors for the nominal size of the transformer, and in practice I would expect it is most commonly done this way, but it is not required. Size the conductors for the load and the overcurrent protection for the conductors. If it has to get changed in the future because load was added, so be it. From a code standpoint, we don't have to design for future loads. From a good design standpoint, we should at least consider them.
 
cppoly said:
How does the NEC NOT require you to size the primary conductors based upon the transformer FLA?
Click to expand...
Because it requires that whatever size conductors you use, they be properly protected per Article 240.

We could flip the question around--suppose your 225 kVA transformer fails, and the replacement that is handy is 300 kVA, but none of the loads have changed. Should we be required to upsize the primary conductors and OCPD just because the transformer rating increased, even though the primary and secondary loads will be the same?

cppoly said:
If the NEC only requires you to size primary conductors based upon the current load and not the nameplate, then couldn't this be a major problem in the future when another load is added 10 years down the road and everyone is assuming that the full transformer nameplate kVA is available to use? If I see an existing 225 kVA transformer, I’m assuming the secondary is good for full the full 225 kVA and not limited to 180 kVA feeder on the primary.
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That would be an oversight--you need to check the primary side OCPD and the primary and secondary conductor sizes.

Cheers, Wayne
 
Thanks Wayne. I sort of disagree with this analogy. If I see an 800 amp distribution board, I'm assuming the feeder and board are good for 800 amps (but of course that is a code requirement). If I see an existing 300 kVA transformer, I'm assuming it's good for 300 kVA. But I understand your point that it's not required.
 
cppoly said:
If I see an 800 amp distribution board, I'm assuming the feeder and board are good for 800 amps (but of course that is a code requirement).
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That's not actually required. It could plausibly be supplied by a 600A or 700A feeder. [It could be supplied by a 20A feeder, but that's not plausible.]

Cheers, Wayne
 
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