Transformer secondary - sizing OCPD incorrectly in panel

C

cppoly

Senior Member
Location
New York
For a 30 kVA transformer, 480V primary 120/208V secondary, is having a 150A main breaker on the transformer secondary 120/208V panel a code violation or just bad engineering practice??

30,000 VA / 360 = 83.3 amps.
 
You need more info than that.

450.3 for protecting the transformer

240.21(C) for protecting the secondary wire

408.36 for protecting the panelboard it feeds

If it directly feeds a load, then you would need to evaluate that equipment's protections requirement.
 
The transformer secondary side protection can be any size you want it to be. The primary side protection can not be greater than 125%, per 450.3, except when you limit the secondary side to 125%.

So like elect117 said, more information is needed.
 
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So let's the secondary conductors are protected at their ampacity at the 150 amp MCB that they terminate in.

The issue I'm having is you can't get more than 83 amps out of the 30 kVA transformer. So when someone sees 150 amp main breaker on a panel connected to the transformer secondary, they are probably assuming the panel handle 150 amps of non continuous load.

So if it's not a code issue, isn't this a really bad design issue then?
 
cppoly said:
The issue I'm having is you can't get more than 83 amps out of the 30 kVA transformer. So when someone sees 150 amp main breaker on a panel connected to the transformer secondary, they are probably assuming the panel handle 150 amps of non continuous load.

So if it's not a code issue, isn't this a really bad design issue then?
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No. I don't design my electrical installations around making them brother in law proof. The unqualified idiot will outsmart you every time so don't even bother.
 
Let's say instead of a 150 amp MCB, there's a 500 amp MCB in the panel connected to the transformer secondary with 600 kcmils and no code issues - right? How could this be allowed on a 30 kVA transformer secondary??
 
cppoly said:
Let's say instead of a 150 amp MCB, there's a 500 amp MCB in the panel connected to the transformer secondary with 600 kcmils and no code issues - right? How could this be allowed on a 30 kVA transformer secondary??
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You would need a panel board rated 500 amps or above to comply with 408.36. would certainly be kind of a strange design, but I don't see any issue whatsoever
 
cppoly said:
Let's say instead of a 150 amp MCB, there's a 500 amp MCB in the panel connected to the transformer secondary with 600 kcmils and no code issues - right? How could this be allowed on a 30 kVA transformer secondary??
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The voltage matters here. Single phase, 3 ph, delta-delta or delta-wye. Read section 240.21(C).


To protect the transformer -
Please see 450.3. With the presence of secondary protection you are limited to 125% (and rounding up if not a standard size) for over 9A. If the secondary is less than 9A, 167%.

Though, it could be argued, and probably will be, that if your primary protection is sized to 125% then there is no need to have secondary protection that suitably protects the transformer. You would, however, be required to still protect the conductors and the load accordingly. I read the section as saying that the presence of secondary protection, whether that is the form of a panelboard MCB or fused disconnect for a nonmotor load, requires you to size it to 125% of the transformer's rating.

You will note that they do not mention FLA. I believe that is because they prefer nameplate rating and there are transformers that are made to exceed their FLA for continuous durations. Though, not likely to be found on a standard 30kVA dry type. Most dry types, however, can safely exceed their rating for a short duration without harm.

I wouldn't design anything that would intentionally fail. So putting a 500A MCB might not be totally illegal, but if the load is 500A, the transformer will fail.
 
cppoly said:
Let's say instead of a 150 amp MCB, there's a 500 amp MCB in the panel connected to the transformer secondary with 600 kcmils and no code issues - right? How could this be allowed on a 30 kVA transformer secondary??
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I agree with your question something just seems wrong about it. In your example you would need to use a 400 amp OCPD with the 600's not a 500 amp.
 
Elect117 said:
I read the section as saying that the presence of secondary protection, whether that is the form of a panelboard MCB or fused disconnect for a nonmotor load, requires you to size it to 125% of the transformer's rating.
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Huh? Certainly not.

Table 450.3(B) says that if the primary OCPD is sized to at most 125% (rounding up to a standard size is allowed), secondary protection is not required, so you can use whatever size you want. If you size the primary OCPD higher, then it must be at most 250% (no rounding up), and you must provide secondary OCPD of at most 125% (rounding up allowed).

As a practical example, say I have a 50 kVA transformer on hand, and it will work in the short run, but my expansion plans will require upsizing it to 150 kVA, which is on order with a long lead time. I can install primary and secondary conductors and a secondary panelboard and OCPD sized for 150 kVA (using either option in Table 450.3(B)), as long as I size my temporary primary OCPD for 125% of the 50 kVA. Then when I change transformers, I just need to change the primary OCPD, nothing else.

Cheers, Wayne
 
wwhitney said:
Huh? Certainly not.

Table 450.3(B) says that if the primary OCPD is sized to at most 125% (rounding up to a standard size is allowed), secondary protection is not required, so you can use whatever size you want. If you size the primary OCPD higher, then it must be at most 250% (no rounding up), and you must provide secondary OCPD of at most 125% (rounding up allowed).

As a practical example, say I have a 50 kVA transformer on hand, and it will work in the short run, but my expansion plans will require upsizing it to 150 kVA, which is on order with a long lead time. I can install primary and secondary conductors and a secondary panelboard and OCPD sized for 150 kVA (using either option in Table 450.3(B)), as long as I size my temporary primary OCPD for 125% of the 50 kVA. Then when I change transformers, I just need to change the primary OCPD, nothing else.

Cheers, Wayne
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Which I already alluded to in the same post. That either could be argued. It would be on the person enforcing it to read it to mean whether the secondary protection is required or whether the secondary protection exists.

As simple and vague the section is, it does not say whether or not you could size the secondary protection higher than that shown in the table. Where primary and secondary protection exist, it says the secondary protection shall not exceed 125% (with rounding up and ignoring the less than 9A).

Obviously you are arguing that the secondary protection is not required and therefore not required to be sized to table 450.3.
 
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