How to use 450.3(b)

goodoboy

Senior Member
Location
Houston
Hello,

I am using this table correct.

I have a 1kVA transformer and need to calculate OPCD size.

Data:
480V 3 phase to 120V 1 phase transformer
8 amp load for secondary side.

1. I calculate 3.5amp OPCD size. 1.67 x (1000/480) = 3.479amp. Is this correct?
2. How do i determine to use OPCD on primary and secondary?
3. Do I need a fuse on the secondary side to protect the conductors?

Thank you.
 
Last edited:

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hello,

I am using this table correct.

I have a 1kVA transformer and need to calculate OPCD size.

Data:
480V 3 phase to 120V 1 phase transformer
8 amp load for secondary side.

1. I calculate 3.5amp OPCD size. 1.67 x (1000/480) = 3.479amp. Is this correct?
2. How do i determine to use OPCD on primary and secondary?

Thank you.
Please read the notes under the table carefully. especially note 1.
 

augie47

Moderator
Staff member
Location
Tennessee
Keep in mind Art 450.3 is for transformer protection only and has little to do with the conductors.
(Thus the FPN at 450.3).
Decide if you plan to have primary and secondary transformer protection or primary protection only (your choice) and follow the guidelines in the Tables and the Notes.
You calculated your primary (only) protection at 3.5 amps, so you can use a 3.5 amp fuse for transformer protection.
If you placed and 8 amp fuse on your secondary, you could increase your primary protection to 250% or 5 amps.
After you have decided on the transformer protection go to Art 240.21(C) to determine conductor protection.
 
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templdl

Senior Member
Location
Wisconsin
Thanks, I been reading that all morning. That's why I am here asking the questions.
This is the way I have always thought of it:
Think of the Pri. protective device as actually providing short circuit protection should the transformer fail. It's the device that takes the transformer off line so that it doesn't affect the upstream distribution system. Sometimes the rating of the pri. protective device seems to be unreasonably high but it really isn't intended to provide overload protection for the transformer as that's the purpose of the secondary OCPD.
 

goodoboy

Senior Member
Location
Houston
Keep in mind Art 450.3 is for transformer protection only and has little to do with the conductors.
(Thus the FPN at 450.3).
Decide if you plan to have primary and secondary transformer protection or primary protection only (your choice) and follow the guidelines in the Tables and the Notes.
You calculated your primary (only) protection at 3.5 amps, so you can use a 3.5 amp fuse for transformer protection.
If you placed and 8 amp fuse on your secondary, you could increase your primary protection to 250% or 5 amps.
After you have decided on the transformer protection go to Art 240.21(C) to determine conductor protection.
Thanks,

Shouldn't the fuse on the primary side protect the conductors on the secondary side going to load?
 

augie47

Moderator
Staff member
Location
Tennessee
Keep in mind, if this particular transformer is a compenent of a piece of assembled Listed equipment it may not be installed per NEC rules but will be installed per the listing requirements.
 

templdl

Senior Member
Location
Wisconsin
With our MCCs for a comon 480-120v CPTs we used (2) KTK-R primary fuses and (1) FNM secondary fuses.
If the VA of the CPT were to be provided I could provide the PRI. And SEC. Fuses that were provided with our MCCs. As an example a 150va CPT was supplied with a 2-1/2a FNM sec. fuse and 3a KTK-R PRI. fuses. The max pri. size allowed is 20a but was never supplied as such that I was aware of. Matter of fact a 20a KTK-R was allowed from a 50va-1500 KVA but we supplied a 1500va with a 10a.
 

goodoboy

Senior Member
Location
Houston
Keep in mind, if this particular transformer is a compenent of a piece of assembled Listed equipment it may not be installed per NEC rules but will be installed per the listing requirements.
Thanks,

The 480V is coming from the MCC. It A/C unit is powered fromthe 480V and then the 480V is tied to the transformer for the 120V single phase loads. There secondary side only supplys two loads.

Thanks,
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Thanks,

The 480V is coming from the MCC. It A/C unit is powered fromthe 480V and then the 480V is tied to the transformer for the 120V single phase loads. There secondary side only supplys two loads.

Thanks,
not all that long ago (maybe 25 years) UL allowed MCC control transformers to have no primary protection at all, other than what was in the bucket.
 

Smart $

Esteemed Member
Location
Ohio
Thanks,

The 480V is coming from the MCC. It A/C unit is powered fromthe 480V and then the 480V is tied to the transformer for the 120V single phase loads. There secondary side only supplys two loads.

Thanks,
With 480-120 single phase transformer (2-wire secondary), you can use primary protection to protect secondary conductors. See 240.21(C)(1) and Table 450.3(B). Max ocpd rating is 167% (1kVA xfmr rated primary current is slightly greater than 2A). One possible problem is that a 3.5A fuse/breaker is not a standard rating IAW 240.6. Depending on whether the AHJ will permit you to round the calculation result to 3.5A, you may get stuck using a 3A fuse/breaker.

What is the calculated load on the secondary? Never mind, I see from your other thread it is 8A load. JMO, but I suggest you go a little larger on xfmr rating if the lighting is inductive (a good chance your fuse will smoke at turn on).
 
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augie47

Moderator
Staff member
Location
Tennessee
With 480-120 single phase transformer (2-wire secondary), you can use primary protection to protect secondary conductors. See 240.21(C)(1) and Table 450.3(B). Max ocpd rating is 167% (1kVA xfmr rated primary current is slightly greater than 2A). One possible problem is that a 3.5A fuse/breaker is not a standard rating IAW 240.6. Depending on whether the AHJ will permit you to round the calculation result to 3.5A, you may get stuck using a 3A fuse/breaker.

What is the calculated load on the secondary? Never mind, I see from your other thread it is 8A load. JMO, but I suggest you go a little larger on xfmr rating if the lighting is inductive (a good chance your fuse will smoke at turn on).
I agree that if your secondary load is actually 8 amps, you are cutting it awfully close.
Of course, as you up your transformer, your 15 amp circuit may not be adequate.
 

Smart $

Esteemed Member
Location
Ohio
I agree that if your secondary load is actually 8 amps, you are cutting it awfully close.
Of course, as you up your transformer, your 15 amp circuit may not be adequate.
Must have been in the other thread, which I did not read in its entirety...

...but given the load isn't changing, I don't see how the feeder circuit rating will affect overall operation other than the power surge at turn on.
 

augie47

Moderator
Staff member
Location
Tennessee
Must have been in the other thread, which I did not read in its entirety...

...but given the load isn't changing, I don't see how the feeder circuit rating will affect overall operation other than the power surge at turn on.

Probably was.. he has two similar threads going and, giving it more thought, you are correct..same load.
 

templdl

Senior Member
Location
Wisconsin
Hello,

I am using this table correct.

I have a 1kVA transformer and need to calculate OPCD size.

Data:
480V 3 phase to 120V 1 phase transformer
8 amp load for secondary side.

1. I calculate 3.5amp OPCD size. 1.67 x (1000/480) = 3.479amp. Is this correct?
2. How do i determine to use OPCD on primary and secondary?
3. Do I need a fuse on the secondary side to protect the conductors?

Thank you.
I worked for a major MCC manufacturer and for 1kva 1ph 480v-120v CPT we would supple (2) 10a 600v ktk-r fuses for the primater and (1) 250v fnm 15a fuse for the sec.
 

goodoboy

Senior Member
Location
Houston
I agree that if your secondary load is actually 8 amps, you are cutting it awfully close.
.
Can you explaing this statement a bit more? How am I cutting it close with this 1kVA transformer? Is this becaus my fuse will be 3-3.5amps and could open during start-up of the secondary 8 amp load?

Thank you for the help
 
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