transformer protection

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lquadros

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Hi,
I have a 2KVA transformer at 480V 1 phase, with available fault current of 30KA upstream.I have 125 pecent secondary protection and Primary needs a breaker of 12.5A. There are no breakers available under 15A with high interrupting capacity and 15A turns out to be over 300 percent limit. My options I am thinking are to upsize the transformer to adjust the breaker or go with Fuses. Is there any other way?
Or am I missing something here in the whole picture? Please advice. Thanks in advance.
 
Re: transformer protection

lquadros
A 2kva transformer Primary amps at full load is
4.2 amps. The max OC protection is 250% or 10 amps. Why the need for a 12.5 amps?
 
Re: transformer protection

I don't do transformers like this so if I say something stupid ignore me.

The 4.16 amps is on the secondary.

The primary if it is 240v would have 10.42 amps on it at 125% of the 2kva.

Editted cause I goofed

Editted again: I can't tell which is the primary and which is the secondary from the terminology.

[ November 30, 2004, 01:52 PM: Message edited by: physis ]
 
Re: transformer protection

Physis
You might be correct. The post read as follows:
"I have a 2KVA transformer at 480V 1 phase, with available fault current of 30KA upstream."
It sounds like the primary is 480 volts and a fault current of 30ka.

Bob
 
Re: transformer protection

According to Table 450.3(B) if you are using 125% secondary protection then your primary current must be 9 amperes or more. In that case, your primary protective device is limited to 250% and not 300%. As bob has said, 250% would give you 10.42 Amps. I would use a 10 Amp fuse. :)
 
Re: transformer protection

I assume the transformer is 480 volts primary and secondary 240 volts, 1-phase, 2Kva. The primary curren is 2000 divided by 480 = 4.17 amperes. Table 450.3(B) of the 2002 NEC provides the maximum overcurrent protection allowed. For a transformer that has a primary overcurrent of less than 9 amps, the primary overcurrent device must not exceed 167 percent of primary current. In this case 6.96 amps. Secondary protection of the transformer is not required. However, overcurrent protection must be provided for the conductors supplied by the 240 volt secondary. If primary and secondary protection is supplied for the transformer, the primary OCD cannot exceed 250 percent of 4.17A = 10.4A. The secondary OCP must not exceed 167 percent of 8.33A = 13.9A. In this situation, you are not allowed to go up to the next higher size OCD.
 
Re: transformer protection

Thanks for your replies.
I agree to the 250% rule. I do have a 125% secondary protection. The main problem is, I am unable to find a breaker that is 10Amp with 35kA interrupting capacity. I will need to go the next size up that is 15Amp.
Also I have a situation where I have to power up a 250VA. 480V 1ph. primary transformer. I will have no choice but to use fuses instead of circuit breaker. Because the fault current in this case is 22kA. and the appropriate breaker is not available.
Thanks for your replies
 
Re: transformer protection

I agree to the 250% rule. I do have a 125% secondary protection. The main problem is, I am unable to find a breaker that is 10Amp with 35kA interrupting capacity. I will need to go the next size up that is 15Amp.
Click to expand...
lquadros
You are not allowed to goto the next size breaker.
250% is the max allowed under 450.3. You will need to use a fuse to meet the fault conditions.
 
Re: transformer protection

Another thing to consider is not only the practicality of the size of the OCPD which is available but the 250% rule as it related to inrush and nuisance tripping the OCPD. With smaller transformers you can expect the inrush current to be over 20x the transformer FLA. The last time that I had compared trip curves the magnetic trip calibration of small residential breakers often is as low as 6-7x the breaker rating up to around 10x and somewhat higher with industrial breaker.
 
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