I came across a new installation in the industrial facility I work in. It is a 480 volt isolation transformer 45kva feeding a 100 amp panel for heat trace loads. No wiring is installed yet just the transformer and the panel. If the primary is fed with a 50 amp breaker will this be a violation since the panel will be capable of exceeding the transformer capacity even if the load isn't over the capacity at the time of installation? Hope I make sense here.
Proper Transformer Sizing???
- Thread starter tpd
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So if it is primary only protection then it would be 54 amps for a 3 phase 480 volt 45 kva @ 125% and that would be a 70 amp OCPD. If it is primary and secondary protected then it would be 250% on the primary side which would be 135 amps next size up 150 amp. But the secondary would be at 125% which would be 70 amp and not the 100 amp that the panel now has. And that would be a violation correct? Am I missing something? I know I'm not beyond missing things!
Your evaluation is correct... except there is nothing to say it is primary and secondary protection. Your second post included that it is three phase, which changes the input and output current compared to single phase. Nevertheless, the 50A protected primary is still within primary only protection maximum. The 100A OCPD of the panel may just be the required panel protection, and not necessarily secondary protection.
Another dependency is secondary conductor protection. If the secondary is a 4-wire system, the secondary conductors are not considered protected by primary OCPD. Yet if the secondary conductors are protected at their ampacity rating by the 100A panel OCPD, then all is fine...
Not necessarily. 240.21(C)(1) is regarding secondary conductor protection... not the transformer secondary.
Where transformer secondary protection is a single OCPD, it's purpose can be threefold: 1) protect the transformer secondary, 2) protect the transformer secondary conductors, and 3) protect the panel.
Where primary-only transformer protection is used, the secondary conductor's must still be protected per 240.21(C), and the panel must also be protected (can't recall the exact section at present). The 100A panel I assume has a 100A MCB, so it is protected, and being it is the first OCPD, it must protect the secondary conductors... so they must have an ampacity of 100A or greater.
Thank you for your time. I'm not involved in many transformer installations so I assumed it should be sized according to the panel being used. I've seen many instances at this older facility that 200 amp panels are used with 45 kva transformers! 208/120. They may or may not be sized correctly on the primary side but they sure run very hot! Awhile back we had one of these smaller transformers burn up and we made them get a 75 kva to replace it. Which is a 200 amp capacity. I just figured they were under sized because of it being an old plant. In my experience I am not to impressed with the engineers doing these installs because most don't know code at all and seem to fly by the seat of their pants!
kwired
Electron manager
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I'm having some confusion over what you have. You say isolation transformer and 480 volts. That kind of means 480 volts in and 480 volts out, and transformer capacity for 45kVA would be 54 amps. But something tells me the secondary may very well be 208/120 wye, which calling it a step down transformer instead of an isolation transformer would be more technically correct.
I guess my question is what is primary and secondary voltages. As far as the secondary being able to overload the primary breaker, not really a code issue to some extent, what is the intended load to be connected? If it results in more than 50 amps on the primary then you are undersized, but there would not be any violation in relation to the primary circuit if the connected load never reached 50 amps on the primary side.
It is a 480 volt to 480 volt delta wye transformer. I think it must be 277 volt heat trace that is being installed. I guess then I think this is a poor installation even if it meets Code requirements. If the calculated load at the time of install is 40 amps and this uses even 3/4ths of the panel breakers and down the road it gets added to because there are breakers available then we are going to overload the transformer and create heat until it has to be upgraded. Instead of properly sizing a transformer to match the capacity of the panel in the first place and trying to avoid excess heat ect. This install will never really be an issue as far as causing a fire being in an industrial location but if it was in a situation with more chance of causing a fire then it could really become a problem even though it meets code!
The primary OCPD should trip if the panel is overloaded... provided loads are balanced. The reason secondary conductors are not considered protected for 4-wire systems is because when unbalanced, the primary line currents are not identical to that of the secondary. You'll have to prove this for yourself to realize what happens. Consider an extremely unbalanced secondary loading, and determine the primary current.
Also, FWIW, it is up to whomever adds load in the future to balance the load and not overload the transformer, or any associated equipment.
Good information thank you. I guess that's my main concern is from what I see nobody, engineers included ever looks at any of this when adding loads in the future. I'll have to keep an eye on this installation and see if everything gets sized per NEC. That's about all I can do about it!
o
So Article 240.4(F) & 240.21(C)(1) both address 4 wire systems and not using primary protection for secondary conductors. So then 240.21(C)(2)(1) says if less than 10 feet ect. that secondary OCD not required. Does this include 4 wire systems? If not then where exactly would the OCD be located? The panel has an OCD and is probably 7 feet from secondary termination. It isn't real clear about this as it seems to contradict. I have read numerous posts on this and find it is confusing to many.
So Article 240.4(F) & 240.21(C)(1) both address 4 wire systems and not using primary protection for secondary conductors. So then 240.21(C)(2)(1) says if less than 10 feet ect. that secondary OCD not required. Does this include 4 wire systems? If not then where exactly would the OCD be located? The panel has an OCD and is probably 7 feet from secondary termination. It isn't real clear about this as it seems to contradict. I have read numerous posts on this and find it is confusing to many.
240.21(C)(2)(1) DOES NOT say secondary OCPD is not required. It says conductors must have an ampacity a) not less than the calculated load and b) the supplied device or OCPD rating. The part in "b)" seems to say no OCPD is required, but what it really means is that where conductors are protected by the primary, they must still have an ampacity not less than the supplied device. ...or the OCPD rating when not protected by the primary.
What it amounts to in the example you gave is as I said earlier, the secondary conductors must have an ampacity not less than the 100A panel they supply.
I was reading the NEC handbook on Article 240.21(C) and it said that 1 thru 6 permit secondary conductors without an overcurrent protective device at the point the secondary conductors receive their supply under one of the conditions. That's where I got that secondary OCD wasn't needed. I guess it is referring to location of the OCD. I'm just trying to understand if it actually needs to be secondary protected by Code requirement. Which would mean 125% on the secondary side and to me that would have to be located before the 100 amp panel. A 70 amp OCD. I'm also good with just sizing the secondary like you said and sizing primary side at 125% max. I've looked through Mike Holt understanding the NEC plus in the Handbook and on different forum posts. Just drives me crazy trying to find the answer. Basically does 240.4(F) and 240.21(C)(1) mean that 4 wire always need secondary OCPD. I really appreciate the information and patience!
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Yes, The title of 240.21 is "Location in Circuit." 240.21(C) tells you where the location of the OCPD can be for transformer secondary conductor protection.
Protecting the secondary conductors doesn't necessarily mean 125% on the secondary. If the transformer primary is protected at 125% (or next size up), then the transformer secondary doesn't need protection, then the transformer secondary conductors can be protected at any level you want. What throws people off is that when the transformer is required to secondary protection, that secondary OCPD also protects the secondary conductors. In other words, the OCPD required by 450.3(B) also provides the protection required by 240.21(C).
Also, the secondary conductor OCPD doesn't need to be located before the panel. The panel MCB can serve as the required OCPD from 240.21(C). You must comply with the conductor length requirements in 240.21(C), though.
Yes, 240.4(F) and 240.21(C)(1) mean that a 4 wire secondary will always need a secondary OCPD for conductor protection.
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