Breaker installation with 75kva transformer
- Thread starter LisP2921
- Start date
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- Location
- Massachusetts
Yes the panel needs a breaker and so do the secondary conductors.
Interesting that inquiry omited advising what the secondary configuration was. One would assume that it is a 480Y/277 or more commonly a 208Y/120 if in fact it was a 3ph transformer. as such secondary protection is required. If it was a 240 delta secondary it is not required as I understand the NEC requirements.
Again, the question is abbiguous in that it cold be a 1ph transforer and if so wold most likely have a 120/240 1ph3w secondary which would require secondary protection. If the secondary was 1ph2w which is somewhat uncommon then it would not.
It sould be noted that if secondary protection is not required the primary OCPD must be sized accordingly.
As a former application engineer I have become very cautious about answering ambiguous questions and questions that are asked in a way that the person who is asking the questions is trying to get an answer which they want to hear which may be the incorrect answer. I have become very gun shy.
Again, the question is abbiguous in that it cold be a 1ph transforer and if so wold most likely have a 120/240 1ph3w secondary which would require secondary protection. If the secondary was 1ph2w which is somewhat uncommon then it would not.
It sould be noted that if secondary protection is not required the primary OCPD must be sized accordingly.
As a former application engineer I have become very cautious about answering ambiguous questions and questions that are asked in a way that the person who is asking the questions is trying to get an answer which they want to hear which may be the incorrect answer. I have become very gun shy.
Volta
Senior Member
- Location
- Columbus, Ohio
You might be thinking straight 480, not 480Y/277 240.4(F). The wye-delta needs secondary protection.
At 480 volts / 75 kVA, it is probably not single-phase.
- Location
- Massachusetts
To my point, trying to address an ambiguous qustion is a formula for an accident. At this point the transformer configurtion is only conjecture. I stay away from "probably" as a basis for an answer and become very concerned that the person who is asking the question may not understand electrical distribution and protection and with all due respect has an appreciation os safe electrical practices and procedures. Otherwise the transformer rating would have been completely described with more than the KVA and only one response to his question would be required.
For the technicians safety it is extremely important that they have an appriciation and inderstading of that they are trying to do. If I don't feel comfortable I decline to aid and assist a posible accident waiting to happen.
For the technicians safety it is extremely important that they have an appriciation and inderstading of that they are trying to do. If I don't feel comfortable I decline to aid and assist a posible accident waiting to happen.
kwired
Electron manager
- Location
- NE Nebraska
Sometimes it is not that they are trying to get the answer that they want to hear but just don't know what questions to ask, or what information they still need to provide to get the correct answer. If they did know they probably would not have a question to start with.
Instead of giving an answer that may be wrong or not answering at all the proper response is to ask more questions.
What is voltage/phase of secondary circuit?
Does it supply line to neutral loads or just single phase two wire or three phase three wire.
Is secondary ungrounded or grounded and which conductor if it is grounded.
What is setting of primary overcurrent device - especially if there is not to be a secondary device.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
It would be very plausible to have 480V, 2-pole (single phase) breaker feeding a 75KVA single phase transformer stepping the voltage down to 120/240V.
Not enough info in the OP to answer question as stated.:happyno:
But IMO the answer would be yes you need a breaker regardless, not because the NEC may or may not require it, but simply because it is a good practice and better design. The NEC is not a design guide, it is a safety manual. Quality, and well thought out designs will in most cases surpass the minimum NEC safety requirements.
Not enough info in the OP to answer question as stated.:happyno:
But IMO the answer would be yes you need a breaker regardless, not because the NEC may or may not require it, but simply because it is a good practice and better design. The NEC is not a design guide, it is a safety manual. Quality, and well thought out designs will in most cases surpass the minimum NEC safety requirements.
- Location
- Connecticut
- Occupation
- Engineer
There's certainly enough information in the OP to answer the question. Whether the transformer is single phase or three phase is irrelevant to the question as stated.
The secondary panel needs either a main breaker (or fuse) or it may be a main lugs panel with a breaker (or fuse) upstream of the panel on the secondary side of the transformer, per 408.36. The single phase/three phase discussion is a MacGuffin.
- Location
- Massachusetts
If it is single phase the primary breaker could serve all the functions of a breaker on the secondary side.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Good call:thumbsup:
- Location
- Connecticut
- Occupation
- Engineer
I disagree. I don't see how a c/b on the primary of a transformer could serve the function of providing overcurrent protection for a panelboard on the secondary side.
If it is single phase 2 wire, the primary breaker can function to protect the secondary conductors per 240.21(C)(1).
kwired
Electron manager
- Location
- NE Nebraska
I agree.
- Location
- Massachusetts
Can you cite a code section to back that position?
kwired
Electron manager
- Location
- NE Nebraska
How about 240.4(F).
This section does not address the panelboard but does address the secondary conductors essentially making them no different than a feeder when it comes to protection of the panelboard.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
75KVA with 480V 3ph pri and 208V 3ph sec
75KVA with 480V 1ph pri and 240V 1ph sec
The primary breaker in the 1ph graph would trip and protect a panel at 240V. Every instance may be different and coordination would need to be checked on a case by case basis. It could be very possible that the fault current would be low enough that the primary breaker, when adjusted to not trip on inrush of the transformer, is set higher than the fault current at the LV panel.
All in all, I would say that to avoid complications, it would be a good practice to utilize primary and secondary protection regardless of the transformer configuration.
75KVA with 480V 1ph pri and 240V 1ph sec
The primary breaker in the 1ph graph would trip and protect a panel at 240V. Every instance may be different and coordination would need to be checked on a case by case basis. It could be very possible that the fault current would be low enough that the primary breaker, when adjusted to not trip on inrush of the transformer, is set higher than the fault current at the LV panel.
All in all, I would say that to avoid complications, it would be a good practice to utilize primary and secondary protection regardless of the transformer configuration.
lauraj
Senior Member
- Location
- Portland, Oregon
240.4(F)
(F) Transformer Secondary Conductors. Single-phase
(other than 2-wire) and multiphase (other than delta-delta,
3-wire) transformer secondary conductors shall not be considered
to be protected by the primary overcurrent protective
device. Conductors supplied by the secondary side of a singlephase
transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.
According to the above section, I've always taken it as a 3-phase delta-delta (single voltage) would be allowed to have a MLO secondary panel.
(F) Transformer Secondary Conductors. Single-phase
(other than 2-wire) and multiphase (other than delta-delta,
3-wire) transformer secondary conductors shall not be considered
to be protected by the primary overcurrent protective
device. Conductors supplied by the secondary side of a singlephase
transformer having a 2-wire (single-voltage) secondary,
or a three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary, shall be permitted to be
protected by overcurrent protection provided on the primary
(supply) side of the transformer, provided this protection is in
accordance with 450.3 and does not exceed the value determined
by multiplying the secondary conductor ampacity by
the secondary-to-primary transformer voltage ratio.
According to the above section, I've always taken it as a 3-phase delta-delta (single voltage) would be allowed to have a MLO secondary panel.
- Location
- Massachusetts
Volta
Senior Member
- Location
- Columbus, Ohio
Maybe not the exact intent of the section, but 408.36 (2008) seems to not disallow it.
"... This overcurrent protection shall be located within or at any point on the supply side of the panel board." Several exceptions follow.
So, allow as if there was a 75 kVA, single-phase transformer, 480:240 volt, two-wire primary, two-wire secondary. We could then have a:
156 amp primary, protected at 175 amps. Or 200.
Secondary rating of 312, Let's say a panel board with a 300 amp bus. Or 200. No OCPD.
Illegal?
Strictly speaking, for most two-wire step-down situations it seems acceptable to this section, and 240.4(F) seems ok with it too.
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