I replaced a 480 Y transformer today it had over current protection on the primary side but no protection on the secondary side . The secondary side side off the transformer is supplying 120/ 208 / Y main lug panel . Do I need over current protection on the secondary side of the. Transformer ? The main distribution panel is existing equipment .
Over current protection on the secondary side of transformer
- Thread starter jkrauss
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I found 240.21 c the way I read it is if I meet one of the six applications I will not need over current protection on the secondary
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- New Jersey
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- Journeyman Electrician
You need to comply with 240.21(C)(6) which means you need an OCPD within 25'.
I understand that you have a transformer with a 208y/120v 3 ph4w secondary. As such you do have a neutral and it is because of that neutral that any primary protection that you would provide will not provide secondary over current protection. That goes for 480y/277v 3ph4w, 240d/120v 3ph4w, and 120/240v 1ph3w, the more common systems. It is imperative that you provide secondary protection.
480v and 240v 3ph3w and 480v, 240v or 120v 1ph2w w/o neutrals do not require secondary protection provided that the primary OCPD is sized correctly. You must pay attention to sizing that primary OCPD.
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- Placerville, CA, USA
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- Retired PV System Designer
I do not understand your reasoning on this.
Any scenario that I can think of quickly would result in a high current in one or more of the primary phases whenever there is overcurrent in the secondary regardless of whether that overload involves the neutral or not.
Or do you think that you could get circulating currents in the secondary under the right combination of non-linear loads without corresponding currents in the primary?
Yes, this was something that I didn't understand at first either The reasoning is that should there be a line to neutral overload therimary OCPD would never see it because it is looking at 3ph3w or in the case of a 1ph3w secondary a 2p primary OCPD would never see a line to neutral overload.
line to line current will in no way respond to a secondary line to neutral overload. It's just a proportion thing as we would like to think it would be. If you have 3 phases on the primary which one of the lines would respond to a L-N overload on the secondary?
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If you are going to allow for an unbalanced steady state load (not inrush or short circuit), then it is true that you could load one wye leg to 1/3 the rating of the transformer without getting close to tripping the primary side OCPD. But that is only because that wye load would be fed equally by the two primary phases that meet at the corresponding point to the loaded wye phase wire. The energy has to come from somewhere, and that somewhere is the primary.
If you do some simple equivalent circuits, the flux in the transformer core corresponding to the single phase wye load on the secondary will be balanced by the flux from two primary windings. Because of symmetry, there should be no net current in the third primary winding.
IMHO, the possibility of unbalanced loads in general is one glaring weakness of relying on primary overcurrent protection only, but you can configure just as bad a situation with an unbalanced load on a secondary delta as on a secondary wye.
With the exception, as I said, of non-linear loads. But since the wye connection will still not have OCPD on the neutral, secondary OCPD is not guaranteed to help!
When you are saying is that it isn't guaranteed that OCPD that line on lets say an 'A' ph that a L-N overload load won't respond to the L-N current?
And I agree with you flux reference and that is why the primary is oblivious to a L-N over loads.
This has been a great discussiion.
Oh, I just looked at the clock and its getting late.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
What I am saying is that whether the secondary is wound delta or wye and whether the loads are inherently line-to-line or line-to-neutral, the overloads will measure current in the phase wires, not currents running specifically from one phase wire to another. And the current in the neutral will not be separately measured.
They do not make separate CBs for use with three phase wye and three phase delta (unless you are talking about GFCI, that is.)
Goodnight!
You brought a delta secondary, how will there be a L-N in a delta?
To clarify, this conversation would be for common application such as D-D and D-Y not with with wye primaries.
With D-D the secondary can be protected with a primary OCPD because the current is proportional except with a 3ph4w common 240/120 delta.
With a 480y/277 and 208y/120 you are not guaranteed that the primary and secondary currents will be proportional because of unbalanced L-N load. As an extreme example it is entirely possible to have a 100% L-N overload on one phase and it will never be seen by primary OCPD.
There are three issues: secondary protection, secondary feeder protection, and panelboard protection.
You need overcurrent protection on the secondary side to protect the main lug panelboard. The panelboard should have been had main breaker or be protected by a fused or circuit breaker disconnect at the transformer.
secondary feeder protection is addressed by 240.21 (C)(1) - (C)(6). How long are the secondary feeders?
Depending on the primary protection size, the secondary would not
I posted something similar a few years ago "ART 450 Secondary Protection confusion" after I encountered the same installation you described.
You need overcurrent protection on the secondary side to protect the main lug panelboard. The panelboard should have been had main breaker or be protected by a fused or circuit breaker disconnect at the transformer.
secondary feeder protection is addressed by 240.21 (C)(1) - (C)(6). How long are the secondary feeders?
Depending on the primary protection size, the secondary would not
I posted something similar a few years ago "ART 450 Secondary Protection confusion" after I encountered the same installation you described.
JDBrown
Senior Member
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- California
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- Electrical Engineer
That's mostly correct. However, NEC 408.36 Exception says that, when the transformer is feeding a panelboard, the overcurrent protection can only be on the transformer's primary side if the installation meets 240.21(C)(1).
Since you've got a wye secondary, the secondary conductors are not considered to be protected by the primary OCP. You would be okay if your panelboard had a main circuit breaker, but since it's a main lug panel, you will still need an overcurrent protective device "having a rating not greater than that of the panelboard" (NEC 408.36) "located on the secondary side of the transformer" (NEC 408.36(B)).NEC 240.21(C)(1) said:
Adding to that, I have been told that the Six Disconnects Rule applies to SDSs, which would rule out using a MLO panel with more than 6 breakers on the secondary of a transformer. But I've been unable to find an actual code reference for this, so it might be an urban myth. Or I might not be looking in the right places.
in NEC 2008, six disconnect rule is 230.71. 408.36(b) requires ocp on secondary side of transformer. before 2008 when lighting and appliance and power panelboards were defined separately, you may have been able to have a mlo power panelboard, but in this situation, the required panelboard overcurrent protection is missing.
JDBrown
Senior Member
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- California
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- Electrical Engineer
That's the one I've seen as well, but it looks like it only applies to services. I've been told that it applies to separately-derived systems as well, but I've never been able to find anything that says that.
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