transformer secondary & primary OCPD
- Thread starter Ponchik
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- New Jersey
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- Journeyman Electrician
Mostly because it's required with just a few exceptions. Typically a 3Ø transformer which is Delta/Wye requires both. Look at the notes to T450.30(A).
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- Licensed Electrician
With primary only protection on a delta/wye transformer an overload or fault on the secondary may not trip the overcurrent device.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
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- Electrician
I'm confused. We are talkin about the transformer itself here not the conductors right? I don't see any restrictions with primary only protection as long as we comply with the ocpd percentages given in 450.3 (b). What is 450.30 (a) I don't have that in my 2011 (job site beater codebook), was there a change?
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- Licensed Electrician
But you can't really use a transformer with no conductors. 240.4(F) tells you that you need secondary protection.
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- New Jersey
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- Journeyman Electrician
I agree, it's sort of a play on words but the end result is protection on both sides which is what the OP is asking.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
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- Electrician
Ok y'all were talking about conductors, got it. It's not entirely an academic differentiation. One could have big conductors and ocpd for VD or maybe something that was existing, and that wouldn't meet the 450.3 requirements.
If we are talking under 1000 volts primary, it's designers choice... really [Table 450.3(B)].
What is not designer's choice is when secondary conductor or panelboard protection is required to be on the secondary side [240.21(C)(1), 408.30(B)].
Where protection is required on the secondary side, many designers choose to use primary and secondary protection versus primary only.
The primary is 240V and secondary is 480V with a 45KVA transformer. Is the primary fuse required on the primary side?
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Electric-Light
Senior Member
The answer was given to you. Do so within options allowed by art 450 while satisfying 240. It gives you more than one options to satisfy them.
Where else would you put the primary fuse(s)? :slaphead:
A transformer always requires, at the very least, primary OCPD (on the primary side :lol
.PS: If you are back feeding a transformer with a PV system, for example, you are required to evaluate OCP both ways, i.e. each side as a primary [705.30(B)].
Where else would you put the primary fuse(s)? :slaphead: A transformer always requires, at the very least, primary OCPD (on the primary side :lol
PS: If you are back feeding a transformer with a PV system, for example, you are required to evaluate OCP both ways, i.e. each side as a primary [705.30(B)].
I know. Wasn't that a dumb question. :ashamed1:
Electric-Light
Senior Member
Where else would you put the primary fuse(s)? :slaphead:
A transformer always requires, at the very least, primary OCPD (on the primary side :lol
PS: If you are back feeding a transformer with a PV system, for example, you are required to evaluate OCP both ways, i.e. each side as a primary [705.30(B)].
Wouldn't that only be true if the export capacity is physically able to break the transformer or conductor limits? It's possible if there's a short on the utility side and there's 9,000 lbs of rotating mass in a large mechanical inverter but there's practically no energy stored in the static converter and none in the panel.
Analyzing each side as primary for primary OCP is a requirement. The whole point is that neither side is oversized. Neither side can exceed 125% of the transformer rated current. This is for nominal operation, not a fault condition.
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