NEC 240/450 conflict, transformers

[Caligula]

(2005 code) 240.21(C)(1) states that "multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device."

450.3(B) shows that you do not need secondary overcurrent protection for transformers if the primary is no larger than 125% of the transformer rating.

Does 450.3(B) only apply to non-multiphase transformers? 450 doesn't specify the table to apply to any specific type of transformer (except <600V).

If I have a transformer in room 'A', and I am serving panel(s) in another room 'B' in another part of the building, do I need to provide disconnects at the transformer, or is the MCB suficient of the served panels provided I do not exceed 125% on the transformer's primary?

Everyone seems to give conflicting answers on this.

 

[mpd]

art. 240 applies to the conductors, art. 450 applies to the transformer

 

[ron]

It is double speak IMO.
I consider that 450.3(B) only applies to non-multiphase transformers.
I generally use 3ph delta wye transformers, so I always have primary and secondary OCPD. Technically, there will be comments that the device on the secondary of the transformer only protects the conductors, but it is there on the secondary of the XFMR anyway.

 

[Caligula]

Can anyone tell me WHY this is?

If you have sized your conductors correctly, your secondary conductors are never going to carry more current than their capacity. If your secondaries are overloaded at 208V, that current will still show up as an overload on primary breaker on the 480V side.

Does this have to do with an overcurrent of X% at 208 being much a smaller overcurrent by % at the 480 side, thus taking potentially longer to trip per the time-current curves?

 

[jim dungar]

450.3(B) is about sizing the primary protection only. It has nothing to do with sizing protection for the secondary.

Very briefly 450.3(B) says:
The primary protection must not exceed 125% unless you have some specific secondary conductor protection.

The rules of 450.3 apply to all transformers, except as noted. It makes little difference if the output is: 3-phase or 1-phase, delta or wye, or single or multi-voltage. However, the rules of 240 are dependent on the construction of the transformer windings.

 

[augie47]

Can anyone tell me WHY this is?

My understanding is that on a two-wire to two wire or delta-delta the secondary and primary current are directly proportional whereas in a transformer with a midpoint secondary, the load on the primary may not be reflected in direct proportion on the primary so secondary conductor protection is needed.

 

[StephenSDH]

Delta-Wye a fault on the secondary, is reflected on two phases on the primary, plus the transformer impedance. So you might not trip the instantaneous trip on the primary breaker.

Center-Tap single phase transformer, if you have a Hot to Neutral fault on the secondary it will only show up as 1/2 the current on the primary and again might not trip the instantaneous trip.

 

[Cold Fusion]

Edit: I was typing while Stephen was posting. This is exactly what he is saying.

Can anyone tell me WHY this is?

If you have sized your conductors correctly, your secondary conductors are never going to carry more current than their capacity. If your secondaries are overloaded at 208V, that current will still show up as an overload on primary breaker on the 480V side.

Does this have to do with an overcurrent of X% at 208 being much a smaller overcurrent by % at the 480 side, thus taking potentially longer to trip per the time-current curves?

See if this is what you are asking. My example is single phase, but that is just to make the math easier (for me - I know you got it)

Example: Single phase xfm, 240V, 20A primary; 120/240V, 20A secondary

Case 1: Consider a 240V secondary load drawing 20A. The primary is drawing 20A. Primary and secondary are fine.

Case 2: Consider a 120V secondary load drawing 40A. The primary is drawing 20A. Primary is fine, secondary is overloaded.

The principle is the same for 3ph D-Y xfm, the math is a lot more complicated.

cf

 

[Caligula]

Edit: I was typing while Stephen was posting. This is exactly what he is saying.
See if this is what you are asking. My example is single phase, but that is just to make the math easier (for me - I know you got it)

Example: Single phase xfm, 240V, 20A primary; 120/240V, 20A secondary

Case 1: Consider a 240V secondary load drawing 20A. The primary is drawing 20A. Primary and secondary are fine.

Case 2: Consider a 120V secondary load drawing 40A. The primary is drawing 20A. Primary is fine, secondary is overloaded.

The principle is the same for 3ph D-Y xfm, the math is a lot more complicated.

cf

Wow, that is a great answer. Makes complete sense.

 

[StephenSDH]

Motors and transformers both have inrush when you start them up. Commonly size a motor breaker up 250%. Just like here you size the transformer up 250 if you have secondary protection.

Does anyone have trouble with breakers tripping when you put a 125% breaker on a transformer primary? I would think the inrush depending on the transformer you pick would give you trips at 125%, but never heard anyone complain.

Thanks, Steve

 

[pfalcon]

(2005 code) 240.21(C)(1) states that "multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device."

450.3(B) shows that you do not need secondary overcurrent protection for transformers if the primary is no larger than 125% of the transformer rating.

Does 450.3(B) only apply to non-multiphase transformers? 450 doesn't specify the table to apply to any specific type of transformer (except <600V).

If I have a transformer in room 'A', and I am serving panel(s) in another room 'B' in another part of the building, do I need to provide disconnects at the transformer, or is the MCB suficient of the served panels provided I do not exceed 125% on the transformer's primary?

Everyone seems to give conflicting answers on this.

Ow, some of these explanations make my head hurt.
450.3(B) does not tell you when you need or can use secondary protection. It only tells you the maximum OCPD values when you have such a circuit.

 

[Cold Fusion]

---Does anyone have trouble with breakers tripping when you put a 125% breaker on a transformer primary? I would think the inrush depending on the transformer you pick would give you trips at 125%, but never heard anyone complain.---

Limiting my response to small standard stuff, say 480/208, 30kva - 300kva:

I haven't had any trouble sizing the primary at 125%.

Now, if I were putting in a 480/208, 200ft run, and the wire was marginal for 125% primary OCP, I'd probably be looking at up sizing the wire and the primary CB - just cause there is no way to recover if this one decides to be the problem child of the decade.

CF

 

[Cold Fusion]

Ow, some of these explanations make my head hurt.---.

I understand that. The laws of man are a lot harder to understand than the laws of physics. It really gets tough when an AHJ thinks the NEC trumps the laws of physics.:-?

--450.3(B) does not tell you when you need or can use secondary protection. It only tells you the maximum OCPD values when you have such a circuit.

It tells you when you need it. You can also install it just because it is good design even if the NEC does not require it.

(Caution - Rant ahead. Delete before reading is absolutely permissible)

The NEC is not a design guide, nor a receipe for a safe, efficient, low maintenance installation that meets the customers specs. Take a look at 90.1.A, B, C - especially the "essentially free from hazards" piece.

The NEC is a list of minimums the state government believes one must follow while designing and installing a system. If the customer wants the system to actually be "safe, efficient, low maintenance, and meet the intended application specs - you are on your own. the NEC is not going to help.

(Thank you for listening - rant over)

cf