Transformer protection less than 9Amps.

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lquadros

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Sections 450-3(b), 725-21(a)(1)] and Section 240-3 (f) and 240-21(c)(1)

1. Transformer secondary can be loaded enough (167%) that they can burn. (Door bell circuit). How is this handled?

2. There is a thought that larger than FLA for primary protection for transformers is done to provide inrush currents. Is this correct?

I do understand the conductor protection part. What about transformer itself? Does it safely withstand higher full load currents?

Sorry, I started this thread in "electrical calculations and engineering? - I am continuing it here. I think this is the right place.

I Thank all in advance for your replies.
 
I always have thought of it this way.
The transformer only is subjected to the load on the secondary side. When one determines the load to be served the the transformer can be selected. Then a secondary OCPD is applied.
So, the transformer is actually protected from overload by the secondary OCPD.
Then the primary device? As far as overload protection for the transformer the secondary OCPD has already covered that. But what if the transformer just plain fails, the winding smoke, fault, etc. I always have looked at the primary OCPD as simply short circuit protection which is there to take the transformer off line should it fail.
The only time that I consider the pri OCPD as providing overload protection is when the NEC allows it such as with a delta/delta transformer or a single phase transformer with a 2 wire secondary in cases where a secondary OCPD is not required.
 
Purpose of OCPD - clarification

Purpose of OCPD - clarification

Thanks for the reply templdl,

Here is an answer from Mike Holt in one of his newsletters.

Primary Protection [Sections 450-3(b), 725-21(a)(1)].

Control circuit transformer having a primary current rating:

Where the transformer has a rating less than 2 amperes, the overcurrent protection on the primary must be set at not more than 300 percent of the rated primary current.

Where the transformer has a rating between 2 and 8.99 amperes, the primary protection should not exceed 167% of the primary rating.

Where the primary current is greater then 9 amperes, the primary protection should not exceed 125% of the primary rating.

Secondary Protection. Section 240-3 (f) and 240-21(c)(1) identifies that the secondary conductors of a single-phase transformer having a 2-wire (single-voltage) secondary is considered protected by the primary overcurrent protective device, provided the primary protection is in accordance with Section 450-3.

Now lets say we have a 1KVA 480v primary and 110v secondary 1ph. transformer. The FLA would be 1000/480=2.08A at pf=1. This transformer can be potentially loaded above its FLA and can burn, due to oversized OCPD selection. :?

Why is it done this way? Is this for inrush currents?

So am I correct in assuming that
1.The OCPD is only used to protect the conductors against short circuit and/or overload and not protect the transformer itself. :?:

2. In case of protecting switching power supplies or any electrical devices - "The OCPD only protects conductors against overload and/or short circuit and does not protect the device itself that it is connected to". :?:
 
"Transformer secondary can be loaded enough (167%) that they can burn. (Door bell circuit). "
Door bell transformers are impedance protected. The impedance of the windings prevents overload.
 
lquadros,
Inrush currents do go up as the transformer gets smaller.
The 1kva transformer example you had referred to, did you say there was both primary and secondary protection?
Normally when we supplied a 1kva 480-120 CPT in an MCC bucket it had a (2) 10A KTKR fused primary (my factory spec sheet shows 20a max allowed) and a single 15a FNM for the secondary.
So what protected the transformer from overload? The secondary fuse.
That primary 10a fuse takes the transformer off line should the transformer fail.
I agree that 167% seems a bit high from a theoretical point of view but if it were a problem the NEC would change the "plays."
Dave
 
protection.

protection.

templdl,
I do not intend to use secondary protection if I do'nt have to.

By the way, do you agree with my second statement? Afterall, the breakers or fuses can always be upsized to the next higher available value. In that case do they protect the device from failing internally :roll: ? I do not think so.
 
From a theoretical point of view one would be it to be obvious that 167% would be too high to provide adaquate OCP. However I would like to believe that this number was determined to provide the OCP required. I would like to see how 167% was arived at though. Being from the electrcal industry myself, 3 years as a dry type distribution transformer application engineer, I have not found that the 167% rule causes a tranformer failure.
 
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