480V to 240/120V single phase

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hhsting

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I have 15kVA dry type 480V single phase to 240/120V single phase transformer.

Following:

1. Does the primary breaker protect the secondary conductors or does it need to have secondary main breaker?


2. Is the transformer separately derived need grounding electrode or not?

3. Would rules of 450 apply sizing primary breaker and secondary breaker?
 
1. Read 240.21(C)(1).
2. Read the definition of a separately derived system in 100 and requirements in 250.30.
3. Read 450.3 and 450.3(B).
 
1. How many wires are in your transformer secondary? The code referenced states "Single phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device." Is there still something in that requirement that you don't understand?
2. A separately derived system is "An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections." Do you have a direct connection, other than grounding and bonding, between the primary and secondary of your transformer?
3. 450.3 states, "Overcurrent protection of transformers shall comply with 450.3 (A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit." What makes you believe that this would not apply to your transformer?
 
d0nut said:
1. How many wires are in your transformer secondary? The code referenced states "Single phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device." Is there still something in that requirement that you don't understand?
2. A separately derived system is "An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections." Do you have a direct connection, other than grounding and bonding, between the primary and secondary of your transformer?
3. 450.3 states, "Overcurrent protection of transformers shall comply with 450.3 (A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit." What makes you believe that this would not apply to your transformer?
Click to expand...

1. There are two wires shown but the secondary conductors feed 3 wire 240/120V single phase panel board. The panelboard does have brrakers where neutral is switched. Can you have 2 wires feed the panelboard? I think that is a mistake should be 3 wires two hot one neutral??

2. That i dont know i dont have internsls of transformer wiring. What is it typically for this type of transformer?

3. Well the part transformer shall mean transformer. Thats not really clear
 
"Yea. Whats wrong? Well the downstream panelboard is feeding gas dispensers, gas tank pumps, lights, air compressor"

"1. There are two wires shown but the secondary conductors feed 3 wire 240/120V single phase panel board. The panelboard does have brrakers where neutral is switched. Can you have 2 wires feed the panelboard? I think that is a mistake should be 3 wires two hot one neutral??"

Just curious, find it interesting thank you.
 
Are you doing plans review or designing ?
240/120 would require L-L-N & appropriate grounding.
240.21(C) fpr secondary conductors, 450.3 for transformer protection, 250.30 for grounding, 408.36 will also play apart
 
hhsting said:
I have 15kVA dry type 480V single phase to 240/120V single phase transformer.

Following:

1. Does the primary breaker protect the secondary conductors or does it need to have secondary main breaker?


2. Is the transformer separately derived need grounding electrode or not?

3. Would rules of 450 apply sizing primary breaker and secondary breaker?
Click to expand...

1. There is a very limited selection of qualifying topologies that enable you to protect the secondary conductors indirectly by the primary OCPD. Anything with a center-tap connected to your loads, or a WYE system, does not qualify, because there is a possibility that an overload on the secondary will be in the "blindspot" of the primary OCPD.

The qualifying topologies are a 2-wire to 2-wire transformer, or a delta-delta 3-wire on both sides, the overcurrents are going to line up winding-to-winding, and not get re-distributed, which is why the primary OCPD can indirectly protect the secondary conductors. Given a 480V primary, and a 240V secondary for a qualifying topology, a 100A breaker on the primary effectively acts as if it were a 200A breaker on the secondary. Another example: 277V to 120V 2-wire to 2-wire 1-phase transformer. In this case, a 100A breaker on the 277V primary, would effectively protect the secondary conductors at 231A. It's the voltage ratio that governs this calculation

2. Most transformers produce a separately-derived system, because transformers can only transmit a change in voltage across the isolation through the magnetic coupling. They cannot transmit absolute voltage, if the two sides are electrically isolated from one another. You need to bond to ground and to use a grounding electrode connection, if you want to re-establish the secondary as a grounded system of voltages.

The kind of transformers that do not produce a separately-derived system, are called autotransformers. The secondary of an autotransformer shares a reference to the same grounded conductor as the primary.

3. Article 450 applies for protecting the transformer itself. Article 240.21(C) is for protecting the transformer secondary conductors.
 
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