To make a short story long...
OT:Transformer Conductor Sizing Question
First, the OP is in the United States, so we can exclude the replies from the International forum constituents. Here in the United States if installing transformers in Residential, Commercial, or Industrial premises (buildings), then the National Electrical Code NFPA 70 MUST be adhered to as a minimum.
If the transformer is installed on an industrial machine, NFPA 70 is not the code, but following NFPA 70 minimums will ensure sufficiency.
Many novice laymen making electrical installations often make an egregious, incorrect assumption that overcurrent protection devices (fuses circuit breakers) are strictly sized directly to conductor size in all instances… THIS IS NOT CORRECT for transformer installation.
Most US based novice laymen garner their simplistic correlation of overcurrent protection sizing locked with conductor sizing based on residential application. In the US, most domestic residential homes are equipped with multi-outlet general purpose circuits. Upon building construction, it will NEVER be known how much dynamic load will be applied by the end-user (resident) to the multi-outlet circuits. The strategy put forth by NFPA 70 board was to write a code where 14 awg multi-outlet circuits SHALL be protected by a 15 amp overcurrent device, and 12 awg circuits SHALL be protected with a 20 amp overcurrent device. These rules will open the circuit when the end user possibly overloads the circuit with adding to many appliances to any individual circuit. The opening of the overcurrent device will prevent a FIRE. That is why electrical codes in the US are written by the National Fire Protection Agency (NFPA).
A Transformer is NOT a multi-outlet general purpose circuit, so the novice laymen rules are not valid.
A Transformer is a FIXED, STATIC engineered load, and has an entire section in the NEC NFPA 70 to allow the design engineer to use simple math and percentage rules to design a SAFE, SUFFICIENT, and economical sizing of conductors, and overcurrent protective devices.
NFPA 70 Article 450 is what is used for minimum Transformer requirements. A basic summarization of the code is:
1.Size the conductor to serve the transformer primary ampacity, size the conductor to server the secondary ampacity.
2. PROTECT the transformer by sizing the secondary overcurrent protective device at exactly 125% of secondary ampacity. (Note transformer protection comes from secondary protection, transformer protection does NOT come from the primary overcurrent device)
3. Size the Primary conductors at 125% of the transformer kva rating
4. Size the Secondary conductors at 125% of the transformer kva rating
5. Size the Primary overcurrent protective device at 250% of transformer kva rating (regardless of conductor ampacity)
My take on the OP post… 5kva, 480v single phase primary, 240/120v secondary
Primary Calculations:
5000 va divided by 480 = 10.42 amps fla multiplied by 125% =13 amps primary minimum circuit 14 awg
Primary fla 10.42 amps multiplied by 250% = 26 amps (next allowable trade size breaker is 30 amps)
SO Here is a perfectly code compliant installation with 14awg conductor connected to a 30amp breaker.
Secondary Calculation:
(Note if the transformer were to be wired at 120v 2 wire only you use 120v as a divisor, but due to the fact that the secondary is going to be wired as a 240/120 secondary, the phase conductor votage 240 volt is used for the calculation).
5000 va divided by 240 – 20.84 amps fla multiplied by 125% = 26 amps MAX overcurrent device 30 amps
26 amps minimum circuit conductor size 10 awg.
Wire size for secondary, I would use 10 awg on the phase conductors and the neutral conductor for the 240v/120v secondary of this transformer.
OT:Transformer Conductor Sizing Question
First, the OP is in the United States, so we can exclude the replies from the International forum constituents. Here in the United States if installing transformers in Residential, Commercial, or Industrial premises (buildings), then the National Electrical Code NFPA 70 MUST be adhered to as a minimum.
If the transformer is installed on an industrial machine, NFPA 70 is not the code, but following NFPA 70 minimums will ensure sufficiency.
Many novice laymen making electrical installations often make an egregious, incorrect assumption that overcurrent protection devices (fuses circuit breakers) are strictly sized directly to conductor size in all instances… THIS IS NOT CORRECT for transformer installation.
Most US based novice laymen garner their simplistic correlation of overcurrent protection sizing locked with conductor sizing based on residential application. In the US, most domestic residential homes are equipped with multi-outlet general purpose circuits. Upon building construction, it will NEVER be known how much dynamic load will be applied by the end-user (resident) to the multi-outlet circuits. The strategy put forth by NFPA 70 board was to write a code where 14 awg multi-outlet circuits SHALL be protected by a 15 amp overcurrent device, and 12 awg circuits SHALL be protected with a 20 amp overcurrent device. These rules will open the circuit when the end user possibly overloads the circuit with adding to many appliances to any individual circuit. The opening of the overcurrent device will prevent a FIRE. That is why electrical codes in the US are written by the National Fire Protection Agency (NFPA).
A Transformer is NOT a multi-outlet general purpose circuit, so the novice laymen rules are not valid.
A Transformer is a FIXED, STATIC engineered load, and has an entire section in the NEC NFPA 70 to allow the design engineer to use simple math and percentage rules to design a SAFE, SUFFICIENT, and economical sizing of conductors, and overcurrent protective devices.
NFPA 70 Article 450 is what is used for minimum Transformer requirements. A basic summarization of the code is:
1.Size the conductor to serve the transformer primary ampacity, size the conductor to server the secondary ampacity.
2. PROTECT the transformer by sizing the secondary overcurrent protective device at exactly 125% of secondary ampacity. (Note transformer protection comes from secondary protection, transformer protection does NOT come from the primary overcurrent device)
3. Size the Primary conductors at 125% of the transformer kva rating
4. Size the Secondary conductors at 125% of the transformer kva rating
5. Size the Primary overcurrent protective device at 250% of transformer kva rating (regardless of conductor ampacity)
My take on the OP post… 5kva, 480v single phase primary, 240/120v secondary
Primary Calculations:
5000 va divided by 480 = 10.42 amps fla multiplied by 125% =13 amps primary minimum circuit 14 awg
Primary fla 10.42 amps multiplied by 250% = 26 amps (next allowable trade size breaker is 30 amps)
SO Here is a perfectly code compliant installation with 14awg conductor connected to a 30amp breaker.
Secondary Calculation:
(Note if the transformer were to be wired at 120v 2 wire only you use 120v as a divisor, but due to the fact that the secondary is going to be wired as a 240/120 secondary, the phase conductor votage 240 volt is used for the calculation).
5000 va divided by 240 – 20.84 amps fla multiplied by 125% = 26 amps MAX overcurrent device 30 amps
26 amps minimum circuit conductor size 10 awg.
Wire size for secondary, I would use 10 awg on the phase conductors and the neutral conductor for the 240v/120v secondary of this transformer.
