Transformer Feeder Length

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xformer

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Dallas, Tx
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Master Electrician
Hello all, Is there a calculation for a transformer's maximum feeder length? Or is it just assumed that 240.21(B)(1) takes over?
 
I think what the OP is asking, is about the the feeder (secondary tap conductors) from the transformer to the first OCPD.

If I am mistaken and they are only asking about the conductors that are feeding the primary of the transformer, than as David stated.
 
I was trying to calculate a 1000' feeder to a 240/120V 3KA Transformer. Probably Thinking too hard about. Considering Vd.
 
xformer said:
I was trying to calculate a 1000' feeder to a 240/120V 3KA Transformer. Probably Thinking too hard about. Considering Vd.
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Not sure what 3KA means. Depending on primary voltage, I would definitely consider voltage drop.
 
I figured. So you just have a 240volt 1000’ run to a transformer that steps down to 120volt?
 
If you were to base your 240volt 1000’ conductors on the full capacity of the transformer I would use at least #3awg.

kcmils=2xKxIxD/Volts dropped

now just using the calculated load (500w) for voltage drop shows you could use #10awg.

Numbers figured on 3% voltage drop.

Now someone who knows more than me might be able to answer wether the inrush current of transformer powering on will have any big impacts.
 
Use the transformer taps to help adjust for voltage drop; gets you a 5% bump on the 120V side; in other words the voltage on the 240V side could be as low as 228V when it gets to the transformer, the taps will get you back to 120V on the LV side.
 
With ~540 Watts (includes transformer losses 0.9 PF) the in primary current will be 2.5A the feeder will operate at temperature only slightly above ambient. Adjusting the wire resistance for 45 degree operating temperature will produce results as follows:

For # 12 CU or # 10 AL ~ 3.4 % voltage drop

For or # 10 CU or # 8 AL ~ 2.15 % voltage drop

Since primary breaker should be no more than 15A,2p both of the above are possible, however, ground fault at transformer location will produce only 52A (assuming you provide #6 bonding conductor) that would delay breaker operation beyond 5 seconds. The site and the 3 kVA transformer local ground may improve the time, if really good – but typically the path via both grounds is over 10 Ohm. And since livestock is particularly sensitive to even small step voltage (don’t have shoes :=) we do not want to have currents flowing in earth anyway.

2 # 10 CU + #6 CU bonding conductor will provide 77 A cct. Which should clear fault under 5 seconds.

#10 AWG CU - resistance = 1.12 Ohm/1000ft for conductor temperature of 45°C

#6 AWG CU - resistance = 0.443 Ohm/1000ft for conductor temperature of 45°C

A single phase to ground fault 120 V / (1.12 + 0.443) Ohm =~ 77 A (98% of all faults are phase to ground)

You may find precise data on wire resistance at particular operating temperature using the Voltage drop calculator found at https://www.mc-group.ca/voltage_drop_calculator.htm It is free for the data you may need.

I am not sure what the bonding requirements in your jurisdiction are but #6 CU is pretty common.

If you do a lot’s of work on farms you may find this link useful.

https://esasafe.com/assets/files/esasafe/pdf/Electrical_Safety_Products/Bulletins/10-23-3.pdf
 
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