SCPD and Wire Sizing for Single-Phase Center Tapped Transformer

[kloc5]

Hello,

We are using a 10KVA single-phase 480VAC to 120VAC transformer for our automation equipment. However, we are center-tapping the secondary so that we can get 240VAC across both hot legs for some devices, and balancing the 120VAC devices between the two 120V circuits.

My question is about sizing the OCPD for the secondary. Since we will have two hot legs, do I treat this as two 5KVA transformers to 120VAC and size the fuses accordingly (60A fuses I believe)? Do the wire sizes need to be sized as if each hot leg had the total possible current if it was completely unbalanced (83.3A)? What about the neutral leg wire sizing, since it is the return wire for both hot legs, but still the maximum current possible remains 83.3A if I'm not mistaken.

Thanks for your help!

 

[gar]

220602-1616 EDT

kloc5:

I would suggest that a center tapped transformer secondary is never designed to carry full rated power of the transformer on only 1/2 of the secondary for very long.

Thus, maximum secondary current for 100 % duty cycle is based on total secondary voltage times secondary current anywhere in the secondary. For your case 41 A is the maximum load current.

Think about the problem. If you load 1/2 of the secondary with 82 A and the secondary wire has been designed as 41 A being the maximum current, then you pass 82 A thru 1/2 the secondary and the power dissipation in that 1/2 secondary winding will be 4 times the design value and you will certainly burnout that half of the secondary.

.

 

[jim dungar]

Chose your protection based on 10kVA at 240V. Use a 2 pole switch or breaker.

 

[kloc5]

220602-1616 EDT

kloc5:

I would suggest that a center tapped transformer secondary is never designed to carry full rated power of the transformer on only 1/2 of the secondary for very long.

Thus, maximum secondary current for 100 % duty cycle is based on total secondary voltage times secondary current anywhere in the secondary. For your case 41 A is the maximum load current.

Think about the problem. If you load 1/2 of the secondary with 82 A and the secondary wire has been designed as 41 A being the maximum current, then you pass 82 A thru 1/2 the secondary and the power dissipation in that 1/2 secondary winding will be 4 times the design value and you will certainly burnout that half of the secondary.

.

Ok, so when the 10K transformer is NOT center-tapped, the two secondary windings are connected in parallel and therefore share the 83A between the two windings. When we ARE center-tapping the transformer, the two secondary windings are connected in series (with the center-tap in the middle) and therefore if the load is unbalanced and you have 83A, it was not designed to handle this, correct?

As for the conductor sizing for a center-tapped transformer, I'm assuming the hot wires should be sized to be protected by these 60A fuses, and the neutral to handle the current for both windings? However, if I'm not mistaken, the currents in the neutral from each half of the center-tap cancel therefore I could use the same wire size as the hot wires, correct? Then the ground wire would be based on Table 250.122 for the secondary fuses (10AWG), correct?

Thanks!

 

[gar]

220603-1334 EDT

kloc5:

Starting from scratch.

In an isolation transformer with more than one winding, and a single primary winding, the distribution of power dissipation within the windings should be such that the power dissipated in the windings producesis approximately a uniform temperature rise that is about the same anywhere in the transformer when fully loaded. The absolute temperature should be less than allowed for the wire insulation for desired average life time.

This does not mean you can properly protect a transformer with a fuse in series with a winding that is greater than a value that will protect that or other windings.

So, if you only protect a transformer on the primary side with a fuse based on the total power capability of the transformer, and load 1/2 of the secondary to full power capability of the transformer primary, then you will not protect that secondary from burnout.

Roughly you treat each half of your 240 V secondary as a 5 kW 120 V secondary which means the maximum current allowed is 41 A, and really should be fused at some lower level than 41 A.

For a balanced resistive load on the secondary there is no neutral current.

If I load 1/2 of the secondary with a capacitive current of 41 A, and nothing on the other 1/2, then I see the same current in both the neutral , and the one loaded hot line. Same is true for a pure inductive load.

Now if 1/2 of the secondary has said capacitive load, and the other secondary 1/2 has said inductive load, then you will have 2 times the current in the center tap conductor of either hot line.

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