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I apologize for the basic question, but I just bought a transformer with the following specifications. On the input side, there are two black wires and two red wires. Here is a photo.

enter image description here And here the specs of the transformer. enter image description here

I have some doubts about the connection. I assume the two red wires should be connected to the L (live) and N (neutral) of the domestic power supply, and the black wires to ground. However, should both black wires be connected to ground, or just one of them?

For a connection in series will be this correct? enter image description here

Thank you!

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    \$\begingroup\$ Looks like dual primary windings to wire in series or parallel depending on your voltage (115 or 230). What is the line voltage where you live? \$\endgroup\$ Commented Sep 10, 2024 at 18:38
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    \$\begingroup\$ The primary windings are probably intended to be connected in parallel for 115 V operation and in series for 230 V operation. So the connection will be different depending on the mains voltage you have. \$\endgroup\$ Commented Sep 10, 2024 at 18:38
  • \$\begingroup\$ Where I live is 120V 60 Hz \$\endgroup\$ Commented Sep 10, 2024 at 18:43
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    \$\begingroup\$ We don't know what you bought and how to connect it. Either the transformer has proper specs like a data sheet with instructions about the wiring, or, we can guess things and it might blow up on you. You need to be precise what you bought and from where, was there any data or instructions? Generally, this is not a support site for random components bought from online stores that are unable to give details on what they sell. \$\endgroup\$ Commented Sep 10, 2024 at 18:54
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    \$\begingroup\$ @EdinFifić No it isn't. Even if we only assume the wiring colours and winding polarity, it would be incorrect for OPs 115VAC mains, but correct for 230VAC mains. Also the OPs idea of using red wires for Live and Neutral and black wires for earth/ground is likely completely incorrect. Unless the transformer has a shield connection. But then it would have more primary wires. \$\endgroup\$ Commented Sep 10, 2024 at 19:53

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Your assumption about connecting red with red to the live (hot) side is correct.
The two black wires on the primary also go together, to the neutral side (although it shouldn't matter whether reds go to live or neutral side).
So, for 120V, the reds go together, and the blacks go together, that is, red with red on one side, black with black on the other side.

If you were connecting them in series for 230V operation, one red wire would go to the hot side, its black counterpart would be shorted with the red one from the other pair, and the black one from that other pair would go to the neutral side of your power.

Your connection schematic/drawing in your question is correct.

To clarify with an explanation from my comment: Yes, the reds shorted together, and the blacks shorted together, then red ones going to the hot side, and the black ones going to the neutral side.
They are two separate primary coils wound for 120V each, but also for half the total current at 120V.
If you use only one pair of them, they will get warmer/hotter at the full rated load on your secondary because they will have twice their rated current and thus 4 times the amount of heat they were supposed to dissipate, so they could burn out.
Power (wasted as heat) = resistance x the current squared, so double the current means 4 times the heat across the same resistance.

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  • \$\begingroup\$ It's only likely correct, and only if OP had 230VAC mains which OP does not have. \$\endgroup\$ Commented Sep 10, 2024 at 19:57
  • \$\begingroup\$ @Justme He is aware of that, and he was wondering how he should wire them in either case. \$\endgroup\$ Commented Sep 10, 2024 at 20:08
  • \$\begingroup\$ so for 120 V I should connect both reds and both black cables? \$\endgroup\$ Commented Sep 10, 2024 at 20:31
  • \$\begingroup\$ @Zlmanuel87 Yes, the reds shorted together, and the blacks shorted together, then red ones going to the hot side, and the black ones going to the neutral side. They are two separate primary coils wound for 120V each, but also for half the total current at 120V. If you use only one pair of them, they will get warmer/hotter at the full rated load on your secondary because they will have twice their rated current and thus 4 times the amount of heat they were supposed to dissipate, so they would likely burn out. Power = resistance x the current squared, so double current means 4 times the heat. \$\endgroup\$ Commented Sep 10, 2024 at 20:43
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    \$\begingroup\$ Thanks. It works! 😊👍 \$\endgroup\$ Commented Sep 10, 2024 at 20:51

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