+/- 15 v PSU transformer question (is 12-0-12 OK?)

For bipolar power supplies, specifically +/- 15VDC, I usually see an 18-0-18  or so transformer in the schematic. However, I just built one but get ~15v on the positive rail and ~20vdc on the negative.  With a full wave bridge and 2200uF capacitors I'm getting about 30vdc at the input to the regulators. The datasheet indicates it should be no more -23v so I'm assuming that's the problem.

Using a 12.6-0-12.6 transformer I get ~15 on both rails.  I've always read the transformer should be higher than the expected regulated voltage, but if voltage = 1.4x input just after a full wave bridge why? And with tube circuits it seems expected that the B+ voltage will be higher than the transformer voltage. 

The rectified d.c voltage will be 1.414 x the nominal a.c voltage of the transformer. In this case you will have around 17v.  If you are using regulators Lm317 7815 etc they normally need 3v above the voltage you are regulating to.  In your case you only have 2 volts above the regulated d.c voltage you want. 

Dropout voltage is a function of output current and temperature.  For low currents and higher temps the drop can be lower than 3V.

But 3V is good design practice as it works under all operating conditions.  A 12.6VAC secondary may not give 3V across the regulator under high currents, with lower mains voltage.  An 18V secondary is much safer for 15VDC output.

Matador said:

Dropout voltage is a function of output current and temperature.  For low currents and higher temps the drop can be lower than 3V.

But 3V is good design practice as it works under all operating conditions.  A 12.6VAC secondary may not give 3V across the regulator under high currents, with lower mains voltage.  An 18V secondary is much safer for 15VDC output.

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The current demands are relatively low for what  I need  the PSU,  the transformer is rated at 2A, and the heatsinks oversized.  I'm getting a little over 18vdc at the input of the regulators.

The issue I was running into was with the 18-0-18  I was getting about 30vdc at the input of the regulators and 20vdc from the 7915. Not sure why that is or what I could do about it.