Married filing jointly, combined 2018 income $160k, me $138k, her $22k. Her W2 box 2 only $180. Owe $3,200 Fed. Is her employer not withholding enough federal tax?

[StephenL1]

Check the allowances on her W-4.  If it is set to married, then the withholding is correct as it would assume the full standard deduction of $24,000 which would fully offset her income.  Consider changing the allowances on your W-2s to single to offset individual income.  The W-4 calculator that TurboTaxJulieCo linked to can help you with this.

[TPKR]

Your spouse's withholding percentage is less than 1%.  (180/22,000)  However, your marginal tax rate is likely above 20%.  Your spouse's employer is likely withholding taxes under the false assumtion this is the only income for the family.  Either of you could increase Federal Tax Payments to resolve this issue...spouse thru changes in Form W-4...or you presumably thru quarterly estimated payments.

[JulieCo]

It depends.  Due to the lower income tax rates in several tax brackets, the withholding tables were lowered for 2018 and employers were instructed to take out less taxes.  This impact is affecting many tax returns in 2018 and is part of the reason the IRS lowered the underpayment penalty threshold to 80% of the tax amount due in March of this year (as opposed to the original 90% of tax due threshold). 

See more information here:

https://ttlc.intuit.com/replies/3301788

You may want to consider changing the allowances on your W-4 forms, you can do this utilizing TurboTax’s W-4 Withholding Calculator.