The Reagan administration's proposal to limit the deductibility of interest payments will have little or no impact on most taxpayers, and even in 1990 will increase tax burdens by only $1.5 billion.

However, tax experts say that an important, broad new principle would be established if the provision became law: Interest payments by consumers on car loans, charge accounts and other borrowing, such as for a vacation home, no longer would be deductible automatically, as is now the case.

Once that new principle is set, the proposed limits on interest deductions could well be lowered in future legislation, the tax analysts point out. For instance, the administration also wants to make all unemployment benefits fully taxable. A few years ago, such payments were not subject to tax at all and now are taxed only if the taxpayer's income is above a certain level.

At the same time, the new limit on deducting interest payments would be subject to two interacting phase-in rules over the next 10 years that would delay its impact considerably.

The primary targets of the new limit are investors in tax shelters whose share of the interest paid by partnerships is used to shelter other business and employment income. Reaching that target will add a new complexity to the tax returns of the individuals affected, perhaps even requiring preparation of a new form to determine what share of interest payments can be deducted.

Only interest paid on a debt secured by a taxpayer's principal residence -- a home mortgage -- would remain fully and unquestionably deductible by taxpayers who itemize their deductions. Under the new proposal, all other interest payments by individuals -- other than those made in connection with a trade or business managed by the taxpayer, including rental real estate -- generally would be deductible only up to $5,000 plus the taxpayer's net investment income.

The $5,000 threshold alone would be enough to keep intact the interest deductions of the vast majority of taxpayers.

In 1983, deductions were itemized on 35.2 million, or just over one-third, of the 96.3 million individual income tax returns filed. Taxpayers claimed deductions of $307.1 billion, with interest deductions heading the list at $132.5 billion, or about 43 percent of the total.

But more than 85 percent of the interest deductions were on home mortgages. About $7.7 billion was deducted for interest on charge accounts and credit cards and $36.5 billion for "other" interest payments, a category not broken down further by the Internal Revenue Service.

A breakdown of interest-paid deductions other than for home mortgages is available only through 1982. IRS statistics show an average deduction of about $1,000 for taxpayers with adjusted gross incomes of $20,000 to $25,000 that year. The average was still only $2,237 for taxpayers in the $50,000 to $75,000 group.

Of course, some taxpayers in any income group could be deducting more than $5,000 worth of interest paid other than for home mortgages. On the other hand, interest deductions tend to rise along with income, and the higher a taxpayer's income, the more likely that there will be investment income to be added to the $5,000 threshold to determine the new limit on deductibility.

There already is a limit of $10,000 on the deductibility of certain types of interest payments, primarily if the debts were incurred to make investments, such as in stocks or bonds. Interest cannot be deducted if the proceeds of a loan are used to buy tax-exempt securities.

Interest paid in connection with the production of income should be deductible, and then only when the income is received, according to the administration's argument. If the interest is incurred to generate income that will not be taxed -- or perhaps taxed long in the future -- it should not be deductible from current income. That is why, the argument goes, consumer interest payments -- say on a car loan -- should not, in principle, be deductible at all. No income tax will ever be paid on the benefits of having a car, unless it is used in a business, and then its expense could be deducted anyway.

The same argument applies with equal force to home mortgage interest deductions, but for political reasons the administration chose not to make them subject to any limit.

Specifically, the new limit would work like this:

Three types of interest payments not now subject to the $10,000 limit would be subject to it in the future. They are consumer interest, the shares of interest paid by partnerships and deducted by the partners on their personal returns, and interest paid by certain types of closely held small corporations known as Subchapter S corporations whose shareholders, for tax purposes, are treated as if the corporation were really a partnership.

In the latter case, the interest would not be subject to the limit in the case of a shareholder who is also a manager of the corporation. By the same rule of involvement in management, interest paid by a sole proprietorship and reported on a personal return would not be subject to the limit.

Under the first of the two phase-in rules, the limit on deductibility of payments would remain at $10,000 plus net investment income in 1986 and 1987 before dropping to $5,000 plus net investment income in 1988.

The second phase-in rule involves the proportion of all three types of payments that would be subject to the limit for the first time. In 1986, only 10 percent of those types of payments would be subject to the limit. That would rise to 20 percent in 1987 and continue to go up 10 percentage points each year until fully phased in in 1995.

Here is an example:

Say a taxpayer borrows $5,000 in 1985 to buy stocks. In addition, the taxpayer makes interest payments of $1,000 on a car loan, $2,000 on a bank note, $400 on various credit card charges and $7,000 on a mortgage on a vacation home that is never rented. From the stocks and other investments, the taxpayer has net investment income of $3,000.

Currently, only the $5,000 payment for so-called investment interest is subject to the limit on deductibility, which in this case is $13,000 -- $10,000 plus net investment income of $3,000.

Assuming none of the figures changes over the next few years, in 1986 and 1987, under the first phase-in rule, the taxpayer still would have a $13,000 limit on deductibility. In 1986, the $5,000 in investment interest and 10 percent of the other $10,400, or $1,040, would be subject to the limit. The only change for 1987 would be that not 10 percent, but 20 percent of the other payments -- or $2,080 -- would be subject to the limit. In both years, the $13,000 limit means that all the payments can be deducted.

However, things begin to change in 1988. That year, the limit in this example would drop to $8,000 -- $5,000 plus net investment income of $3,000. Meanwhile, the payments subject to the limit continue to rise. In 1988, the total covered would be the $5,000 in investment interest and 30 percent of the other $10,400, or $3,120. Thus, the taxpayer would have $8,120 in payments subject to a limit of $8,000. The extra $120 could not be deducted in 1988 but could be carried over to the following year.

Unless the taxpayer began to rearrange things, even more of the interest deductions would be lost in 1989. That year, $4,160 of the payments plus the $5,000 in investment interest would be subject to the limit. With an $8,000 limit, the taxpayer would lose $1,160 worth of deductions, and in addition still could not deduct the $120 from the previous year. Therefore, $1,280 would be carried forward.

In the case of a mortgage on a vacation home used part of the year by a taxpayer or his family and also rented part of the time, interest payments would be partly subject to the limit. The part not limited would be equal to the number of days the property was rented divided by the number of days in a year.

For instance, if the property were rented 30 days out of the year, 8.2 percent of the interest payments could be deducted without regard to the limit. The remaining 91.8 percent of the payments would be subject to the limit. Apparently, holding down personal use to no more than 14 days a year would not preserve full deductibility, as is now the case for some other types of costs, such as depreciation, incurred for such property.

The figures for personal interest deductions claimed in 1982 and 1983 understate total interest deductions such as those also available through real estate syndications. That's because the resulting losses are shown as a partnership loss on Schedule E of Form 1040 rather than on Schedule A, which is used for itemizing personal deductions. Schedule E also is used to report income and losses from S corporations.

Similarly, some interest deductions that would be subject to the new limit also could show up on Schedule F, which is used for reporting farm income.

Real estate tax shelter syndicators believe that the new limit could cause many well-to-do but not wealthy investors to shun such shelter deals. Sufficiently wealthy individuals probably would have enough investment income to add to the $5,000 threshold to benefit fully from receiving a share of a partnership's interest payments.

In addition, income from partnerships and S corporations would be added at the same time to the types of investment income that serve to raise the limit of interest deductibility. They are not so counted now.