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It may not be right at all and I look forward to hearing from you, but my experience is that Chase will give you a total credit limit across their cards of half your income, while Amex does one third. For an annual income of $48000, that's $24000 and $16000 respectively. I don't know to which extent credit companies look at how much credit you have elsewhere.
On the Amex 3x CLI thread, someone calculated the maximum is 40% of income with a single Amex revolver. Not sure how much it is for those with multiple Amex revolvers (+/- charge cards).
I strongly doubt that there is a single set hard limit for a given creditor. More likely within each creditor there is a sliding scale of maximum total limits based on your income, spending profile, length of history with them, etc.
This probably translates into observable plateaus where many borrowers observe "Hey, Creditor X is only allowing me total credit up to 25%/30%/40%/etc of my income."
Field of employment also probably factors into it, since working in construction is seasonal, in the service industry is unpredictable, but in a salaried office job is pretty stable cash flow. The more stable the industry you're in, the higher the highest plateau you can reach. An opthalmologist might eventually be able to reach the 60% plateau while I can only get to 40% or 50%.
Consider this an educated guess - I have a professional background in risk scoring and bucketing.
But not in anything to do with FICO or directly to do with credit card issuers, so no PMs asking arcane FICO questions. I'm as semi-clueless as everyone else on that one, and most of what I do know I learned here.
With Chase and US Bank I am at about 49 and 48% respectively. I recently applied for another US Bank card and they said I would have to shift limit around because I had reached the maximum credit they could extend. I have not applied for another Chase card, but I reckon that I woud have to do the same thing. I also believe that the longer you are with a lender, the higher they will go. However, I haven't even been with US Bank for one year, so was shocked they allowed obtaining so many cards in such a short period of time with decent limits.
I should clarify that the 40% limit for Amex (if true) would only be available for those with a spotless credit report and sufficient history length. Those with less than perfect scores, short AAoA, etc. would not be able to get to 40%. But there does seem to be a percentage hard limit for those with no negatives for certain banks.