You may view all of the margin allowance limits per currency here.
If you see "Margin allowance exceeded" when trying to place a margin order, it means your order would require extending more of a currency than is allowed.
Margin allowance is the maximum amount that can be extended for a specific currency (for example: USD, EUR, BTC). These limits are set per currency.
Margin allowances are independent for each currency. Hitting your USD extension limit does not necessarily mean you’ve hit your EUR borrowing limit.
This typically happens when one of the following is true:
You already have open margin positions that are using some (or all) of the extension capacity for that currency.
You're trying to submit a new order, but the size would push your extension above the remaining allowance.
Allowance limits for that currency have been reduced due to market conditions.
Reduce the size (or leverage) of your margin order
A smaller order (or lower leverage) requires less extension, which may keep you within your remaining allowance.
Free up borrowing capacity for the currency you need
If you have existing positions using the same extension currency, you can close one or more of those open margin positons. (Or reduce their size)
Use a different margin pair that extends a different currency
Since allowances are per currency, this means switching to a pair with a different quote currency may work.
Example:
If you are long BTC on the BTC/USD market, this would require an extension of USD. Now, let's say you've reached the USD allowance limit, and can no longer open a new long position on the BTC/USD market.
In order to further "go long on BTC", you could then open a long position on the BTC/EUR market.
This is because on the BTC/EUR market, EUR is extended, and does not increase the amount of USD that has been extended.
This method is not as effective for short positions. Since short positions require an extension of the base currency. Then all markets including that same base currency are affected by the margin position limit. (You cannot change from BTC/USD to BTC/EUR to short more BTC passed the limit)
Be sure to check margin position limits before opening a position
Position management is critical, it is always best to be aware of margin position limits before trading. All margin position limits can be found here.