Why Are Ammeters Connected in Yellow-Phase Only?

On a 3 phase motor all phase will read the same amperage. It does not matter which phase you choose to attach the amp-meter.

This is only true until there is a problem with the load. You can troubleshoot 3-ph. motors by taking amp readings across all 3 legs:

1. all legs equal but high indicate mechanical bind
2. one leg pulling high or low electrical = prob with motor

In general, it is done out of (unwritten) convention. If everyone is trained to do it that way, you will know what to expect when you go somewhere else. But other than that, it makes no difference.

Unless you have a monitoring system that can check all phases constantly and you want to check for phase imbalance using 3 ammeters on a small motor is not really needed. usually it is a larger motor that will have the three phase monitoring, and even then, not all the time. Our process has very few times that allow for planned outages, so being able to anticipate a motor failure is pretty important to us. I doubt we would even put a single phase ammeter on a motor that small.

Maybe off-topic..

What country uses Yellow as one of the phases?

Australia, we use Red-White-Blue

middle east and asian contrys yellow phase is use intseted of ur withe phase. in ur country in which phase ammeter connecting for small 3 phase loads.

Prior to April 2005 it was red/yellow/blue in the UK, with a black neutral. Since then all new installations have had to fall in with pan-European harmonisation, i.e. brown/grey/black with a blue neutral. BS7671 refers. So it is quite possible to find both styles in one installation, in which case an appropriate warning label has to be placed at the distribution board so as not to confuse electricians from non-UK European origin with the conductor colours.

In my experience in the US, there aren't really any hard and fast rules for 3 phase wire coloring. A lot of the time we use black wire and label them L1, L2, L3 for supply voltage.

Other than the grounded and grounding conductors, only the high leg of a center-tapped delta configured transformer bank is mandated; that being the color orange by the NEC in the USA.

I'm not sure of the color you're describing. In the trade we use Brown, Orange, or Yellow/purple to indicate Medium voltage (480-600 V.A.C.) and Black, Red, and Blue for 120-240 V.A.C.

In three phase you would measure on the high leg.

There is no hard and fast rule to fix Ammeter on yellow phase only. For a balanced load circuit, the current flowing in all the three phases will be same. You can connect Ammeter on any phase. But for the symmetry purpose manufacturers used connect Ammeter on Yellow Phase.