We are trying to find a reasonable solution to a problem we are having with MCP instantaneous breakers installed in a combination starter tripping during inrush of a high effeciency motor. The motor is a 480V 150hp motor with a FLA of 175A. The breaker that is installed currently is a 250A instantaneous breaker set to the maximum setting giving us a 2500A trip point. Several times when we try to start this motor we trip this breaker on inrush and have measured inrush currents in excess of the 2500A setting (2800-3000A). We are trying to come up with possible solutions for getting over this inrush issue and still staying code compliant.
I know the NEC allows a maximum setting of 17x FLA with an instantaneous breaker for a high efficiency motor. A 17x setting for this motor would equate to a setting of 2975A. Would it be possible to install a 400A instantaneous breaker (next size up) and dial the breaker down to 3000A? Even with this i'm not sure it would be enough to get over the inrush.
The other option we looked at was installing a 250A thermal-magnetic breaker but I noticed that even with this breaker the adjustable instantaneous trip setting only allowed a maximum of 2500A which puts us in the same position as the origonal breaker. The code allows for a inverse time breaker to be used up to 250% of motor FLA. Does this inverse time breaker pertain to a thermal-magnetic breaker or a thermal only type breaker. If it applies to a thermal-magnetic type breaker with an adjustable instantaneous setting does the 17x that applies for an instantaneous breaker still apply to the instantaneous setting on the thermal magnetic breaker. In other words if we install a 400A thermal-magnetic breaker are we restricted in how high we adjust the instanteous setting? Would this violate any NEC or UL ratings?
Are there breakers that are stricly thermal elements on dont have an instantaneous element but rather have a trip curve similar to a fuse that would allow us have a longer delay on this high inrush.
What is typcially the solution to this inrush problem for these high efficiency motors. Others are stating that we need to install soft starters or VFD's that are very costly and I find it hard to believe that a soft starter that may cost upwards of $10k is the only solution to this problem. Are there any additional provisions in the NEC that allow us to get around this inrush issue with upsizing the short circuit protection while not violating the UL isting of the starter?
I know the NEC allows a maximum setting of 17x FLA with an instantaneous breaker for a high efficiency motor. A 17x setting for this motor would equate to a setting of 2975A. Would it be possible to install a 400A instantaneous breaker (next size up) and dial the breaker down to 3000A? Even with this i'm not sure it would be enough to get over the inrush.
The other option we looked at was installing a 250A thermal-magnetic breaker but I noticed that even with this breaker the adjustable instantaneous trip setting only allowed a maximum of 2500A which puts us in the same position as the origonal breaker. The code allows for a inverse time breaker to be used up to 250% of motor FLA. Does this inverse time breaker pertain to a thermal-magnetic breaker or a thermal only type breaker. If it applies to a thermal-magnetic type breaker with an adjustable instantaneous setting does the 17x that applies for an instantaneous breaker still apply to the instantaneous setting on the thermal magnetic breaker. In other words if we install a 400A thermal-magnetic breaker are we restricted in how high we adjust the instanteous setting? Would this violate any NEC or UL ratings?
Are there breakers that are stricly thermal elements on dont have an instantaneous element but rather have a trip curve similar to a fuse that would allow us have a longer delay on this high inrush.
What is typcially the solution to this inrush problem for these high efficiency motors. Others are stating that we need to install soft starters or VFD's that are very costly and I find it hard to believe that a soft starter that may cost upwards of $10k is the only solution to this problem. Are there any additional provisions in the NEC that allow us to get around this inrush issue with upsizing the short circuit protection while not violating the UL isting of the starter?