MCP selection along with NEC and UL508A

[Mehdi]

10HP Motor name plate shows volts 230/460 -Amps 21.6/10.8. We requested 208V motor and manufacturer said it is compactable but due to some reason they can provide 208V/Amps information on motor name plate. We ran the motor on 208 and it was drawing 19.8 Amps. In the beginning of the project, we looked into NEC table and found 10hp motor full load amps 30.8 at 208 volts. we selected the breaker 30-42 amps. I was wondering why motor is taking same amps on 208 what are on nameplate for 230 volts. I was not sure if we met the UL508a and NEC code in a right direction. Kindly help me to understand and learn. Thank you.

 

[retirede]

I was wondering why motor is taking same amps on 208 what are on nameplate for 230 volts.

The actual current draw is a result of the shaft load. The fact that it’s the same as the 230V rating nameplate is a coincidence.

The full load current at 208V should be pretty close to the inverse ratio of the voltages times the nameplate amps. I get about 23.9A.

 

[petersonra]

The nameplate is the amps at max load.

 

[Jraef]

You can't select an MCP anyway, it is only able to be used as part of a factory assembled and listed starter. So the breaker size is part of the combination starter listing.

 

[Mehdi]

You can't select an MCP anyway, it is only able to be used as part of a factory assembled and listed starter. So the breaker size is part of the combination starter listing.

I did not understand your answer, sorry. Are you saying that only panel builder can select MCP? We are panel builder shop and one of my colleagues has selected the CB and I was wondering if its correct selection. We make same panels every time and if something does not correct maybe I can fix it. Thank you.

 

[Mehdi]

The nameplate is the amps at max load.

name plate FLA is 21.6 per manufacturer and NEC is saying FLA for 10hp motor at 230V is 28A, which one should we follow? When we select breakers that time, we do not know what will be on motor's nameplate?

 

[Mehdi]

The actual current draw is a result of the shaft load. The fact that it’s the same as the 230V rating nameplate is a coincidence.

The full load current at 208V should be pretty close to the inverse ratio of the voltages times the nameplate amps. I get about 23.9A.

NEC says 10hp motor at 230V FLA is 28A, motor nameplate shows 21.6, your calculation says 23.9. Which value should be considered keeping in view of UL508A. Thank you.

 

[Jraef]

I did not understand your answer, sorry. Are you saying that only panel builder can select MCP? We are panel builder shop and one of my colleagues has selected the CB and I was wondering if its correct selection. We make same panels every time and if something does not correct maybe I can fix it. Thank you.

Read the NEC, article 430.52 and what it says about “Instantaneous Trip” circuit breakers (the official term for an MCP). They can only be used in a factory tested and listed motor starter assembly. UL 508a panel shop listing will only allow you to use an MCP if you “add it to your procedure”, because they are never UL listed, they are only UR, Recognized Components. UR means they must be used in a specific manner as part of a larger listed assembly under the conditions of use in the component recognition, and the condition of use for an MCP requires short circuit and overload testing of the combination. In theory a panel shop COULD perform the required additional testing of a combination they made up, but that test and documentation will likely cost around $50k per line item (I’m guessing here on the cost because it was $25k last time I was involved, 20+ years ago).

MCP are technically only sold to the general public as like-for-like REPLACEMENT PARTS for factory assembled and listed starter assemblies. There is no other valid use for them in the field.

 

[petersonra]

A panel shop can also assemble them from parts as long as the listing shows what parts to use. I don't know if any panel shops would bother to get mcps added to their procedure. There really is no point when ul would not allow the addition except for already listed assemblies.

It used to be that mcps were substantially cheaper than tm breakers but it's not that way any more. There is little difference in the oem price.

The main benefit was for soft starters and drives, some of which had high sccr ratings with mcps. Some drive and soft start manufacturers were able to find a handful of mcps that they could make in droves and sell them as a sort of package pretty cheap. But ul stopped listing them that way maybe a decade ago so that went away. For a while you could buy a 400 amp frame mcp for less than $200 and team it up with a specified drive and get 65k sccr. Very cost effective.

Ironically at least to me, is that UL now requires an IT trip in molded case switches to get a higher SCCR. So it is basically an mcp, but labeled as a mcs.

 

[Mehdi]

Read the NEC, article 430.52 and what it says about “Instantaneous Trip” circuit breakers (the official term for an MCP). They can only be used in a factory tested and listed motor starter assembly. UL 508a panel shop listing will only allow you to use an MCP if you “add it to your procedure”, because they are never UL listed, they are only UR, Recognized Components. UR means they must be used in a specific manner as part of a larger listed assembly under the conditions of use in the component recognition, and the condition of use for an MCP requires short circuit and overload testing of the combination. In theory a panel shop COULD perform the required additional testing of a combination they made up, but that test and documentation will likely cost around $50k per line item (I’m guessing here on the cost because it was $25k last time I was involved, 20+ years ago).

MCP are technically only sold to the general public as like-for-like REPLACEMENT PARTS for factory assembled and listed starter assemblies. There is no other valid use for them in the field.

Thank you, I was calling CB as MCP. Now I understand the difference.

 

[Jraef]

It used to be that mcps were substantially cheaper than tm breakers but it's not that way any more. There is little difference in the oem price.

Yep, and also, unlike in the past, most breaker mfrs now offer T-M breakers with adjustable mag trips. They usually have the same mag trip adjustment rages as the MCPs (until you get below 15A), so there is no point in messing with an MCP.