Wire size for 400 amp Motor Circuit Protector

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BURGH

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Customer is using a Siemens 3VA1440-5MH32-0AA0 with a Thermal Magnetic Trip Unit.

He asking what wire size should be run to this breaker.

He is being told the lugs will handle 50 … 300 mm² wire
 
This does not appear to be an NEC application. Are you asking about the line side or the load side conductors? Typically the load side conductors are sized based on the rating of the connected motor.
 
Not to many of us will know the IEC regulations.
The NEC code section would associate the wire size to the motor full load amperes as Don mentioned.

If the MCP is 400A, the NEC allows the MCP to be as large as 1100% of FLA for a Design B motor but it could be smaller, so guessing at the motor size from the MCP is too wild of a guess.
 
Actually, there is no valid field use of an "MCP" (Instantaneous Trip Only) breaker per the NEC, it can ONLY be used as part of a factory assembled and listed starter assembly. So what the NEC says becomes irrelevant if they are going to ignore that basic issue. It's also highly likely that this 3VA breaker is not UL listed and may not be suitable for anything other than a solidly grounded Wye system (they don't have Delta systems overseas), so be sure to check that out. You may have to change the breaker, and if it is indeed an MCP being built in the field, it would have to be changed to a Thermal Mag breaker. I tried looking up that part number, but because it is an IEC 400V breaker, it's difficult to wade through the Siemens Ginglish (German translated to English) catalog data.

Wire sizes to feed a motor starter circuit are dictated in Article 430, but in a nutshell, you must use the motor HP from the NEC tables, read the FLC value for that HP and size the conductor for 125% of that value. Assuming this is some equipment that came from overseas and has IEC components, you would take the motor kW rating, divide it by .746 to get an equivalent HP, then go UP to the nearest NEC table HP value and use that. So for example let's say it says the motor is 110kW, then you go 110/.746 = 147.5HP, so you use the NEC table for 150HP that says 180A, then x 1.25 = 225A rated conductor minimum (then adjust for any voltage drop of course). Be careful though with the motor nameplate because IF they used a 400V 50Hz motor and are using it as 480V 60Hz, the motor will work fine, but will turn 20% faster, so the kW rating at 60Hz will be 120% of the 50Hz kW rating. If they SHOW a rating at 60Hz, you can just use that though.

As to the lug capacity, 50 to 300mm2 = #1 to 600kCMIL as the AWG range; more than adequate for any motor that would run behind a 400A MCP.

PS: After wading through that website, it does appear that the trip unit is adjustable magnetic only, no thermal trip. It is also rated 380/415V max and 55kAIC max (but of course, at 415V, not 480V...)
 
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ron said:
Not to many of us will know the IEC regulations.
The NEC code section would associate the wire size to the motor full load amperes as Don mentioned.

If the MCP is 400A, the NEC allows the MCP to be as large as 1100% of FLA for a Design B motor but it could be smaller, so guessing at the motor size from the MCP is too wild of a guess.
Click to expand...

IEC is pretty simple. It was done by electrical engineers though instead of mechanical ones so motors are rated in kW, contactors in A plus a utilization class. Utilization class exists in NEMA too but you have to pull the standard out to find it and they only have two instead of a dozen. Unlike NEMA you don’t have to make an extra trip to the NEC to look up FLA. Amps are Amps.

At the smallest size. You can adjust up to 1700% maximum which often needs to be done for energy efficient motors.

You’re also being rather small minded in assuming that everything must be Listed because that is simply not true. Utilities, mining, marine, and FAA are just a few United States jurisdictions where NEC is often simply a design standard and Listing is either not mandatory or may be required in some cases but NEC compliance isn’t (MSHA). I have personally worked on a plant site that had ALL of these jurisdictions on one site.

So either way though it should be obvious. We rate wiring according to thermal limitations or ampacity. In a motor circuit that means it is based on the motor FLA (or NEC FLA table) and a multiplier that depends on the utilization. The overload protection protects the wiring from overloads while the overcurrent device (MCP in this case) protects it from short circuits. So to size wiring even on the feeder side we look to the load, not the starter or the MCP which are both normally either open or a very low resistance.
 
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