Motor nameplate 'usuable at 208v 14' sizing branch conductors

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brother

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We are in an old building that had things that were done by others that were 'somewhat' questionable to say the least. lol There was a debate about the motor sizing of the branch circuit conductors calculatin from the small vfd they are thinking of bypassing, (its feed by 15amp fused disconnect that is fed by a 20amp breaker).
Voltage present is 208volts 3 phase.

I didn't have my code book at the time, but my 'handsome/ugly book' says a 3phase 208volt 5hp 16.7 FULL load current is what we should use plus 125%. So technically 16.7x125%= 20.875 is the calculation I get and Since they plan to bypass everything (which again I dont recommend, as the vfd had overload protections adjustable). . .

Others ae saying since its just a 14 amp nameplate on motor they can use #14 for the 208v even though they said sometmes the motor start up would blow the 15amp fuses. . I tried to explain Full load current (the nec chart) vs full load amps (nameplate amps) is different in sizing the branch circuit.

The overload protection are sized at the nameplate amps (ie. the 15amp fuses) or setting on the vfd possibly? can go up to about max 125% possibly 140%? (need my NEC to sure of the 140%) if trip setting is still too low. 115% of name plate is what appears to be set at 115%x 14=. 16.1 if we do 125%x 14=17.5 140%x14= 19.6 But if they bypass everyting which I dont recommend, I think they might can use the 20amp breake even though that is above the 140% , but I have always rounded down for overload protection, and round UP for groundfault/short circuit protection and it appears since everything else is bypassed the 20amp breaker will function as both the overload and ground/fault short circuit protection. Did I make a mistake on this calculation? can you technically use the 20amp breaker (no fuses) even though its 19.6amps max calculation? Also, I couln't remember what the 010A means beside the 'usuable at 208v 14. I assume the 14 was the running amps of the motor itself at 208v.

When I get a chance I can double check my NEC code book 2023
Here is the nameplate
 

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ptonsparky said:
The branch circuit conductors to the VFD is sized at 125% of the VFD label. Not the motor. The VFD manual most likely suggests fusing. A CB ahead of those fuses is OK.
Conductors to the motor are sized via tables in 430.
Overcurrent protection is sized per the FLA on the label of the motor.
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And short circuit/ground fault protection is per 430 part IV. Across the line starting will very likely require more than 20 amp device if you wish it to hold during starting and 430 part IV allows up to 2.5 times motor rated current for inverse time breakers.
 
kwired said:
And short circuit/ground fault protection is per 430 part IV. Across the line starting will very likely require more than 20 amp device if you wish it to hold during starting and 430 part IV allows up to 2.5 times motor rated current for inverse time breakers.
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I think some people missed The part about l that they plan to REMOVE/BYPASS the VFD and fused disconnect . So it's just the 20 amp breaker. So that breaker will have serve as overload protection as well which is generally smaller than the ground fault protection.

Thanks for respondong.
 
brother said:
I think some people missed The part about l that they plan to REMOVE/BYPASS the VFD and fused disconnect . So it's just the 20 amp breaker. So that breaker will have serve as overload protection as well which is generally smaller than the ground fault protection.

Thanks for respondong.
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...short circuit protection

brother said:
The VFD is being removed/bypassed regardless. They said it was broken.
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Sounds like they don't know to me.
 
ptonsparky said:
...short circuit protection


Sounds like they don't know to me.
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Yes both groundfault and short circuit protection.

As for the VFD , lol, they claimed that they tested it and the VFD. Is just not working, they say it's an old one.

I'm just curious, is my thinking correct on the 20 amp breaker being ok since will have to function as overload protection?
 
Apologies if it's been mentioned previously - but if the original intent of the VFD was to slow the motor down or have some smooth accel ramp, those features will be lost with across the line starting. The motor will run at 60 Hz (1750 rpm according to that nameplate) and it will "jump" to that rpm as fast as it can.

I am sure the OP knows this - and I hope the client does too.
 
brother said:
I think some people missed The part about l that they plan to REMOVE/BYPASS the VFD and fused disconnect . So it's just the 20 amp breaker. So that breaker will have serve as overload protection as well which is generally smaller than the ground fault protection.

Thanks for respondong.
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I did not miss it. 20 amps likely not going to hold when starting across the line, soft start nature of running it on the VFD does allow it to hold when using the VFD.
 
brother said:
So that breaker will have serve as overload protection as well which is generally smaller than the ground fault protection.
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It will not provide overload protection unless motor rated current was right at 20 amps.

You sort of can get away with fuses in some cases providing both overload and SCGF protection but is rather limited. I see it with fertilizer/pesticide injector pumps on irrigation systems, they typically are 1/2 HP three phase motors @ 480 volts. Motor rated amps is right about 1 amp, 2 amp fuse is commonly used though 1.25 or 1.5 typically would be better protection, but hard enough to get these farmers to replace them with a 2 amp fuse. They tend to put 6 amp fuses in which will basically let it run at locked rotor current - then ask you why it burned the motor out. When I have put a motor overload on it they complain about that being unnecessary. I guess they like to throw $$ away and replace the motor when it burns out rather than figure out what is overloading it.

The 2 amp will do a decent job of blowing to protect from single phasing, which is probably what happens when they first up it to the 6 amp. Then further mechanical failures or more single phasing eventually burns it out.
 
brother said:
Also found out for FYI. The highest starting amps they measured was 73amps. Lowest is 66. They claim the 20 amp breaker is holding during their test run on bypassing .
I didnt know 20amp 3 pole breaker had that high of a tolerance i 3 times its rating would be the max.
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First hurdle is the magnetic trip point, if motor doesn't exceed that when first energized then acceleration is the next hurdle for starting current. Acceleration of a particular motor generally is dependent on characteristics of the driven load. Faster it can accelerate to running speed the quicker current draw will drop to whatever is the normal operating current.
 
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