Sizing OCPD Pump

[mto12345]

good morning

I am looking to size a Circuit Breaker for a Pump @208V Motor, rated current is 11.9A, Starting current is 76.8 (see attachment) . Per NEC 240.6 (A), FLA *1.25 should be used. However, since there is a much gap between rated and starting current what do i use as my FLA. My concern is at the staring current, the CB might trip every time if sized at rated current.

 

[Dsg319]

good morning

I am looking to size a Circuit Breaker for a Pump @208V Motor, rated current is 11.9A, Starting current is 76.8 (see attachment) . Per NEC 240.6 (A), FLA *1.25 should be used. However, since there is a much gap between rated and starting current what do i use as my FLA. My concern is at the staring current, the CB might trip every time if sized at rated current.

Size your OCPD off of NEC FLC tables that correlates with nameplate HP. FLA are used for sizing overloads.

 

[infinity]

good morning

I am looking to size a Circuit Breaker for a Pump @208V Motor, rated current is 11.9A, Starting current is 76.8 (see attachment) . Per NEC 240.6 (A), FLA *1.25 should be used. However, since there is a much gap between rated and starting current what do i use as my FLA. My concern is at the staring current, the CB might trip every time if sized at rated current.

Use the motor table value to size the conductors at 125%. If you're using a circuit breaker it can be up to 250% of the table value. That should never have a problem starting.

 

[tom baker]

I can’t see the attachment, what is the HP?

 

[infinity]

From the OP:

 

[kwired]

I can’t see the attachment, what is the HP?

11 amp @ 208 three phase should be a HP.

single phase would maybe about 1.5 HP.

20 amp breaker is likely fine for either those, though if for some reason it won't hold during starting you could go up to 30 amp and still be code compliant.

Apparently I can't read. OP says 11.9 amps. (and I missed the "3" as in 3 HP above as well)Then we see information infinity posted and this apparently is not a standard motor that fits NEC tables either so kind of have to go with nameplate instead of tables. Still comes out to max breaker being a 30 amp, I'd probably install a 25 myself in most instances.

 

[mto12345]

I can’t see the attachment, what is the HP? 4.1HP NEMA Code H

 

[Jraef]

When you see something like “4.1HP”, that means it is a kW rated 50Hz motor and when converted to 60Hz, the HP comes out odd like that. The NEC doesn’t really have a way to respond to that so what I have always done is just round to the nearest NEC Hp in the tables. So in this case it would be 5Hp in the tables for sizing the OCPD and conductors.

 

[wwhitney]

the HP comes out odd like that. The NEC doesn’t really have a way to respond to that so what I have always done is just round to the nearest NEC Hp in the tables.

430.6(A)(1) tells you to interpolate when you have the reverse problem--a motor marked in Amps and you need to determine the HP. So interpolating in this situation would be entirely reasonable.

Cheers, Wayne

 

[kwired]

When you see something like “4.1HP”, that means it is a kW rated 50Hz motor and when converted to 60Hz, the HP comes out odd like that. The NEC doesn’t really have a way to respond to that so what I have always done is just round to the nearest NEC Hp in the tables. So in this case it would be 5Hp in the tables for sizing the OCPD and conductors.

Not always a 50 Hz based motor in my experiences. On the farms I see a lot of motors made by same manufacturers that make a lot of NEMA motors but not with NEC table horsepower ratings. One the most common is crop drying fan motors. You might have a motor with a lot of similar physical dimensions as a general duty 10 HP motor but this one is rated air over and has nameplate of 12.5 or even up to 15 HP, single or three phase are that way. NEC only goes up to 10 HP in the single phase table.

Been seeing many general use motors in recent years, not air over rated anyway maybe they still considered to be an OEM application motor, that are single phase and rated 12, 15 and even 16 HP. Again not in NEC tables. Some are even dual volt 230/460 single phase.

 

[mto12345]

11 amp @ 208 three phase should be a HP.

single phase would maybe about 1.5 HP.

20 amp breaker is likely fine for either those, though if for some reason it won't hold during starting you could go up to 30 amp and still be code compliant.

Apparently I can't read. OP says 11.9 amps. (and I missed the "3" as in 3 HP above as well)Then we see information infinity posted and this apparently is not a standard motor that fits NEC tables either so kind of have to go with nameplate instead of tables. Still comes out to max breaker being a 30 amp, I'd probably install a 25 myself in most instances.

Thank you - I used Table 430.250 @ 5HP, 208V =16.7 *125% (per 430.22)=20.8 so a 30AMP CB would work

 

[mto12345]

Thank you to all -
I used Table 430.250 @ 5HP, 208V =16.7 *125% (per 430.22)=20.8 so a 30AMP CB would work

 

[kwired]

Thank you - I used Table 430.250 @ 5HP, 208V =16.7 *125% (per 430.22)=20.8 so a 30AMP CB would work

A 16.7 amp motor could have up to 45 amp breaker before you have to have it not start before you can go any higher.

Rule of thumb I was taught long ago is to go with 2 times motor rated current and then next standard size up. That seems to be in about the same ballpark that many manufacturer tables suggest you use as well. Example I have a slide chart from Square D that for a 5 HP motor @ 208 recommends a 35 amp breaker.