# 10 Romex overcurrent protection

[electrofelon]

To summarize today's lesson, ampacity and OCPD size are two different things and may be vastly different

 

[noon9elec]

What kind of load is the #10 supplying?

so it’s actually a sub panel. minimal lighting load. however. 4 small window a.c. i know what right and are going to change. i just wondering what true load a #10 could carry. i also know it dependent on type insulation. to everyone who answered thank you you guys are awesome

 

[LarryFine]

so it’s actually a sub panel.

That makes it a feeder, not a branch circuit, which means regular ampacity limits apply; 30 amps.

 

[wwhitney]

Don't the various "motor load and other load(s)" sections in 430 permit an arrangement like this example:

15 amp non-motor, non-continuous load
15 amp conductors sufficient for the motor load
25 amp OCPD allowed for the motor load
Subpanel fed with 30 amp conductors protected by a 40 amp OCPD

And if so, would all the non-motor loads have to all be on a simple 15 amp branch circuit, or could they be spread across multiple 15 amp branch circuits, as long as the calculated load is 15 amp or less?

Cheers, Wayne

 

[LarryFine]

Subpanel fed with 30 amp conductors protected by a 40 amp OCPD

I don't agree. The motor-load allowances don't apply to feeders, just branch circuits.



Unless I'm wrong, of course.

 

[electrofelon]

Don't the various "motor load and other load(s)" sections in 430 permit an arrangement like this example:

15 amp non-motor, non-continuous load
15 amp conductors sufficient for the motor load
25 amp OCPD allowed for the motor load
Subpanel fed with 30 amp conductors protected by a 40 amp OCPD

And if so, would all the non-motor loads have to all be on a simple 15 amp branch circuit, or could they be spread across multiple 15 amp branch circuits, as long as the calculated load is 15 amp or less?

Cheers, Wayne

I'd have to digest your example, and the code, so for now I'll be lazy and say the answer is in 430.62

 

[kwired]

I'd have to digest your example, and the code, so for now I'll be lazy and say the answer is in 430.62

430.62 can be used if all the loads are motor loads, and is somewhat simple, feeder with motors with other loads isn't necessarily quite as simple IMO - 430.63.

 

[Another C10]

so it’s actually a sub panel. minimal lighting load. however. 4 small window a.c. i know what right and are going to change. i just wondering what true load a #10 could carry. i also know it dependent on type insulation. to everyone who answered thank you you guys are awesome

Ahh Ha ... Its a Sub Panel ... that's a game changer in my book, think growth ... give the 10/3 a rest, put in big brother #8 or #6 .. Is the engineer cheap or what. No offence to our beloved engineers.

 

[noon9elec]

Ahh Ha ... Its a Sub Panel ... that's a game changer in my book, think growth ... give the 10/3 a rest, put in big brother #8 or #6 .. Is the engineer cheap or what. No offence to our beloved engineers.

just typical engineer. if i replace going to #6. or i walk. TU

 

[noon9elec]

thank you Gents. My call is running #6. great responses

 

[winnie]

That is true, a number 10 can probably handle 40 amps as mentioned for short periods before the copper starts turning blue and baking the insulation a tad. My thought on that short spurt duration clause is .. is it really worth pushing a conductor to its molecular ability.

Actually code is quite conservative (IMHO for good reason), and in many situations a #10 can probably handle 40A on a continuous basis. Hell, the 90C 'free air' rating of #10 is 55A. The original install will _likely_ work just fine and is still wrong.

Note that I am not arguing about making code less conservative, and the OP is correct that the installation should be changed. This is simply a game of probabilities; imagine the number of fire if code was not conservative and decided that a 0.01% (one out of 10,000) chance of fire per circuit in a given year was acceptable.

-Jon

 

[paulengr]

Don't the various "motor load and other load(s)" sections in 430 permit an arrangement like this example:

15 amp non-motor, non-continuous load
15 amp conductors sufficient for the motor load
25 amp OCPD allowed for the motor load
Subpanel fed with 30 amp conductors protected by a 40 amp OCPD

And if so, would all the non-motor loads have to all be on a simple 15 amp branch circuit, or could they be spread across multiple 15 amp branch circuits, as long as the calculated load is 15 amp or less?

Cheers, Wayne

You are mixing loads here so it’s a feeder. On feeder breakers and even fuses to some extent we have a lot more options. First is the long term trip setting or tap in old school terms. That is dictated by NEC and ampacity. Can’t change it unless we change feeders. Second is instantaneous. This can usually cause coordination issues. It’s nice to have (ZSI or series ratings for instance) but most of the time we turn it off or don’t order it. Then we have options to select a speed which is usually called a time dial setting and sometimes a curve type. This is chosen to coordinate with the downstream devices. This is where we are coordinating with motor inrush. Except that if multiple simultaneous motor starts are possible and/ or they use fused protection (generally does not coordinate well with breakers) we often must coordinate to inrush rather than the protection device curves. This is tricky territory where an engineer (power cad jockey) can be beneficial, I cab do a lot of napkin math to get it pretty close but then I can be a power cad jockey too.

 

[wwhitney]

Don't the various "motor load and other load(s)" sections in 430 permit an arrangement like this example:

15 amp non-motor, non-continuous load
15 amp conductors sufficient for the motor load
25 amp OCPD allowed for the motor load
Subpanel fed with 30 amp conductors protected by a 40 amp OCPD

OK, since no one else went through it, I finally got around to checking. The above example works as follows, say for a 2HP, 230V single phase motor:

430.22 requires minimum 15 amp conductors for the motor branch circuit, based on the tabular FLC of 12 amp.
430.24 then requires minimum 30 amp conductors for the feeder.
430.52 allows up to a 30A inverse time circuit breaker to protect (SC & GF) the motor branch circuit conductors.
430.63 allows up to a 45A inverse time circuit breaker to protect the feeder.

Cheers, Wayne

 

[electrofelon]

You are mixing loads here so it’s a feeder.

I don't think I like that wording

 

[LarryFine]

Remember, gents, that rated conductor ampacity is about protecting the insulation from damage, not the conductor.

 

[noon9elec]

You are mixing loads here so it’s a feeder. On feeder breakers and even fuses to some extent we have a lot more options. First is the long term trip setting or tap in old school terms. That is dictated by NEC and ampacity. Can’t change it unless we change feeders. Second is instantaneous. This can usually cause coordination issues. It’s nice to have (ZSI or series ratings for instance) but most of the time we turn it off or don’t order it. Then we have options to select a speed which is usually called a time dial setting and sometimes a curve type. This is chosen to coordinate with the downstream devices. This is where we are coordinating with motor inrush. Except that if multiple simultaneous motor starts are possible and/ or they use fused protection (generally does not coordinate well with breakers) we often must coordinate to inrush rather than the protection device curves. This is tricky territory where an engineer (power cad jockey) can be beneficial, I cab do a lot of napkin math to get it pretty close but then I can be a power cad jockey too.

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