Question about using #6 THHN wire for a 30HP RPC

[AmericanMaker]

80A is 3 phase the full load current of a 30HP motor. It’s from the NEC required size, not from the motor nameplate or some unloaded current reading. Just using the numbers that are required by the code.

Got it, thanks!

 

[Just a Sparky]

This is my point of contention. The NEC ampacity chart temperature ratings are broken down by wire type. The chart does not impose a limit of 60º C on THHN. Only on type TW and UF. If the temp ratings are to be imposed on all wire types, then why list the wire type on the chart at all? This is what I find confusing! Sorry if I was not clear on that point.

Nowhere in the code (that I have found yet) does it say "the 90C column is only to be used as the basis for derating calculation's, never directly."

Skimming.

You're looking for 110.14(C), which is a general rule. General rules apply to the entirety of the code (except Chapter 8 - see 90.3) unless explicitly modified elsewhere.

Chapter 1: General, Illinois Electrical Code 2020 | UpCodes

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110.14 Electrical Connections

[...]

(C) Temperature Limitations
The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
(1) Equipment Provisions
The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.12.
(a) Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, shall be used only for one of the following:

    Conductors rated 60°C (140°F).
    Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used.
    Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.
    For motors marked with design letters B, C, or D, conductors having an insulation rating of 75°C (167°F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity.

(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:

    Conductors rated 75°C (167°F)
    Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

(2) Separate Connector Provisions
Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity at the listed and identified temperature rating of the connector.
Informational Note: With respect to 110.14(C)(1) and (C)(2), equipment markings or listing information may additionally restrict the sizing and temperature ratings of connected conductors.

The source of the statement asserting that the 90*C column of 310.16 is only used for derating stems from the fact that 90*C rated terminations are rare in practice outside of heat-generating loads and devices like light fixtures, transformers, etc. You will almost never encounter a 90*C rated circuit breaker or fuse lug in installations below 1000 volts, and since all branch circuits and feeders originate at either a circuit breaker or fuse, that limits the wire ampacity in the overwhelming majority of installations to that of the 75*C column. Sometimes 60*C when dealing with older equipment below 100 amps that doesn't feature a 60/75*C dual rating - which has only become ubiquitous in modern times.

The wire types listed at the top of table 310.16 are more or less there for reference only. The actual articles which "officially" define these insulation temperature ratings are elsewhere. No consideration is made in Table 310.16 for external conditions which might influence the allowable temperature rating of the installation as a whole (read: not just the conductors) such as termination and equipment temperature ratings - that burden rests upon the code user. There is mention of this in 310.15(A):

https://up.codes/viewer/illinois/nfpa-70-2020/chapter/3/wiring-methods-and-materials#310.15_(A)

(A) General
Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15(A) through (F) and 310.12. Under engineering supervision, ampacities of sizes not shown in ampacity tables for conductors meeting the general wiring requirements shall be permitted to be determined by interpolation of the adjacent conductors based on the conductor's area.
The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).
Informational Note No. 1: Table 310.16 through Table 310.19 are application tables for use in determining conductor sizes on loads calculated in accordance with Part II, Part III, Part IV, or Part V of Article 220. Ampacities result from consideration of one or more of the following:

    Temperature compatibility with connected equipment, especially the connection points.
    Coordination with circuit and system overcurrent protection.
    Compliance with the requirements of product listings or certifications. See 110.3(B).
    Preservation of the safety benefits of established industry practices and standardized procedures.

Informational Note No. 2: For conductor area see Chapter 9, Table 8, Conductor Properties. Interpolation is based on the conductor area and not the conductor overall area.
Informational Note No. 3: For the ampacities of flexible cords and cables, see 400.5. For the ampacities of fixture wires, see 402.5.
Informational Note No. 4: For explanation of type letters used in tables and for recognized sizes of conductors for the various conductor insulations, see Table 310.4(A) and Table 310.4(B). For installation requirements, see 310.1 through 310.14 and the various articles of this Code. For flexible cords, see Table 400.4, Table 400.5(A)(1), and Table 400.5(A)(2).

Bear in mind that wiring methods themselves can also impose their own temperature limitations, sometimes dependent on their conditions of use. For example, NM cable (Romex) contains THHN insulated conductors rated 90*C, but 334.80 explicitly limits the allowable ampacity of NM cable to that of the 60*C column.

https://up.codes/viewer/illinois/nfpa-70-2020/chapter/3/wiring-methods-and-materials#334.80

Also some insulation types have multiple temperature ratings dependent upon the environmental conditions to which they are exposed. (Dry/wet locations, etc.) Type THHW for example.

https://up.codes/viewer/illinois/nfpa-70-2020/chapter/3/wiring-methods-and-materials#310.4

All of these are examples of why electrical work is best left to professionals.

 

[AmericanMaker]

@

You're looking for 110.14(C), which is a general rule. General rules apply to the entirety of the code (except Chapter 8 - see 90.3) unless explicitly modified elsewhere.
....
All of these are examples of why electrical work is best left to professionals.

@Just a Sparky, thanks for looking up and posting all that detail. Very helpful.
I'm slowly learning my way around UpCodes to better understand the codes.

 

[Garwood]

Jamie is just short drive from me. Been running one of his 30hp RPC's for 15 years. He's a good guy.

His numbers are probably OK, but I would go 3awg and be prepared to install a 100a breaker if the 70 trips on startup.

The just a sparky doom and gloom in this thread is too much! Hilarious!

"Run from them black magic pixies, run!!!!"

 

[Just a Sparky]

The just a sparky doom and gloom in this thread is too much! Hilarious!

"Run from them black magic pixies, run!!!!"

Oh, sorry for trying to be helpful. Don't know why I even bother sticking my neck out here anymore. I've got better things to do with my time than hand out free insight if that's the kind of thanks I'm going to get for my trouble.

Fucking douche.

 

[AmericanMaker]

Jamie is just short drive from me. Been running one of his 30hp RPC's for 15 years. He's a good guy.

His numbers are probably OK, but I would go 3awg and be prepared to install a 100a breaker if the 70 trips on startup.

@Garwood, yeah man, Jamie is a good guy and he has been an awesome help. I will always recommend him and his products.

I've started the RPC plenty of times on the 70 with zero problems. In fact, we don't even see lights in the house dim. If it wasn't for that loud contactor, you wouldn't even know I turned it on. Based on some previous conversations I had about RPC's, when I first got it and started it up, I was expecting to blackout the whole subdivision!

Thanks to all the comments on this thread, I will move up to a larger conductor, have an inspector come in and check my work, and take my losses ... beginner machinist tuition!!!

 

[Almost Retired]

American maker :: you are on the correct table for sizing wire
but you are not using it correctly
the 90*C column is the ampacity at that temperature
in other words if you run that many amps thru it it will heat up greatly

your breakers are normally rated for 60*C
look on the side of the breaker
you will see either 40*C or 75*C , no marking is 60*C
(this is according to the square d website)
you must choose the corresponding column to determine the ampacity and temperature that the breaker can operate at normally

if the wire temp goes over the rated breaker temp, it will heat up the breaker and cause nuisance tripping
(the breaker has two ways to trip :: short circuit-short time , just over max current rating will be long time
there is a coil inside for the short time trip , there is a bimetallic heater strip for the long time trip)

you also have to consider the termination at the other end of the wire, it also has a temp rating
which if exceeded will cause overheating, corrosion, and failure

common rules of thumb
for conduit and THHN wire use the 75*C column, for romex use the 60*C column
next is the distance , unless the round trip (there and back) measurement (of the wire itself) is over 100 feet dont worry about it

how do i know this ??? i have been an electrician since 1990 from residential to commercial, industry and offshore drilling all as an electrician

so #6 romex is 55 amps (NEC will allow a) 60A breaker
#6 THHN is 65A = 70A breaker
#4THHN is allowed a 90A breaker
they dont sell 80A breakers

 

[Almost Retired]

you are most likely thinking only the single phase calculations, 3 phase at 70A you still need to multiply by 1.73 to get full KVA. Which is 240V x(70 x 1.73) = 29 KVA
then to get single phase amps is 29.064 kva / 240V if that is what the input single phase is to get 121.1A! So 125% of 121.1 = 151.4A
and can easily double upon start up which gives the motor a large voltage drop if the conductors are undersized, which then drives up the incoming amps which heats things electrically. More heat = more resistance and causes a downward spiral until something melts.
I like to spend my $ wisely once, even if the difference is $7/ ft up to $10/ ft.
Size to the Max load at 60 C, or 75C max normally
T90(THHN) wire is only rated at 60C when exposed to oil, including vapor from the machines.
Also need the derated wire if inside a conduit which ususally requires sizing up one size because of that also.
that brings it to a min 1/0 cable for single phase in.

Cerrowire Resources - Ampacity Charts

Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.

www.cerrowire.com

3 phase out to the machines only needs to be 4 GA Max. and min 6 Ga if its just the one mill.

amps stated as amps is just that. no need to worry about 1 or 3 phase or other calculations
if you put an ammeter on it that is what it will show
the inrush current during startup is short duration comparatively speaking
the wire will not heat up any more than the motor, put your hand on the motor .. if you cant leave it there after starting
something else is wrong
derating for number of conductors only happens when there are more than 3 current carrying conductors
for a 3phase motor or rpc that is 3 hot legs. ground does not get counted
if you put anything else in a conduit with motor wiring in it, you are asking for trouble. no matter how big the wires or conduit

 

[AmericanMaker]

American maker :: you are on the correct table for sizing wire
but you are not using it correctly
the 90*C column is the ampacity at that temperature
in other words if you run that many amps thru it it will heat up greatly

your breakers are normally rated for 60*C
look on the side of the breaker
you will see either 40*C or 75*C , no marking is 60*C
(this is according to the square d website)
you must choose the corresponding column to determine the ampacity and temperature that the breaker can operate at normally

if the wire temp goes over the rated breaker temp, it will heat up the breaker and cause nuisance tripping
(the breaker has two ways to trip :: short circuit-short time , just over max current rating will be long time
there is a coil inside for the short time trip , there is a bimetallic heater strip for the long time trip)

you also have to consider the termination at the other end of the wire, it also has a temp rating
which if exceeded will cause overheating, corrosion, and failure

common rules of thumb
for conduit and THHN wire use the 75*C column, for romex use the 60*C column
next is the distance , unless the round trip (there and back) measurement (of the wire itself) is over 100 feet dont worry about it

how do i know this ??? i have been an electrician since 1990 from residential to commercial, industry and offshore drilling all as an electrician

so #6 romex is 55 amps (NEC will allow a) 60A breaker
#6 THHN is 65A = 70A breaker
#4THHN is allowed a 90A breaker
they dont sell 80A breakers

@jeff10049, thanks for all the comments.

• Both the breaker and the connector at the RPC are rated at 75ºC.
• My wire length from the panel to the RPC is 35 feet.
• I'm only running 2 current carrying conductors in the conduit from the panel to the RPC.
• For now I'm running #4 THHN from the 70 amp breaker to the RPC and #6 THHN from the RPC to the VMC.
• My clamp meter has not show any draw great than 18 amps at this point, but the spindle has not be under load ... the machine has not cut any chips yet.
• For now, I'll use the clamp meter to watch my load and a laser infrared digital thermometer to watch heat at the breaker and motor.

In the next month or two, after I get all my tooling purchases behind me, I'll drop the coin on a 90 amp breaker, #3 THHN, and hire an inspector to come in and check my work.