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

[markz528]

I see comments about using #1, #2 and #3 wiring here which I find confusing when, according to the NEC 310-16, the allowable ampacities for #4 and #6 THHN are 95 amps and 75 amps respectively.

Based on the chart below, the #4 AWG coming from the panel is well within the range of 125% of circuit size (70*1.25 = 87.5 amps). And so is the #6 AWG between the VMC and the RPC, considering the VMC's rating of 44 FLA (44*1.25 = 55 amps). And knowing that the RPC and VMC are the only two loads on this circuit, if I combine the RPC's idling 20 amps and the VMC 44 FLA, I'm still at 64 amps (if that is the correct math).

So considering all that, and knowing that the manufacturer of my RPC said "the RPC can peak at 70 amps during startup, but it will never draw above 70 amps, even under load," why would I need to use #1, #2 or #3 wiring? This seems pretty straight forward to me, but I'm not an electrician and I'm always willing to learn from people with more experience than me. Am I missing something here?

But the connections will be rated 60 deg C in those sizes so you have to use the 60 degree ampacities column. Whatever derating of the conductors that might be required can be subtracted from the 90 deg C column, but you cannot exceed the 60 deg C rating. You have to use the 60 degree rating for circuits rated 100A or less or for #14AWG through 1 AWG. See NEC 110.14(C)(1).

 

[JST]

110.14 (C) (1) (a) (3):
"Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors."

So it depends on the equipment, and whether or not you can locate a terminal temp rating. If you can, you can use that rating.

The de-rating applies. You can use 90C wire if you need to in order to pass some hotter area. But you can only use it at the current rating either listed for the equipment, or the 60C rating.

The 60C is the "default" if no other information is available.

 

[BT Fabrication]

I see comments about using #1, #2 and #3 wiring here which I find confusing when, according to the NEC 310-16, the allowable ampacities for #4 and #6 THHN are 95 amps and 75 amps respectively.

Based on the chart below, the #4 AWG coming from the panel is well within the range of 125% of circuit size (70*1.25 = 87.5 amps). And so is the #6 AWG between the VMC and the RPC, considering the VMC's rating of 44 FLA (44*1.25 = 55 amps). And knowing that the RPC and VMC are the only two loads on this circuit, if I combine the RPC's idling 20 amps and the VMC 44 FLA, I'm still at 64 amps (if that is the correct math).

So considering all that, and knowing that the manufacturer of my RPC said "the RPC can peak at 70 amps during startup, but it will never draw above 70 amps, even under load," why would I need to use #1, #2 or #3 wiring? This seems pretty straight forward to me, but I'm not an electrician and I'm always willing to learn from people with more experience than me. Am I missing something here?

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.

 

[AmericanMaker]

Sorry, I'm not trying to be stubborn or argumentative. I genuinely want to understand this.

Based on the charts, here's why I think (I'm not yet 100% convinced) that #4 THHN copper is plenty safe for my setup.

According to Table 310.15(B)(16)

• THHN has a temperature rating of 194° (90º C).
• #4 THHN is rated at 95 amps at 194° (90º C).
• I'm not using TW, UF or any wire rated at 147º (60º C).

With A/C, the ambient temp in my garage will never exceed 86º (30º C). According to Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F), the adjustment factor for 86º (30º C) is 1.0. So, #4 THHN is still rated at 95 amps.

And according to Table 310.15(B)(3)(a) - Adjustment Factors for More Than Three Current-Carrying Conductors, the derating factors start at 4 current carrying conductors in conduit. From my panel to the RPC there are 2 current-carrying conductors and 1 ground, so there appears to be no need to derate the conductors.

Just to confirm that I understand the charts and the derating process, I read this post on How to Derate Conductors, and I went out and purchased a clamp meter and watched the single phase conductor load.

• The RPC idles at around 14 amps.
• With the mill under air and power, the single phase conductor load was at 16 amps.
• Tool changes bumped it up to 17 amps.
• With the spindle running at 500 RPM, but not under load, the single phase conductor load maxed at 18 amps.

Clearly the spindle was not brought up to 10k RPM under load. That's a test I still need to do.

Given there are two hots for the 240 amp circuit coming from the panel, I'm not sure I have to double those loads or not. But even if I do, the max load would have been 36 amps for my tests.

My conclusion is that the #4 THHN 95 amp rating on a 70 amp circuit seems safe and sufficient for my setup. It seems pretty straight forward to me. What am I missing here that is causing others to say I need #1, #2 or #3 AWG conductors?

 

[gustafson]

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.

Good lord, the Manufacturer says to protect it with a 70 amp breaker
PERIOD
all that math is wrong
holy crap he is not pulling 70 amps 3 phase, he is pulling 70 amps SINGLE phase
60C? Do you anticipate the wires are going to be too hot to touch? I think not
Exposed to oil? it is not inside the machine in oil vapor, if there was enough oil vapor to affect the wire, you would die of lung failure much sooner

 

[markz528]

110.14 (C) (1) (a) (3):
"Conductors with higher temperature ratings if the equipment
is listed and identified for use with such conductors."

So it depends on the equipment, and whether or not you can locate a terminal temp rating. If you can, you can use that rating.

The de-rating applies. You can use 90C wire if you need to in order to pass some hotter area. But you can only use it at the current rating either listed for the equipment, or the 60C rating.

The 60C is the "default" if no other information is available.

That is true. I find it rare to find components rated higher than 60 deg C so I just default to it. But if it is rated higher you can use that

Sorry, I'm not trying to be stubborn or argumentative. I genuinely want to understand this.

Based on the charts, here's why I think (I'm not yet 100% convinced) that #4 THHN copper is plenty safe for my setup.

According to Table 310.15(B)(16)

• THHN has a temperature rating of 194° (90º C).
• #4 THHN is rated at 95 amps at 194° (90º C).
• I'm not using TW, UF or any wire rated at 147º (60º C).

With A/C, the ambient temp in my garage will never exceed 86º (30º C). According to Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F), the adjustment factor for 86º (30º C) is 1.0. So, #4 THHN is still rated at 95 amps.

And according to Table 310.15(B)(3)(a) - Adjustment Factors for More Than Three Current-Carrying Conductors, the derating factors start at 4 current carrying conductors in conduit. From my panel to the RPC there are 2 current-carrying conductors and 1 ground, so there appears to be no need to derate the conductors.

Just to confirm that I understand the charts and the derating process, I read this post on How to Derate Conductors, and I went out and purchased a clamp meter and watched the single phase conductor load.

• The RPC idles at around 14 amps.
• With the mill under air and power, the single phase conductor load was at 16 amps.
• Tool changes bumped it up to 17 amps.
• With the spindle running at 500 RPM, but not under load, the single phase conductor load maxed at 18 amps.

Clearly the spindle was not brought up to 10k RPM under load. That's a test I still need to do.

Given there are two hots for the 240 amp circuit coming from the panel, I'm not sure I have to double those loads or not. But even if I do, the max load would have been 36 amps for my tests.

My conclusion is that the #4 THHN 95 amp rating on a 70 amp circuit seems safe and sufficient for my setup. It seems pretty straight forward to me. What am I missing here that is causing others to say I need #1, #2 or #3 AWG conductors?

You need to use 60 degree C column unless the connection is specifically rated for 75 or 90 degrees then you can use that that column. This is because the connection cannot carry the current that would cause the wire to get to 90 degrees C.

But I am totally confused on what you have. No way do you have 30 hp capability with single phase input at 70 amps and 240 volts. I have no idea what you have and I am not a RPC guy and have no desire to learn. I do all my single phase to 3 phase conversion via ASDs (VFDs).

 

[goldenfab]

Cooper is better but you can also use aluminum provided the terminals are rated for aluminum, use the proper grease on the terminals, also make sure the bigger sized conductors aren't too big for your conduit and terminals etc. I'm just finishing up building my house and made use of aluminum service cable cable (SER) for some of my sub panels as well as 220 circuits in the garage due to sky high copper prices and availability. The documentation for the SER cable I used iirc said the rating of the strands (THHN or whatever it was).

 

[AmericanMaker]

But I am totally confused on what you have. No way do you have 30 hp capability with single phase input at 70 amps and 240 volts.

markz528, the spec tag shows that, yep, I sure do have a 22Kw (30 hp), 70 amps, 240 volts, single phase input RPC.
And at 400 lbs, it's the largest induction motor I've ever owned.

 

[AmericanMaker]

You need to use 60 degree C column unless the connection is specifically rated for 75 or 90 degrees then you can use that that column.

I don't believe that is correct. The 60º C specifies the temperature rating for a type of wire before the insulation breaks down exposing the conductor and creating a fire hazard. Temperature ratings depend on the heat resistance of the materials used for the insulation and jacket of the wire. The higher a material’s heat resistance, the less likely it will deteriorate in higher temperatures.

In the chart below, it show that the insulation and jacket on TW and UF wire will begin to deteriorate above 147º (60º C). And the insulation and jacket on THHN wire will begin to deteriorate above 194° (90º C). So, you need to use the 60º C column to find the correct ampacity for TW and UF wire and use the 90º C to find the correct ampacity for THHN wire.

And you need to use Table 310.15(B)(2)(a) to find the ambient temperature correction factor for the allowable ampacity for that wire in specific ambient temperature conditions. The hotter the ambient temperature conditions, the easier for the wire to approach its temperature rating if the load on the conductor is too high. So you have to reduce the allowable ampacity to prevent reaching that break down point.

For example, if a TW type conductor is going to be in a high ambient temperature condition, such as an attic, then you need to use Table 310.15(B)(2)(a) to find the ambient temperature correction factor for the allowable ampacity for that wire in those ambient conditions.

 

[SAF]

Sorry, I'm not trying to be stubborn or argumentative. I genuinely want to understand this.

Your actions say otherwise. Several have tried to share with you, but you keep rebutting with your chart. The NEC is a bit more nuanced than that, but you would need to do a bit more study of it to grasp the rest of the rules that apply to sizing circuits.

On the table your using the 90C column ratings are NEVER supposed to be used straight up as is, that column is only to be used for the starting basis for derating purposes. There can be many factors involved in derating besides just room temperature and number of conductors.

The basic rule of the chart is conductors used with circuits below 100A are to be sized on the basis of the 60C column, no matter what they are rated for.

Circuits of 100A or more are to be sized based on the 75C column, in all cases.

The 90C column is only to be used as the basis for derating calculation's, never directly

It’s not as cut and dry as it may first appear by a simple chart may make it appear. Others have mentioned that terminal temperature ratings can also impose limits, no matter the rating of the wire used. But I’m positive you have no idea at this point what the terminal ratings are of your supply breaker or RPC terminals are. You just make the assumption that they have to be listed for 90C, because thats the wire temp you want to use with them. So be it, it’s your gear, your the one thats going to deal with it over the long term.

Same deal with sizing your equipment circuit size. The NEC has rules based on solid history and tested principals. Things are not sized based upon ideal conditions at no load and low RPM. To the contrary they are based on the heaviest possible load under the worst operating conditions and then a reserve margin of 20% added on top of that.

Others have tried to share load numbers from othe reputable RPC manufacturers, but your guy says 70A for everything including load. And thats what you want to hear.

Real numbers should look similar to this. 30HP NEC FLA is 80A @ 230V 3 phase.

Your machine load was listed at 44A @ 230V 3 phase.

If you take them two values as a rough estimate, 80 + 44A = 124 A @ 230V 3 phase then add the 20% headroom as required by the NEC
124A x 1.2 = 148.8A
Then convert that worse case number to single phase 148.8A x1.732 = 257.72 A

Can you begin to see in rough maximum worst case numbers why a #6 AWG rated at 55A @ 60C is cutting it a bit shy?

The #1/0 AWG that was suggested earlier, rated at 150A @ 75C is a little more appropriately sized for your total load, Without the risk of overloading terminals, equipment, and your loads under heavy loads or utility voltage sags.

But your the non believer, so proceed as you wish, and keep telling yourself your not stubborn.

 

[BT Fabrication]

Your actions say otherwise. Several have tried to share with you, but you keep rebutting with your chart. The NEC is a bit more nuanced than that, but you would need to do a bit more study of it to grasp the rest of the rules that apply to sizing circuits.

On the table your using the 90C column ratings are NEVER supposed to be used straight up as is, that column is only to be used for the starting basis for derating purposes. There can be many factors involved in derating besides just room temperature and number of conductors.

The basic rule of the chart is conductors used with circuits below 100A are to be sized on the basis of the 60C column, no matter what they are rated for.

Circuits of 100A or more are to be sized based on the 75C column, in all cases.

The 90C column is only to be used as the basis for derating calculation's, never directly

It’s not as cut and dry as it may first appear by a simple chart may make it appear. Others have mentioned that terminal temperature ratings can also impose limits, no matter the rating of the wire used. But I’m positive you have no idea at this point what the terminal ratings are of your supply breaker or RPC terminals are. You just make the assumption that they have to be listed for 90C, because thats the wire temp you want to use with them. So be it, it’s your gear, your the one thats going to deal with it over the long term.

Same deal with sizing your equipment circuit size. The NEC has rules based on solid history and tested principals. Things are not sized based upon ideal conditions at no load and low RPM. To the contrary they are based on the heaviest possible load under the worst operating conditions and then a reserve margin of 20% added on top of that.

Others have tried to share load numbers from othe reputable RPC manufacturers, but your guy says 70A for everything including load. And thats what you want to hear.

Real numbers should look similar to this. 30HP NEC FLA is 80A @ 230V 3 phase.

Your machine load was listed at 44A @ 230V 3 phase.

If you take them two values as a rough estimate, 80 + 44A = 124 A @ 230V 3 phase then add the 20% headroom as required by the NEC
124A x 1.2 = 148.8A
Then convert that worse case number to single phase 148.8A x1.732 = 257.72 A

Can you begin to see in rough maximum worst case numbers why a #6 AWG rated at 55A @ 60C is cutting it a bit shy?

The #1/0 AWG that was suggested earlier, rated at 150A @ 75C is a little more appropriately sized for your total load, Without the risk of overloading terminals, equipment, and your loads under heavy loads or utility voltage sags.

But your the non believer, so proceed as you wish, and keep telling yourself your not stubborn.

This is exactly my point I was also trying to get across. If the guy wants to burn his things to the ground to save $50, insurance will straight deny any claim if they seen it was wired incorrectly. They will do anything to deny it.
I have a machine on mine and no load 6K rpm runs 8A also per 3 phase leg, machine maxes out at 63A, everything is determined to code by worst case scenerio. Sure you can undersize things and it will still work 99% of the time, but that 1% is what kills things. FLA is based upon stall rating of the motor If i remember correctly.

 

[AmericanMaker]

The 90C column is only to be used as the basis for derating calculation's, never directly

@SAF, I'm not stubborn! I'm persistent. I have read all the comments and I am not disputing that, as you said "It’s not as cut and dry as it may first appear by a simple chart may make it appear."

However, as far as I can tell, no one has explained why the NEC ampacity chart temperature ratings are broken down by wire type.

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." But what I have found explains that you apply the derating calculations on the specific wire type you are using. And that each wire type has a specific temp rating that, when the allowable amperage for that wire type is exceeded, can cause it to overheat and break down the insulating jacket which can lead to shorts and fires.

Can I just do as everyone suggests and leave it at that? Sure, but then I would severely limit what I learn.

While my machine load may be listed at 44 FLA @ 230V 3 phase, that is what the load would be with every servo and induction motor running at once, which will never happen. And the 70 amp load only occurs when the RPC starts up. The VMC is not on at that time otherwise I risk frying VMC components.

But to be cautious, if I take the 44FLA and add it to the RPC idle load of 15 amps that is 59 amps, plus the 20% headroom, that is still 71 amps. So yes, I may need to go from a 70 amp to an 80 amp breaker. But the #4 THHN rating is still 95 amps and (I believe) may well be within the load of my setup.

As a side note, I called and spoke with a North America Rotary Tech yesterday (who I did not buy my RPC from). I gave him my VMC specs. All he was interested in was the spindle rating which is 15HP and he concluded that a 30HP RPC on a 70 amp breaker is what I would need. He did however suggest #3 AWG to be safe, so there is that.

I do appreciate all the input here as well as the over-abundance of caution. To be safe I will hire an inspector to sign off on any electrical work I do for my setup.

Since l have pushed this limits of this conversation, I will leave it at that. Thanks everyone.

 

[BT Fabrication]

I think you are forgetting still about 70A 3 phase doesn't equal 70A single phase.

its single phase input and 3 phase output, thats why large industrial loads are measured in KVA and not just amps as its innacurate.

IE like a transformer that is 30KVA at 600V only needs 28.9 Amps input but if put down to lets say 208V in another panel for 208V parts you would need a panel that is larger. which is 83A.

take that same transformer in single phase and it jumps to 144Amps. as you only have 2 conductors
3 phase has 3 conductors and a larger seperation between each phase shift thus it does more work per Amp/hp

your system is working in reverse. thus you need larger input then output plus the rotation losses.

44 Amps 3 phase RPC output doesn't equal 44A single phase input in power consumed in whatever you want to make it as KVA or watts etc.

 

[BT Fabrication]

See Below on wire temp ratings right from schneider electric that makes panels and wiring products.

https://www.productinfo.schneider-electric.com/0110db9901wiretempratetermdb/0110db9901-wire-temp-ratings-and-terminations/0110DB9901%20Wire%20Temp%20Ratings%20and%20Terminations/English/0110DB9901%20(bookmap)_0000485497.xml/$/TheNECRules-D1995BA7#:~:text=Available%20Equipment%20Terminations,-Remember%20that%20a&text=For%20electrical%20equipment%20rated%20for,its%2090%C2%B0C%20ampacity.

 

[AmericanMaker]

See Below on wire temp ratings right from schneider electric that makes panels and wiring products.

@BT Fabrication, thanks that's a super helpful explanation that I can wrap my head around.
And you couldn't have timed that any better. I was just about to post this when I saw your post come through ....

I did some re-reading of the posts as well as other info online. One thing that has become clear to me is that I failed to pay closer attention to the "connection" comments, because I was so focused on the "conductor" ratings. My bad!

I found plenty of legit references online (cable manufacturers and suppliers) that say "the most common conductor temperature rating is 90°C." But then I found this one sentence on a USA wire manufacturers site "Many connectors are rated at 75°C."

Did I fail to recognize or understand that the conductor must be matched to the connectors? I think that's what happened.

Neither my VMC or RPC have listed connection ratings. I will try to find that out. Someone mentioned that, if the connection rating is not listed, you should assume a 60ºC rating. That helps to better understand why a larger conductor is needed.

Anyhow, I'm glad I persisted!

 

[BT Fabrication]

Article 670 of the NEC (industrial machinery)

The size of the supply conductor shall be such as to have an ampacity not less than 125 percent of the full-load current rating of all resistance heating loads plus 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all other connected motors and apparatus, based on their duty cycle, that may be in operation at the same time.


and yes, I had figured it was just adding up the 2 things till it clicked.
Always assume the worst case scenario with electricity, so if its not stamped anywhere on it 75C or all electrical parts and lugs etc, always go with 60C ratings.

its a bit confusing as they make higher temp wire then 90% never need it that high under 600V yet most things are only good for 60C and maybe 75C if its a starter contactor fed directly from a transformer or some crazy combination.

 

[AmericanMaker]

I know can return the 70 amp breaker to the Home Despot.
Hopefully I can return the two 35' pieces of #4 AWG they cut for me too.

 

[SAF]

Save those pieces of wire, it’s a good reminder of being stubborn cost’s money. The important part is you started to wake up, before it cost you serious money. I’m glad you started taking off your blinders, so the time we all spent on you wasn’t a waste. Best of luck to you in your future endeavors.

 

[AmericanMaker]

Real numbers should look similar to this. 30HP NEC FLA is 80A @ 230V 3 phase.

Your machine load was listed at 44A @ 230V 3 phase.

If you take them two values as a rough estimate, 80 + 44A = 124 A @ 230V 3 phase then add the 20% headroom as required by the NEC
124A x 1.2 = 148.8A

Then convert that worse case number to single phase 148.8A x1.732 = 257.72 A

@SAF the RPC's load at start up may be 70 to 80 amps, but the idle amps are between 15 and 20.

The VMC is not on when the RPC starts up as to not damage the mill's electronics. So the 44 FLA is not a factor when the RPC starts.

I'm curious why you used the 80 amp max surge rating in the calculation instead of the idle amps?

(80A + 44A )*1.2 = 148.8A instead of (20A + 44A) * 1.2 = 79A ?

 

[SAF]

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.

NEC is based on worse case scenario, not what you can squeak by with.