# Is this wire undersized



## maninthesea (Nov 11, 2008)

Hello 
Was asked to quote bringing a compressor back into service at the new fire station today. The compressor has a 10HP 3 phase motor running on 220Volt

MFR of new building had installed a 60 am fused disconnect with 60amp fuses & #8 wire to machine #10 ground.

Its my calculation that the fused disconnect and fuse size is correct based on the motor rated to pull 50amps on startup. 
I understand #8 wire to be rated for 55amps but I belive the wire from the disconnect to the machine should be rated the same or higher than the fuses.

So 55amp wire after 60amp fuses must be a code violation. 

I am thinking I should make them pull the #8 and replace it with #6.

Am I correct or smokeing crack.
Thanks Jim


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## 480sparky (Feb 1, 2009)

Not enough information.

Need the nameplate information of the motor, as well as the type of wiring used.


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## Inner10 (Mar 12, 2009)

I thought regardless of the situation 60A breaker is too high for an 8AWG wire?

...but I guess the question could be can you get away with a 40A breaker...


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## maninthesea (Nov 11, 2008)

480
Sorry I did not realise the nameplate data would make much of differnece. I dont have it without another trip out. What are looking for Full load amps, Efficency or something else. I dont recall Full load amps but do recall it was less than the 60amp fuses and also 55amp # 8 wire. 
As for the wire most of the printing was rubbed off in the pulling but I can tell you it was TH?? The next two letters were vague. 

Inner that was what I was getting at. The cheat sheet I have from the electrical store shows motor at 50 Full load amps and THHN #8(What I assume the wire is) at 55amps which would be fine except the wire is downstream of 60amp fusses which would indicate the wire could fail/melt before the fuse blew. Thats the red flag to me.

I am going to call the fire cheif spearheading this today and recomend he get the public works electician to verify.

Cheers Jim


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## 480sparky (Feb 1, 2009)

Nameplates have a lot of qualified engineering behind them. I've seen motors running on 35 amp breakers wired with #14 wire, and it's legal.

Motor loads really can't use 'cheat sheets'. They require calculations.


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## mrmike (Dec 9, 2008)

maninthesea said:


> Hello
> Was asked to quote bringing a compressor back into service at the new fire station today. The compressor has a 10HP 3 phase motor running on 220Volt
> 
> MFR of new building had installed a 60 am fused disconnect with 60amp fuses & #8 wire to machine #10 ground.
> ...


How did you calculate the 50 amps on start-up? What Rpm is this 10 hp motor? Your talking about #8 & #6 awg. There are many different types with different Temp ratings, etc that affect Amp ratings. Is it Copper or Aluminum wire which will also effect the amp rating........... Need more Info here............. As a rule though, I myself like to be a size bigger than needed-& I know many would debate that.............


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## newenergy (Mar 5, 2008)

480sparky said:


> Nameplates have a lot of qualified engineering behind them. I've seen motors running on 35 amp breakers wired with #14 wire, and it's legal.
> 
> Motor loads really can't use 'cheat sheets'. They require calculations.


90 degree copper THHN in free air? Some kind of fine stranded wire or something? Or is there some specific exception that lets you use #14 copper in conduit on a 35amp breaker.

If copper (nothing fancy like battery cable) conductors aren't in free air here, we're at a minimum size (before any deratings) of #6, no?


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## 480sparky (Feb 1, 2009)

newenergy said:


> 90 degree copper THHN in free air? Some kind of fine stranded wire or something? Or is there some specific exception that lets you use #14 copper in conduit on a 35amp breaker.
> 
> If copper (nothing fancy like battery cable) conductors aren't in free air here, we're at a minimum size (before any deratings) of #6, no?


Not necessarily. Check out Article 240.4(G).


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## Magnettica (Dec 19, 2006)

Motors are inductive loads (as compared to resistive loads, like a light bulb), and have specific rules for many things. 

Things like....

1. Single phase or three phase & horsepower rating
2. Overload protection
3. Branch circuit conductor sizing
4. Short-circuit and ground fault sizing
5. Feeder conductor sizing (for panels and for taps)
6. Overcurrent protection for the feeder (fuses or circuit breaker)

These are all dependent on the nameplate data. 

The key thing to remember about motors is that draw 4-6 times more ampacity at start up than they do once they're up and running. Whenever possible choose three phase motor loads over single phase because the motors are more efficient.


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## maninthesea (Nov 11, 2008)

Good catch on the 50 Full load amps. That came from the electrician at the eletrical supply store. He did a quick lookup on the table and it looks like he used 10HP single phase 230volts.(His mind must have still been watching Youtube) which is 50amps on 430.148 

but it apears to me that the proper table would be 430.150 which gives 10HP 3 phase 208(since our 220 is generaly around 212-213VAC anyway) 30.8Amps X 1.25 = 38.5amps so fuse at 35amps wire must be rated 40 amps or more. 

So the fuses are overrated by these calculations. But I belive the starting current on compressor motors are higher than normal and I am sure the 35amp fuses would blow all the time on startup. Maybe it should be more like 50amps. 

But the fundemental question I had was are you allowed to use 55amp rated wire with 60 amp fuses. 
Either way Ill let their electricians aprove or disaprove but I just wanted to educate myself a little. Those wires just looked much smaller than I am used to. The wires from the overload to the motor are 6 guage.


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## K2 (Jul 8, 2005)

> But the fundemental question I had was are you allowed to use 55amp rated wire with 60 amp fuses.


There are different rules for sizing wires used for motors. It is very common for guys to oversize wires because they don't know the rules but with the current price of wire, it's worth while to learn the rules.


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## Magnettica (Dec 19, 2006)

Is this a single phase or three phase motor? 

If we know that and we know the HP then we can help you do this properly. 

Mind you, the 125% increase applies to the branch circuit sizing (the conductor size), not the full-load current rating of the motor. 


--------------------------

1) A 3Ø, 208 volt motor has a FLC rating of 30.8 amps. Table 430.250

2) Now since this motor is larger than 1HP we have to use overload protection called "heaters." Since we don't know what kind of motor this is we'll assume it has a service factor of 1.5%. Our "heater" protection must be 125% higher then the FLC rating. 430.32(A)

30.8 X 125% = 38.5 amps at a minimum for heater protection. 

3) Branch circuit ampacity must 125% more than the FLC. 430.22

30.8 X 125% = 38.5 amps minimum size conductor (#8 THHN) 75º column, 310.15 (B)(6)

4) The circuit breaker must be sized in accordance to Table 430.52.

So... 30.8 FLC x 250% (inverse time circuit breaker) = 77 amps. 

An 80 amp circuit breaker is required for this motor. 



Now wasn't that easy?


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## 480sparky (Feb 1, 2009)

Magnettica said:


> ............
> Now wasn't that easy?


Not near as easy as the Cheat Sheet down at Home Despot.:w00t:


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## maninthesea (Nov 11, 2008)

Thanks Magnettica (so more than likely the machine will be blowing the 60amp fuses even if they are slo-blow. But the wire size is good and it is safe. So if the public works electician says its good to go there will not be a saftey issue.

Two questions though. 
1. I thought the rule of thumb on fuses and circut breakers was go to the next size down as opposed to conductors which are the next size up?
2. All the math there makes sense but it seems odd to me that the wires from the fused disconnect to the magnetic starter are going to be rated for less current than the fuses. So if there is a problem upstream of the thermal overloads that cause more current draw than the wires are rated for they will fail before the fuses blow or panel breaker trip? 
Thanks for the education


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## 480sparky (Feb 1, 2009)

Thermal overloads are to protect the motor, not the conductors. The fuses protect the conductors.


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## maninthesea (Nov 11, 2008)

& 480 I dont expect to get any good info on anything from the folks at Home Depot. There is one electrical supply store here that I patronize becuase the price is reasonable and they have 1 guy that knows the trade well. He has been out sick this week so I had to ask his boss that I am not as confident in.
Cheers Jim


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## Magnettica (Dec 19, 2006)

maninthesea said:


> Thanks Magnettica (so more than likely the machine will be blowing the 60amp fuses even if they are slo-blow.



Dual Element (Time Delay) fuses aka "slo-blow" fuses have a different percentage multiplier than a normal circuit breaker. Table 430.52


30.8 amps x 175% = 53.9 amps.


So 60 amp "slo-blow" fuses are sized correctly.


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## maninthesea (Nov 11, 2008)

480sparky said:


> Thermal overloads are to protect the motor, not the conductors. The fuses protect the conductors.


not sure what your clearing up?

What I am asking is can a 60amp fuse protect a 55amp wire. 
If 56amps is too much for the wire then I would think we would want a fuse that kept that from happening. Hope this clarifies the question.


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## HARRY304E (Jan 18, 2011)

maninthesea said:


> not sure what your clearing up?
> 
> What I am asking is can a 60amp fuse protect a 55amp wire.
> If 56amps is too much for the wire then I would think we would want a fuse that kept that from happening. Hope this clarifies the question.


You would want a 50 amp fuse for a conductors that are only rated for 55amps.look at 240.6(A) standard fuse sizes..:thumbsup:


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## K2 (Jul 8, 2005)

> What I am asking is can a 60amp fuse protect a 55amp wire?


Yes . Use 430.22, and 430.248 thu 430.250, and 430.52 along with 310.16 and 240.6.


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## John Valdes (Apr 14, 2010)

maninthesea said:


> Hello
> Was asked to quote bringing a compressor back into service at the new fire station today. The compressor has a 10HP 3 phase motor running on 220Volt
> 
> MFR of new building had installed a 60 am fused disconnect with 60amp fuses & #8 wire to machine #10 ground.
> ...


Breaker - 50 amp x 2.50 = 125 amp max.
Conductor Size - FLA x 1.25 = I* 
Overload - FLA

This is based on 3 phase 10 hp motor running at 230 Volt. *Wire size selected from article 310.16.


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## maninthesea (Nov 11, 2008)

Update if anyone cares

Their electician decided to upgrade to #6 wires. 

Nameplate on motor shows 26.4 amps for 208volt(3 phase) sf of 1.15

Main breaker is 60 amps

Wire from breaker to fused disconnect #6. Wire from overloads to motor #6

Voltage at fused disconnect 205, 207 & 205

Thanks for the input.
Jim


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