Slecting the correct wire Ga..

[veno]

.I need to wire a 70 amp circuit..(welder) the distance is 90ft... would I be ok using 6ga. copper or should I jump up to 4ga.? also what size conduit

Also.. have a 7.5 hp 30amp compressor... wire length is 40ft... will 10ga do this?

Thanks

Veno

 

[mrb]

i think youre going to need #8 for the compressor if the FLA is 30amps.

the welder depending on duty cycle you can use the table and use smaller wire. Me, I would just put in #4. Or since its 90ft to the welder, install a subpanel near it so I have more power over there saving me from future long runs.

 

[ArthurPE]

is the 70 and 30A the circuit rating or the operating load current of the devices?

what voltage?

 

[nate379]

I was reading some info and they say a good rule of thumb is it's under 200 ft that it's not an issue.

Now I read something else where they are talking total run, like 100ft run is really 200ft because the juice has to run both ways. I'm not sure.

One calculator I used said 12 guage wire for a 100ft run for 30 amps even... last I checked 10 guage for 30 amp, but I'm not an electrician either.

 

[ArthurPE]

Vd = I Z for AC 1 phase

I = current
Z = impedance (magnitude of R and reactive)
iirc 12 cu is ~2 ohm per 1000'

a good rule is 3 to 5% drop...3 on feeders, 5 is, imho, OK on a branch circuit...
that's why most equipment is rated 115 for use on a 120 circuit...
0.95 x 120 ~ 114...

so let's say we size for 5V of drop...a little margin of error...

what's the maximum run w/#12 that we can make to have 5V (~5%) drop?
let's assume worse case, fully loaded 20A circuit, allowable is 80% or 16A

5 = 16 x 2L/1000 x 2
L = distance 1 leg runs in feet, must account for supply and return path, so 2L

solving L = 5 x 1000/(2 x 16 x 2) ~ 78 ft...

so I would say 100' is a decent rule of thumb...~6V drop
as long as any connections/terminations are done well, they add Z, and therefore Vdrop

another thing to consider: the utility usually supplies a bit over 120

 

[nate379]

Good info. I guess maybe that 200ft rule I read about is both ways so really 100ft of wire.

 

[Aceman]

.I need to wire a 70 amp circuit..(welder) the distance is 90ft... would I be ok using 6ga. copper or should I jump up to 4ga.? also what size conduit

Also.. have a 7.5 hp 30amp compressor... wire length is 40ft... will 10ga do this?

Nobody can answer your question properly without knowing:

Welder type, duty cycle, input amperage, voltage

Compressor voltage

Wire type: THHN for everything or Romex or ???

Conduit size? Is the compressor and welder wire both sharing a conduit?

Need way more info!

 

[cortez]

.I need to wire a 70 amp circuit..(welder) the distance is 90ft... would I be ok using 6ga. copper or should I jump up to 4ga.? also what size conduit

Also.. have a 7.5 hp 30amp compressor... wire length is 40ft... will 10ga do this?

Thanks

Veno

As a general contractor I will always "step up" a gauge unless the customer over rules it (after all they are the boss and do not have to comply with your recommendations). Unless you want to go hungry by losing the jobs to other willing subs, reality forces one to compromise. If you can afford it always step up!!!

On top of this many commercial and industrial customers have a gauge on hand that they want to use and their in-house electrical engineers "OK'' it for usage. The size of the conduit should be adequate enough that it never becomes warm to the touch (again 40 percent full only, but the in-house experts may have you jam pack the conduit for the simple reason that this size is on hand). Always write up the ticket showing that these were "Ok-ed" by the engineers and your recommendations were otherwise.

No one ever reads the ticket except for the "cost" line!!

 

[nate379]

Upsizing is fine if you are close to the limits, but many times there isn't a reason. I mean a guy could run 6 gauge for a 15 amp circuit if he wanted, but why?

 

[mrb]

As a general contractor I will always "step up" a gauge unless the customer over rules it (after all they are the boss and do not have to comply with your recommendations). Unless you want to go hungry by losing the jobs to other willing subs, reality forces one to compromise. If you can afford it always step up!!!

On top of this many commercial and industrial customers have a gauge on hand that they want to use and their in-house electrical engineers "OK'' it for usage. The size of the conduit should be adequate enough that it never becomes warm to the touch (again 40 percent full only, but the in-house experts may have you jam pack the conduit for the simple reason that this size is on hand). Always write up the ticket showing that these were "Ok-ed" by the engineers and your recommendations were otherwise.

No one ever reads the ticket except for the "cost" line!!

most of this makes no sense.
Youre telling me you would run #10 for a single light fixture on a 20amp cir? Upsizing for no reason does nothing but waste copper and money

As far as overfilling conduit, just because someone told you to and you wrote 'ok-ed' on the ticket doesnt make it legal.

 

[cortez]

The "electrician in charge" (who must be licensed) is responsible for the work he directs here in Chicago. Most work in factories and big commercial businesses go un inspected even if a permit is pulled. The rational for these otherwise violations is that the wiring is temporary. And legally if an inspector OK's a job it is legal. Getting an inspector to write off on a job is often done for political purposes.

Chicago has its own sense of what is legal or not and has its own electrical code that is written in legal jargon and can legitimately be interpreted to what seems opposite to its intent.

This is the "city state" mentality of Chicago since day one of its formation as a frontier town that was noted for its legal elasticity.

 

[veno]

Actually the Welder(tig) is a Miller Syncro wave 300.. 98 amps 240v 60% cycle.. but it will never run at full cycle or full amperage... it's max amperage is 375 amps... normal amperage will be between 30 and 200... and only at about 4 to 6 min constant.

the compressor is a 100 gal, 3 cylinder 25 amp @ 240v constant duty...

My ThermalDynamics PacMaster XL75Pus is 60 amp @ 240. 60% cycle, 80%@ 55amp and 100%@ 45amps...

al three will have seperat circuits and never will all three be operated at the same time.. all will have dedicated separate thin wall emt.

the wire length for the welder and plasma will be around 60ft

the compressor will be around 80

 

[sberry]

I would feed the welder 4, it actually requires a 2 I believe. If you are under wiring you should not over breaker it, there is the potential to over heat the wire. If you follow the wire size in the service guide there are allowances for breaker size. Make sense?