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electrifier, Master Electrician
Category: Electrical
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Experience:  Electrical contractor and electrical inspector for over 24 years with phone and networking expertise
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I am wiring two separate 220 volt circuits and one 110 15

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I am wiring two separate 220 volt circuits and one 110 15 amp circuit in the same conduit. One is a 220 volt single phase 40 amp waste water eater the other is a 30 amp 220 volt single phase air compresser. the 110 is an outlet to run two small electric devices I use in this area. Its all to be ran in conduit. This is for a commercial application.

The lenght of wires from the breaker to the actual consumer is 42 to 45 feet.

I was figuring four #8 wires ( 2 blue 2 red or whats available) for the powers to each of the 220 and one single 10 gauge ground wire shared betwen the two for a ground and then three #12 wires one black one white and one bare ground for the 110 outlets. And using 50 amp breakers for the 220 units.

Any tips before I have the wires cut to length.
You have 6 current carrying conductors on a single pipe. The derating for this is 80% and the 90 C. column is used for this derating. The 40 Amp circuit needs to use #8 THHN which is good for 44 amps in this case. The 30 amp circuit needs #10 which is good for 32 amps in this case. The 15 amp circuit needs #14 which is good for 20 amps in this case but can only be protected at 15 amps. The 90 C. table on these three conductors is #8 @ 55 amps, #10 @ 40 amps and #15 @ 25 amps. You will need to protect these circuits with 40, 30 and a 15 or 20 amp breaker for these conductors to be correctly sized. Look at the waste water device and the air compressor to see what the amperage requirements are for these devices and post them back here if you like. We can discuss it further if you like. You don't want too big of a breaker since the devices need the overcurrent protection for their own protection as well as that of the conductors. Sometimes there is a maximum circuit amperage right on the tag for the equipment. We need all these facts to properly protect this equipment and prevent a big problem later.
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Customer: replied 7 years ago.

Sorry I should have posted more details about my needs and question


Ok the Waste water eater manual states it needs to be on a 50 amp breaker I assume 8 gauge wires for power wires to it


the compressor states nothing about amperage for breaker size I actually believe it runs at a higher initial start up amperage than the Waster water eater does so i was figuring its needs would be slightly over 40 Amps the math comes up about 40 for average consumers at the 125% ratio for figuring breaker sizes but giving its start up current is higher than some consumers when doing standard wiring math which is why I was thinking 50 amps as well. And also I was thinking 8 gauge power wires so that if in the future my compressor becomes larger the wires would be there.


next the ground for the two can they be shared and if so what gauge wire if not what gauge wire for each one.


The other 110 outlet is acutally branched off another 20 amp outlet circuit all with low demands. Just a basic 110 outlet on that end of the shop. I am planning on # XXXXX wires because I have them spooled up already??


Maybe this will help but my primary concern is wire sizes for each 220 circuit and sharing the ground on the two 220 circuits. Thanks youve been helpfull

What size is the compressor motor?
What is the amperage requirement for the waste water eater?
Remember your #8 conductors will only support a 44 amp load.
Customer: replied 7 years ago.

the waste water eater is 40 amps


the compressor has two ratings 30 amps and then there is something like peak 36 amps both came directly off the tag on the motor

What size motor is the compressor? How many HP?
Customer: replied 7 years ago.
sorry its a 7.5 HP 80 gallon tank high cubic inch displacement pmp I could not find anything stating size but its a fairly large motor this is a true 7.5 HP rated compressor not one rated at a higher RPM like some of the smaller ones. I found one other part on its label stated Max 40 amps at something like 70 degrees ? however the label clearly shows it as 30 amps 220 single phase
A 7.5 HP motor uses 40 amps as it's full load amperage according to the NEC. The HP rating is to be used instead of the amperage rating on the label according to the NEC. This would mean at 120% the rating would have to be 50 amps. #6 wire would be required for this with the 80% derating factor. 75 amps X 80% = 60 amps. As stated before #8 would only yield a 44 amp rating which is not enough for the required 125% of the amperage.
Now for the waste water eater the #8 is also too small to give 125% for the required amperage. It would also need to be #6.
You are good with #12 for the 120 volt circuit.
If you run a single ground in this conduit for a 50 amp circuit and tap each ground in a junction box you will do fine. The ground only needs to be large enough for the largest load served which would be a 50 amp circuit. #8 will suffice for this ground taking derating into consideration in this conduit.
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Customer: replied 7 years ago.

Thanks you have been very helpful


I am trying to fix my paypal payment I was trying to leave an extra $10 but it didnt take correctly I will try and fix that.



Thank you. I try to be complete and give good information. As an inspector for 20 years I know my information can be very helpful. I appreciate your cooperation here and the extra information you have provided.
Customer: replied 7 years ago.

I put in anotehr $9 for a tip I hope it came through OK The payment page is confusing. Might be me. However your information was more than very helpfull it was educational as well. I needed assurance I was completing this circuit correctly and feel your answeres were rock solid.




Thank you. As an inspector for the past 20 years I've learned to answer questions like yours.