Running new circuit to AC unit, please review my plan.

[dustman]

Running a new circuit to an AC unit, making sure I'm on the right track. The attached chart has the information for this unit which is on column 4.

Maximum overcurrent protection: 30 amps
Minimum circuit ampacity: 19amps
240 volts

It will be running across a rooftop in a pretty hot location.

Here is the current plan: Meyers hub into top of main panel, #10 THHN stranded run through 1/2" emt with a 30amp breaker into j-box near unit, j-box to unit with 1/2" liquidtight flex conduit.

Any reason to run #8 in 3/4" emt? Is tapping into the top of the main panel a code violation? Can I transition to flex conduit without a j-box?

Thanks for your input!

 

[pc1966]

This looks like one for one of our USA members such as @Megawatt as most of us are really only familiar with the UK wiring regulations.

Here we would also be wondering about the total length to check max voltage drop, etc, but on the face of it #8 AWG = 8.3mm^2 looks reasonable for a 30A breaker, but I do wonder about your comments on hot location as you appear to be Arizona and having been there once I know it gets HOT. So cable de-rating and the use of an insulation material rated to, say, 90C like XLPE would be my thoughts. But wait for Megawatt, I don't know the USA code!

 

[dustman]

This looks like one for one of our USA members such as @Megawatt as most of us are really only familiar with the UK wiring regulations.

Here we would also be wondering about the total length to check max voltage drop, etc, but on the face of it #8 AWG = 8.3mm^2 looks reasonable for a 30A breaker, but I do wonder about your comments on hot location as you appear to be Arizona and having been there once I know it gets HOT. So cable de-rating and the use of an insulation material rated to, say, 90C like XLPE would be my thoughts. But wait for Megawatt, I don't know the USA code!

Thank you.

The length will be about 40'.

 

[pc1966]

OK, that is not very long so you should be well under 5% voltage drop with any cable likely to meet the current carrying capacity (ampicity).

Do you have any idea of the max ambient temperature the cable will be subject to? I would imagine it might see 60C on an AZ rooftop (air at 40C or more, plus some direct sun heating) during the time when the A/C is working at its hardest.

 

[dustman]

OK, that is not very long so you should be well under 5% voltage drop with any cable likely to meet the current carrying capacity (ampicity).

Do you have any idea of the max ambient temperature the cable will be subject to? I would imagine it might see 60C on an AZ rooftop (air at 40C or more, plus some direct sun heating) during the time when the A/C is working at its hardest.

Max air temp is likely to be about 46C. Not sure how much hotter it gets in emt in direct sun. It would be west facing in the crook between a wall and white roof, probably the worst possible place, but it's pretty much the only good option.

 

[pc1966]

Max air temp is likely to be about 46C. Not sure how much hotter it gets in emt in direct sun. It would be west facing in the crook between a wall and white roof, probably the worst possible place, but it's pretty much the only good option.

OK you might have to look at 70C ambient for cable de-rating, that may well be fine with #8 cable but I don't know the NEC regulations or the typical cable specs you would work with.

In the UK most conduit wire is PVC and rated to 70C operation, though in some areas the move to LSZH (low smoke, zero halogen in the event of a fire) insulation is bringing the max temperature to 90C.

 

[pc1966]

Just as an example, if this was in the UK and I was looking at the design, my first step would be to look up 90C rated single core cables (Table 4E1A in our wiring regs) and in column 4 we have the rated CCC (ampicity) for 2 conductors enclosed in conduit on a wall (table for ambient of 30C). The choices that look applicable are:

• 2.5mm (4% under #13 AWG = 2.62mm) = 31A
• 4mm (4% under #11 AWG = 4.17mm) = 42A
• 6mm (10% under #9 AWG = 6.63mm) = 54A

Then on the assumption the ambient temperature could be 70C we would refer to Table 4B1 for de-rating and under the "90C thermosetting" column we get a factor of 0.58 for 70C ambient, leading to a new set of CCC values:

• 2.5mm = 31A (at 30C) * 0.58 => 18A at 70C
• 4mm = 42A (at 30C) * 0.58 => 24A at 70C
• 6mm = 54A (at 30C) * 0.58 => 31A at 70C

The spec you have gives minimum ampicity at 19A so we would have to chose 4mm or 6mm for this circuit.

Next we would check voltage drop and from the next page Table 4E1B column 3 has the VD for 2 single phase cables in trunking or conduit as:

• 2.5mm = 19 mv/A/m (but not acceptable on 70C ampicity)
• 4mm = 12 mV/A/m
• 6mm = 7.9 mV/A/m

You give the distance as 40' = under 13m so at the 19A rating for the A/C unit:

• 4mm = 12 mV/A/m * 19A * 13m = 2.9V = 1.2%
• 6mm = 7.9 mV/A/m * 19A * 13m = 1.9V = 0.8%

So both meet VD of 5% for final circuit and so 4mm (approx #11 cable) would probably be our choice.

The other thing we do is compute the earth loop fault impedance Zs which is based on the supply impedance (Ze at source of installation, Zdb at sub-panel) and the resistance of the line conductor (R1) and that of the CPC (circuit protective conductor = ground wire) commonly given the notation R2. We require sufficient fault current in the event of a L-E (hot-ground) fault that the breaker disconnects in under 0.4s, typically this means you have to hit the magnetic trip point.

Here the USA is more complicated as your 240V is actually 120V-0V-120V two-phase and we don't know your supply impedance or that of the EMT conduit you would be using. Assuming the conduit is mechanical protection only and not the CPC (i.e. even though the EMT is grounded you run another #11 ground wire through it for the load), then R1+R2 is going to be the same as the above voltage drop figures:

• 4mm = 12 mV/A/m * 13m = 0.16 ohm
• 6mm = 7.9 mV/A/m * 13m = 0.1 ohm

We don't know the Zs at the DB feeding this, but assuming it is 200A rated and 5% drop on the 240V then the prospective short circuit current = 4kA so impedance around 0.06 ohms. Again, assuming (always a dodgy thing to do...) that the CPC and line are same ratings, then PFC=PSCC so we get our end of circuit Zs values as:

• 4mm case Zs = Zdb + R1+R2 = 0.06 + 0.16 = 0.22 ohm
• 6mm case Zs = Zdb + R1+R2 = 0.06 + 0.1= 0.16 ohm

Worst-case fault to ground would be, say during -10% on your nominal 120V to ground (108V) and fault currents are then:

• 4mm = 108V / 0.22 ohm = 490A
• 6mm = 108V / 0.16 ohm = 675A

I don't know your breaker characteristics, but in the UK/EU the smaller systems (panels typically to 250A in, 63A max out) we have three MCB curves to chose from that alter the ratio of magnetic trip to thermal rating. Here it would be a 32A breaker as specified by the manufacturer and so our choices are:

• B-curve = 3-5 * In = 5*32A max = 160A
• C-curve = 5-10 * In = 10*32A max = 320A
• D-curve = 10-20 * In = 20*32A max = 640A

So on the above assumptions we would need to use 6mm cable if a D-curve breaker was in use, but typically for something like A/C it would be C-curve in which case both 4mm and 6mm would lead to fast enough disconnection in the event of an earth fault.

TL;DR Under UK regs #11 wire with 90C rating would be acceptable (if assumptions true). Disclaimer: the above is not the USA code!

 

[pc1966]

Just to add, there are a couple of other design aspects that should be checked, but in a situation like this we know are not normally at risk. The obvious ones are:

• Make sure the breaker is rated to interrupt the worst-case prospective fault current
• Make sure the worst-case let-through energy of the breaker opening is below the adiabatic limit of the cable(s).

In UK domestic and small commercial/industrial the supply has a HRC fuse of 60-100A rating and that limits the PFC to peak below 10kA irrespective of the supply's PFC, so our 6kA domestic / 10kA commercial rated MCB are almost always going to be safe. Similarly the MCB are energy-limiting types so we know they are normally safe for adiabatic limit on cables of 2.5mm or greater, and often also for 1.5mm and even 1mm (see On-Site Guide table B7).

In a larger commercial/industrial situation you would need to make sure the cascade of DB supply protection and final circuit protection is safe for max supply PFC, and also to check the cable adiabatic (though really only a likely risk if the cable CCC is below the breaker's thermal rating).

 

[Megawatt]

Running a new circuit to an AC unit, making sure I'm on the right track. The attached chart has the information for this unit which is on column 4.

Maximum overcurrent protection: 30 amps
Minimum circuit ampacity: 19amps
240 volts

It will be running across a rooftop in a pretty hot location.

Here is the current plan: Meyers hub into top of main panel, #10 THHN stranded run through 1/2" emt with a 30amp breaker into j-box near unit, j-box to unit with 1/2" liquidtight flex conduit.

Any reason to run #8 in 3/4" emt? Is tapping into the top of the main panel a code violation? Can I transition to flex conduit without a j-box?

Thanks for your input!

The biggest thing I see is you would be required to run 3/4 flex or pipe, but you stated that it would be on a pretty hot roof so having said that you need to derate your wiring and install # 8 instead of # 10 wiring even though it’s on a 30 amp breaker. You can enter the panel on top but be very careful because it could be a top fed panel. If you transfer to flex it has to be continuous all the way if your are using stranded wire instead of romex

 

[dustman]

The biggest thing I see is you would be required to run 3/4 flex or pipe, but you stated that it would be on a pretty hot roof so having said that you need to derate your wiring and install # 8 instead of # 10 wiring even though it’s on a 30 amp breaker. You can enter the panel on top but be very careful because it could be a top fed panel. If you transfer to flex it has to be continuous all the way if your are using stranded wire instead of romex

Thanks, I did a bunch of digging and came to the same conclusion. The #10 is right on the edge considering ambient temperature and proximity to the roof surface. The nec can be confusing, refer to such and such, which refers you to such and such and so on.

According to emt and liquidtight fill charts I could put the 3 required #8 wires in 1/2" conduit. Did I miss something?

Not a top fed panel, going to tape over the feeds to the main breaker before getting to work in there.

During my research, it also seems that I have to use wire rated for wet locations, even though it is in conduit. Thwn or thwn-2? I could swear we were using thhn when I was doing solar installation back in the day.

You are saying the wire has to be continuous or the conduit type? Also, I was under the impression that romex was prohibited on such a circumstance.

 

[Megawatt]

Thanks, I did a bunch of digging and came to the same conclusion. The #10 is right on the edge considering ambient temperature and proximity to the roof surface. The nec can be confusing, refer to such and such, which refers you to such and such and so on.

According to emt and liquidtight fill charts I could put the 3 required #8 wires in 1/2" conduit. Did I miss something?

Not a top fed panel, going to tape over the feeds to the main breaker before getting to work in there.

During my research, it also seems that I have to use wire rated for wet locations, even though it is in conduit. Thwn or thwn-2? I could swear we were using thhn when I was doing solar installation back in the day.

You are saying the wire has to be continuous or the conduit type? Also, I was under the impression that romex was prohibited on such a circumstance.

Thanks, I did a bunch of digging and came to the same conclusion. The #10 is right on the edge considering ambient temperature and proximity to the roof surface. The nec can be confusing, refer to such and such, which refers you to such and such and so on.

According to emt and liquidtight fill charts I could put the 3 required #8 wires in 1/2" conduit. Did I miss something?

Not a top fed panel, going to tape over the feeds to the main breaker before getting to work in there.

During my research, it also seems that I have to use wire rated for wet locations, even though it is in conduit. Thwn or thwn-2? I could swear we were using thhn when I was doing solar installation back in the day.

You are saying the wire has to be continuous or the conduit type? Also, I was under the impression that romex was prohibited on such a circumstance.

If it’s a home you are working on then you can use romex but since you are putting it in pipe and using THHN or THWN is designed for wet locations. Yes you can use 1/2 pipe but it would be a hard pull. My advice is to use 3/4 pipe and the wire can be spliced but the raceway has to be continuous.