Can 3/4" emt conduit support these conductors?


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Old 07-19-15, 08:56 AM
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Can 3/4" emt conduit support these conductors?

I want to run (all will be single phase 120V)

1. 2 x 30 amp circuits on 10 gauge THHN: 4 conductors (2x hot, 2x neutral)
2. 3 x 20 amp circuits on 12 gauge THHN: 6 conductors (3x hot, 3x neutral)
3. Common Ground conductor, 10 gauge THHN

So I think I am looking at 11 total wires in the conduit, 10 of them carrying load.

According to the charts I have looked up, I am allowed to have 10 conductors of 10 gauge THHN in a 3/4" conduit, so considering more than half of these are 12 gauge, I think this should be fine?

But what I am worried about are the complexities of ampacity derating. (I used an online calculator for conduit fill and I look OK there). I'm thinking I might have to drop one of the 20 amp circuits to avoid derating and keep my breakers at 2x30A and 2x20A?

Another option would be to change one of the 20A circuits to 15A (14 gauge) as I think I will only use it for some outdoor flood lights. Would 2x30A, 2x20A, 1x15A keep me under the limits?

These conduit calculations seem pretty straight forward if you are using all conductors of the same gauge, but it seems tricky when you mix gauges...
 
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Old 07-19-15, 09:11 AM
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Where are the circuits being run to? If this is to a garage or something it would be far easier to install a sub-panel.
 
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Old 07-19-15, 09:16 AM
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Everything is within 15 feet of the panel. A sub panel would be silly.
 
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Old 07-19-15, 05:55 PM
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If the run is that short, use 2 conduits and divide the runs. Won't cost much more in materials.
 
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Old 07-19-15, 08:15 PM
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Why not run a multi-wire circuit for your 30 amp circuit? (use a 2 pole breaker) You will save a wire and it will get you at 9 conductors which negates the derating issue. You could also do the same for two of the 20 amp circuits and your down to 8 wires.

You can also skip the ground wire if you want as EMT is a listed grounding path.
 
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Old 07-20-15, 07:56 AM
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Agreed this is a good application for either multiwire circuits or multiple conduits, but what are you using the two 120V 30A circuits for? The only thing I regularly see those required for is RV shore power. Perhaps there's a different way to wire it.
 
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Old 07-20-15, 02:07 PM
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Thanks for the suggestions. The 30amp circuits are for some hefty UPSs (Uninterruptible power supplies).

So if I don't do multi wire, as long as I keep it to 9 conductors or less I should be fine?

I guess I will do 2x30A and 2x20A. That should be 8 conductors and pose no problem in 3/4" EMT, correct?
 
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Old 07-20-15, 04:24 PM
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Yes to all the above.
--------------------
 
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Old 07-21-15, 07:57 AM
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In that case, it is legal by code, but I don't think it's a good design. Code provides a minimum standard. You should run separate conduits for each UPS and oversize the conductors one size. High density switching power supplies cause overcurrent and overheating in the neutral due to harmonic interference. It is also a very good idea to make sure the switching power supply load are balanced out across your service. Even a modest data center load can put significant strain on a transformer and service entrance that is not designed for it.

What size electrical service do you have, and I assume these loads are intended to be continuous use like a data center type application?
 
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Old 07-21-15, 10:06 AM
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I've already run the conduit (including some annoying bends), and I've already cut the 10 gauge THHN to length

I'm all for doing things the right way and the safe way, but I also don't want to make my life doubly difficult, so I will submit the info to your expertise.

I wouldn't go so far to call this a "data center", but it will be used for computer equipment. I am installing 4 UPS batteries in total, of this variety:

Smart App Series - PR3000LCD

They are hefty, but they aren't monsters. I don't think they will regularly stress a standard 30A circuit, but go ahead and look at the specs and tell me.

I will be installing 2 each on each of the 120V phases to distribute the load. The service is your standard 240V biphasic.
 
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Old 07-21-15, 01:31 PM
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How about the number and type of computers powered by the UPSs?
 
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Old 07-21-15, 02:07 PM
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2 UPSs will be handling a 500watt PC + 2 monitors + a thermal label printer each.

2 UPSs will be handling a 600watt server each and a few low power network devices like network switches, modems, and wireless routers.

The draw on the UPSs will not be very much. I got high quality UPS mostly for runtime and durability/dependability.

The only time there will be high draw on the line will be after an extended outage and all four UPSs are attempting to recharge at the same time.

Note that for the particular conduit we are talking about, we are only talking about 2 of the total 4 UPSs, but I have another conduit with the same situation (for a total of 4 30amp circuits).
 
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Old 07-21-15, 02:33 PM
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Ok, the plan sounds fine other than that your power is way over spec'd for the gear you're running. A single 3kVA UPS, hardwired to a distribution circuit if necessary to reach the connected equipment, would be plenty for that load.

BTW, most UPSs limit the charger to something like 5% of rated load (1-2 amps), so even after an outage there is not a significant change in load on the premises circuit.
 
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Old 07-21-15, 09:38 PM
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yes, the UPS are over-specced for their work, but everything they are running is mission critical, so as I said:

1. Runtime during an extended outage
2. Overall dependibility and reliability of the UPS
3. High quality protection for the connected equipment in terms of surge protection / AVR
4. Possible expansion in the future (but I don't see any future expansion being more than double of what I listed)
 
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Old 07-22-15, 08:04 AM
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Just a thought / idea for what we do at commercial sites which have similar requirements to what you stated. The UPS system is sized to operate at about 80% capacity, which is roughly the sweet spot in terms of inverter efficiency and runtime per dollar spent. Note this is actual load, not nameplate max load. The UPS is then backed with a generator which ends up being cheaper per unit time than adding additional UPS batteries which have higher maintenance and replacement expense.

In your case, a viable option is to put in an 8-10 space generator transfer panel at the end of the 3/4 conduit, and then subsequently power your UPS. The money you save in halving the UPS gear could buy a transfer panel and a brand name 7200W generator giving you, essentially infinite run time as long as you can get gasoline. Something to think about anyway.

As far as surge protection, the best thing you can do is make sure your service entrance is very well grounded, that all utilities, telecom or other conductive paths entering the building use the same single point ground, and you install a "whole house" or "panel mounted" surge protector. These are much more effective than end-of-line surge protection like the kind in power strips and UPSs. AVR is good, but it protects against a different set of power quality problems.
 
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Old 07-22-15, 07:24 PM
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Thanks for the recommendations.

A "whole-house" surge protection system is next to install and I already have a location next to the main panel stubbed out.

Reviewing the grounding is also on the list.

And we plan to get a generator eventually as well but that becomes more expensive as we have to think about running the whole building, not to mention finding a place to put it. It would have to go outside, and that would probably require a small concrete slab, and a fence to prevent someone from running away with it. It also complicates the electrical setup because now we need a transfer switch. I think the up front costs end up being quite a bit more than you are suggesting: probably around $10k to do it right, so that is why we are saving that for last.
 
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Old 07-23-15, 07:50 AM
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For a whole house, permanent natural gas unit yes $10k is in the ballpark. I was thinking more along the lines of a portable 7200W unit and a 10 space transfer panel, total somewhere around $1,500 - 2k.
 
 

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