Wire derating/branched circuit question

[nwhomesteader]

I'm wiring my shop with 3/4 emt and #10 wire. So my plan was to run a multi wire branched circuit from a double pole breaker. 2 hots and a nuetral.

I have half the shop done and have wired several junction boxes with 2ea 110v outlets. The left outlets off one leg and right outlets off the other leg with a shared nuetral. So basically 2 separate circuits in each box.

I intend on installing a few 220v outlets off the same circuit. The problem is that the 110 outlets are of course only rated at 20a so I can't run a 30a breaker to take full advantage of the #10 wire. Therefor I installed a 20a breaker. This will probably be fine for most tools but if I get a power hungry tool in the future, I may just run another set of #10's and a 30a breaker for dedicated 220.

It's a one man shop so there won't be multiple things run at once. My questions are: do I need to derate the dedicated 220 circuit from 30a to 25a if I run it in the same conduit as my branched circuit?

There would be 4 hot conductors and a single shared nuetral for the branched circuit.

And do you see any other issues I have missed? All outlets are individually grounded to the boxes and emt is continuous to the sub panel.

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[sberry]

How big is this shop? The number 10 isn't always a great idea for smaller circuits, it usually wont hurt but it doesn't help much either . A lot of these schemes seem better than the are, you will likely find you never need 2 circuits from the same box. Use the 10 you have for a 240 and find some 12 for the generals. You csould not be running dual voltage from the same wire.
A lot of good ideas,,, too many. This should be a simple system with a 120 circuit to a box, you can pass others thru.

 

[sberry]

I have some multi wire branches but used sparingly and where it makes sense. I don't want them on most generals, a chop saw or small air comp trips. A 10 wire makes them worse. You cant modify one if needed without pulling another wire.

 

[Norcal]

How are you going to do the required GFCI's?

 

[pattenp]

Personally I'm not a fan of multiwire circuits just to save a few buck on wire. I think 240V should be an independent circuit from 120V, just my personal choice. If you do use use the MWBC for both 120 and 240 be sure to use a common trip breaker and not two independent trip breakers with handle ties. Since you are powering line-to-line loads the overload device needs to trip both legs at the same time.

 

[wyliesdiesels]

Derating is done with the 90* c ampacity. So 40a * .80 = 32a.

Im with pattenp on this one. Run separate circuits for the 240v outlets.

 

[nwhomesteader]

Thank you guys for your responses.

The shop is 24x48, standard tools, drill press, table saw, chop saw, router table, jointer, etc.

The gfci I forgot about until after I put the first outlets in but I was going to put one the start of each circuit. Unless I have that wrong.

I'm curious why a chop saw would trip your MWBC? Wouldn't that have to do with the breaker size and if you are running other things on the same circuit at the same time? The breaker protects the wire right? Why would #10 make anything worse? Just trying to understand.

I hear what you are saying about #10 not helping much on a regular 110v circuit. So my original plan was to just have 2 double pole beakers (yes they are common trip)..one for each side of the shop running 220v and 110v because right now I have ugly Romex and too much of it. I went overkill on circuits years ago and it made no sense. So I wanted to consolidate and make it look neater. I doubt I'll ever have 2 tools going at the same time with the exception of a dust collector, thats why only 2 circuits.

So my thought was 30a breakers. Then I suddenly realized, hey dummy, 120v outlets can only handle 20a so you can't run 30a breakers for everything. Therefor I used 20a breakers.

This wasn't necessarily about saving money on wire. I bought big spools (I think 250ft) of #10. Plus how many wires can you run in a single conduit before you actually need #10 for a 20a 120v circuit? Out if curiosity. It's probably a ridiculous amount I'm sure.

I may just do what you guys are saying and run separate 220v circuits. But I'll leave the MWBC alone that I have on the #10 since It's already there and working. Buying extra #12 wire would be more expensive at this point.


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[nwhomesteader]

Derating is done with the 90* c ampacity. So 40a * .80 = 32a.

Im with pattenp on this one. Run separate circuits for the 240v outlets.

Thank you! I have no idea where people are getting their amp ratings on wire when derating though! This is exactly why I was asking for help. I see people derating #12 wire down from 30a and now #10 down from 40a? All the tables for amp ratings I saw were 20a for #12 and 30a for #10.

I believe you by the way, but where are these ratings coming from? I try to increase my knowledge base on every project I do so I know for future reference.

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[wyliesdiesels]

Thank you! I have no idea where people are getting their amp ratings on wire when derating though! This is exactly why I was asking for help. I see people derating #12 wire down from 30a and now #10 down from 40a? All the tables for amp ratings I saw were 20a for #12 and 30a for #10.

I believe you by the way, but where are these ratings coming from? I try to increase my knowledge base on every project I do so I know for future reference.

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U need to take a look at an ampacity chart in the NEC- 310.15(b)(16)

Theres different ampacities for different wire types in the same gauges.

Here ya go:

http://www.barr-thorp.com/wp-content/uploads/2011/04/Digest-176-NEC-Tables.pdf

The ampacities u listed are for 60* c terminations. NM-b is limited to 60* c ratings.

#14, #12, and #10 are also limited to 60* c ampacities in certain applications.

However, when derating, u use the 90* c ampacity rating.

 

[nwhomesteader]

U need to take a look at an ampacity chart in the NEC- 310.15(b)(16)

Theres different ampacities for different wire types in the same gauges.

Here ya go:

http://www.barr-thorp.com/wp-content/uploads/2011/04/Digest-176-NEC-Tables.pdf

The ampacities u listed are for 60* c terminations. NM-b is limited to 60* c ratings.

#14, #12, and #10 are also limited to 60* c ampacities in certain applications.

However, when derating, u use the 90* c ampacity rating.

You're awesome, thank you sir!

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[sberry]

The reason I caution about big wire on a circuit like a common saw that close to the panel is it helps "dump" current so quick it adds to breaker trips.
If it is part of a mwbc both legs trip, you trip 2 circuits.

 

[nwhomesteader]

The reason I caution about big wire on a circuit like a common saw that close to the panel is it helps "dump" current so quick it adds to breaker trips.
If it is part of a mwbc both legs trip, you trip 2 circuits.

Ah I see. Does that mean smaller wire or longer runs of wire is almost acting like a resistor during surges?

A friend of mine has mentioned something like this when he hosted wedding at his house and the caterers were plugging all their equipment in. It didn't make sense to me to have it farther from the panel but they said it was better.

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[sberry]

This usually apply with motors or welders. Big surges. Not the same as resistive or constant loads etc.

 

[nwhomesteader]

This usually apply with motors or welders. Big surges. Not the same as resistive or constant loads etc.

Got it. Thank you.

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[sberry]

The 20A breaker on a 12 wire allows some headroom for starts and a bit for combined loads but most work from 120V is done from a 15A plug and a 14 or smaller wire. There is specialized equipment but normally 20A circuits do not have 20 on them,,, there is a bit tendency to forget this with calculations.
Many are looking at V drop tables, 20A at 100 ft, that is not the case here and even a long load will be 60 ft.
Using the right wire fits the listed terminations. The need for multiple outlets on different circuits is blown way out of proportion. I have more stuff than most people have,,, if I don't need it you probably don't either. I had a reception in my shop, 1 case in 20 yrs it would have saved running a cord to an extra coffee pot for 4 hrs.

 

[nwhomesteader]

The 20A breaker on a 12 wire allows some headroom for starts and a bit for combined loads but most work from 120V is done from a 15A plug and a 14 or smaller wire. There is specialized equipment but normally 20A circuits do not have 20 on them,,, there is a bit tendency to forget this with calculations.
Many are looking at V drop tables, 20A at 100 ft, that is not the case here and even a long load will be 60 ft.
Using the right wire fits the listed terminations. The need for multiple outlets on different circuits is blown way out of proportion. I have more stuff than most people have,,, if I don't need it you probably don't either. I had a reception in my shop, 1 case in 20 yrs it would have saved running a cord to an extra coffee pot for 4 hrs.

Yeah that makes sense especially considering there's not going to be multiple things running. I actually didn't realize there were different plugs for 20 amp devices. Wondered about the funny looking horizontal part on one side of the 20 amp 120v receptacle. I've never used anything that had that type of plug on it.

I probably should have just bought #12 wire, 15 amp receptacles, and a couple shorter spools of #10 for dedicated 240v. Oh well though, I've learned things thanks to people willing to answer my questions.

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[sberry]

Its easy to make mistakes at this point, The avg guy doesn't do it often enough or see enuf variations to come up with the easiest way. I have made about every one of the mistakes. I been at it 35 yrs and just starting to get good at it.

 

[bjcouche]

To clarify, your question of how many wires can be in the same conduit with derating:

4-6 conductors = 80%
7-9 conductors = 70%

You already saw that 10awg is rated for 40A at 90C for derating purposes...
This means: you can have 6 10awg current carrying conductors in the same conduit on 30A breakers.
Or you if you had 7-9 conductors, you would only be allowed 40*.7=28A.
Current carrying conductors means the hot and neutral wires but not the ground.

If you have a mix of 10 and 12awg in the same conduit, each wire is derated by the same multiplier. For example, if you had 8 conductors in a conduit, and some were 10awg and some 12awg, the 10awg would be rated for 28A and the 12awg would be rated for 30*.7=21=20A.

These calculations are of course assuming THHN wire rated for 90C....

Brian