Concuctor Sizing

Bkessler

I want to run a 100 amp 3 phase 110 - 208 sub panel in a retail store for a food stand. I am not sure how to calculate my feeder size. The panel is 90 feet from the source, I was planning on running #2's but was not sure if #4 would be ok or not. What is the right way to calculate feeder size?

Magnettica

VA/ Volts x 1.73

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Magnettica

Oh, conductor (feeder) size. My bad.

Assuming all of the equipment is rated for 75º, #3 copper is rated for 100 amps. You could use #4 copper which is good for 85 amps but it would need to be protected by a 90 amp circuit breaker.

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Bkessler

Is that just 100% of max load and the 75c of 310.16?

Magnettica

Not sure what your question is. The 75º column is for equipment rated at 75º Celcius. It means that as long as long as use insulation from that column (RHW, THHW, THW, THWN, XHHW, USE, ZW), and both of your panels are rated for 75º, then you're good to go.

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newenergy

(you are about to witness my ignorance in its full splendor)

How does a 90 amp circuit breaker protect conductor rated for 85 amps?

Also, does this just come from 310.15(B)(6) where 100A feeder rating can use #4 copper?

Is the conductor size allowable smaller for a feeder because it is less likely to be used to capacity? That makes a little sense, but it is also going to be under continuous load which might make you want to derate it???

Magnettica

Great question....

It has nothing to do with derating unless there are more than three current carrying conductors (CCC) in a raceway. Derating depends on the ambient temperature of the area the wires are installed and the insulation of the conductor.

As for the 90 amp breaker protecting a conductor rated for 85 amps.... NEC article 240.4 (B) specifically permits this. It says.....

240.4(B) Devices Rated 800 Amperes or Less.

The next higher standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be
used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a mul-
tioutlet branch circuit supplying receptacles for cord-
and-plug-connected portable loads.

(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rat-
ing (but that shall be permitted to have other trip or
rating adjustments).

(3) The next higher standard rating selected does not ex-
ceed 800 amperes.

So, if your load is less than 800 amps, for instance say 176 amps, the correct circuit breaker size would be 200 amps.

Hope that helped.

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newenergy

Mag,

Interesting. So for lower amps it just doesn't come up in say a lighting circuit (as opposed to a outlet circuit) because the ampacities generally match the standard breaker sizes?

But, say you had a lighting circuit where the conduit ran through an area exposed to 48 degree C and were using 12 AWG copper (75C THWN eg). You would have ampacity = 15A * .82 = 12.3A and you could then use a 15A breaker?

As far as continuous loads and deration go, in Solar PV people derate conductors for continuous load (per 210.19(1)/210.20(A)) to 80% of their ampacity.

This seems to be standard, but I can't swear it's not a misapplication or perhaps most conductors meet the exception of being rated for continuous use at 100 percent.

Thank you sir.

InPhase277

If the conductor were rated only THWN, then you'd start at the table ampacity of 25 A and derate from there. 48 C would be a factor of 0.75. So The conductor would really be able to carry 18.75 A. You would use a 20 A breaker. Lighting circuits are rated at 125% of the load.

The 80% factor comes into play because of the 125% conductor overrating and the overcurrent device itself. The conductors are rated for 100%. It is the rest of the equipment that is not, i.e. a circuit breaker.

InPhase277

rick boyd

Isn't that just nec 316-b

newenergy

Why is this? Table 31.16 still has the "*" to use 240.4(D) for THWN for ampacities for small conductors.


Are these just things that are generally known to be true (by electricians and inspectors), are in the NEC somewhere, or is the info provided from the manufacturer?

(690.8 - The conductors in PV circuits are sized to carry 156% of the short circuit current which is 125% twice - The NEC doesn't explain why but John Wiles, who wrote most of Article 690, explains that once is because of possible periods of very high production from the panels and the other is for continuous load. From one of his articles on suggested practice "Most conductors used in electrical power systems are restricted from operating on a continuous basis at more than 80% of their rated ampacity [210.19, 215.2, 690.8].")

Thanks for your help. I'm not trying to be annoying, just trying to get to the bottom of this.

InPhase277

240.4(D) applies to selecting overcurrent devices for the conductors with the [*] on them, not to the ampacity of the wire itself. I don't have my code book handy at the moment, so I can't verify where in the code to look, but when derating, you start at the table ampacity, then apply adjustment factors such as ambient temp and conduit fill from there. In some cases after derating you may end up with an ampacity higher than that allowed by 240.4(D). But you still must obey that article.

This is one reason we love THHN. We can start derating at 30 A for #12 copper.

InPhase277

newenergy

Ok, but say you require 30A for a circuit and are using THHN. It's running through an environment that never gets hot and doesn't need conduit fill deration. #12 copper can handle the 30A, but from 240.4(D) you would have to use 20A overcurrent protection on #12 copper, right? Wouldn't this force you to use #10 copper anyway?

InPhase277

Yes, it would. But let's say you are running a 20 A circuit through an area where the ambient temp is say 40 C. I think the derate factor for THHN at 40 C is 0.91 (but don't hold me to it), and you have 6 current carrying conductors in the pipe, which means you have to derate to 80%. Well, starting at 30 A for #12 THHN, you derate for temp:

30 A x 0.91 = 27.3 A

Then derate for conduit fill:

27.3 A x 0.80 = 21.84 A

What this means is that, after correction for temp and conduit fill, you can STILL use #12 for your circuit, but you are required to put it on a 20 A breaker. You have already sized the circuit 125% larger than the load. This limits the continuous load to 80% of the circuit rating, which would be 16 A.

InPhase277

newenergy

InPhase,

Thanks very much and assuming I'm reading the right table and reading it correctly you are right about the 0.91.

Magnettica

240.4 (D) says....

(D) Small Conductors. Unless specifically permitted in
240.4(E) or 240.4(G), the overcurrent protection shall not
exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG,
and 30 amperes for 10 AWG copper; or 15 amperes for
12 AWG and 25 amperes for 10 AWG aluminum and
copper-clad aluminum after any correction factors for
ambient temperature and number of conductors have
been applied.

The derating charts in which InPhase277 speaks of can be found in table 310.16 for correction factors (ambient temperature). The adjustment factors (conduit fill) can be found in Table 310.15(B)(2)(a).

Hey...... no one said this would be easy.

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