200 amp rule?

 

220.61 Feeder or Service Neutral Load.
(A) Basic Calculation. The feeder or service neutral load
shall be the maximum unbalance of the load determined by
this article. The maximum unbalanced load shall be the
maximum net calculated load between the neutral conductor
and anyone ungrounded conductor.
Exception: For 3-wire, 2-plwse or 2-phase systems,
the maximum unbalanced load shall be the /'lwximwrl
net calculated load between the neutral conductor and any
one ungrounded conductor rnultiplied by 140 percent.
(B) Permitted Reductions. A service or feeder supplying
the following loads shall be permitted to have an additional
demand factor of 70 percent applied to the amount in
220.61 (B)(1) or portion of the amount in 220.6] (B)(2) determined
by the basic calculation:
(1) A feeder or service supplying household electric
ranges, wall-mounted ovens, counter-mounted cooking
units, and electric dryers, where the maximum unbalanced
load has been determined in accordance with
Table 220.55 for ranges and Table 220.54 for dryers
(2) That portion of the unbalanced load in excess of 200 amperes
where the feeder or service is supplied from a
3-wire dc or single-phase ac system; or a 4-wire, 3-phase,
3-wire, 2-phase system; or a 5-wire, 2-phase system


I don't know how you can size the service, let alone the neutral, if you don't know what the loads would be?

To answer you question, Under part (A) straight 240V loads would be excluded in determining the neutral load. So 240V baseboard heaters, AC condensers & air handlers don't add into the neutral.

(B)(1) allows the 70% reduction under 200A for Ranges, Ovens, Counter mounted cooking units and dryers (which all have small neutral loads). It also says "where the maximum unbalanced
load has been determined in accordance with Table 220.55 for ranges and Table 220.54 for dryers". So the load already has demand factors figured in for multiple appliances, and then you take the 70% reduction.

So I'm guessing that the service neutral load for a 120/240V level 2 car charger would be at 100% for a 200A service?

 

Also, be careful in using the word "derated". To me, that has a different meaning than what you are using it for.

 

Thought about that after the fact.

 

I'm installing 100A services on houses that have had all the wires ripped out by copper thieves. They will have minimum requirements for a green tag. For now it's just a service. I need to return this 2-2-4 wire they slipped me and I need to be on solid ground if they argue with me. Plus, I want to understand the topic better. Really, unless I'm doing a big commercial job, it's cheaper to me to just buy bigger wire than to do all those calculations. Again, if I'm reading this correctly, the unbalanced load only enters into it after 200A which means pretty much never in a single family dwelling.

 

Not mentioned: Code also sets minimums and maximums. The neutral can't be smaller than the ground and does not need to be larger than the phase conductors.

There are some twisty parts to the logic. Everywhere else you must size to 125% of the continuous load plus 100% of the noncontinuous load. But with neutrals, 210.19(A)(1) and 215.2(A)(1) change it to 100% of both. So if you are running the normal ampacity calculations for the phase conductors, the neutral result might be slightly different. That's on top of the 70% reduction above 200 A. I'm not sure why THAT reduction matters though if you are already planning on a 200 A service since the load would be less than 200 A.

Ignoring any true 240 V loads or the reductions for ranges and so forth we are sitting on a calculated load of 200 A. If however you do any of those reductions at all you end up a little lower. If it was exactly 200 A load then you'd be looking at a 3/0 neutral on the 75 C column or just barely 2/0 on the 90 C column. However taking any of these reductions at all into account gets you down to 2/0.

AND just to make it near impossible to correctly reduce the size of a neutral, Code mentions harmonics and suggests to accommodate them or at least not reduce the size of the neutrals when harmonics are present. Residential doesn't seem like a harmonic rich environement but the power supplies for ALL electronics are very harmonic rich: computers, UPS's, solar inverters if you have one, car chargers, HVAC fans and compressors, pool variable speed pumps, etc., all rich in harmonics which could cause an undersized neutral to overheat and burn up. So how do you calculate how much additional current, size, etc., that you need? NEC offers no guidance. In engineering quite frequently you will see references to IEEE 519. BUT if you actually read it two things jump put. The first is that this is not a general purpose standard. It is for utility feeders, not service entrance or general purpose feeders and branch circuits. Second, it sets LIMITS on harmonics, not sizing to accommodate them. There are also some similar things talking about K-factors on transformers with similar nonuseful guidance. So if you can see where this is going, this is why the industrial/commercial practice is tending towards using full size neutrals.

Part of the trouble is the skin effect. As the frequency increases the current tends to flow more along the surface of a cable. At some point the center of the wire isn't even used. In microwave and high frequency work we use coaxial cables and wave guides because the signals are essentially traveling only on the surface of the cable. In aviation they use 400 Hz because it reduces the size and weight of transformers but they have to oversize their cabling.

All that being said I've had 30 years of experience, mostly industrial, with all kinds of crazy power systems. I have never yet experienced an actual bonafide harmonic problem and here is why. Lots of power electronics uses a rectifier for a power supply. So internally the system has a capacitor in series with the diodes in the rectifier. When the line voltage exceeds the capacitor voltage, it starts charging up and we get a current. This ceases as soon as the AC sine wave drops below the capacitor voltage and the diodes block the capacitor from discharging. With a half wave rectifier this happens once per AC cycle but for efficiency reasons we use full wave rectifiers so it happens on both the positive and negative peaks. So we get a double pulse of current or "rabbit ears":

https://3l4sbp4ao2771ln0f54chhvm-wpengine.netdna-ssl.com/wp-content/uploads/2015/11/three-phase-voltage-and-current-input.jpg

Now that obviously looks NOTHING like a sine wave so the %THD is going to be pretty high. As the power draw increases the rabbit ears are going to widen and it will eventually the current just look like a sine wave with a couple odd looking humps. Along with that, the %THD decreases as load increases. Even in systems with no filters at full load %THD drops down to about 6-10% but at very unloaded conditions it can be well over 50%, meaning that over 50% of the power draw is harmonics. Sounds, scary, right?

BUT this is with CURRENT. Normally we don't know what the anticipated load is so we tend to make sure that we make the transformers, service conductors, etc., plenty large. And as long as the transformer and wiring is not overloaded we will see a "stiff bus"...voltage is almost constant. If however we get close to exceeding the transformer capacity then the momentary surge from a large drive (our rabbit ears) can push a transformer to the point where the voltage momentarily dips. When this happens we get voltage harmonics. That's a bad thing and that's why IEEE 519 exists...to give recommendations on when the voltage (not current) harmonics are too high. To be fair it does give limits on current harmonics but they are in terms of %TDD or "demand". %THD limits on current are in terms of transformer capacitor. So if we sized everything correctly though we should have sized the transformer to handle the highest current draw from the drive, which is the point where the %THD is very low (below IEEE 519 requirements). At lower power draw we don't really care about any of this because the transformer capacity will be there. And that's if it is sized very closely to the loads. And that's not accounting for the fact that most of the time you can overload a transformer without harming it but a substantial amount at least for short periods of time. And the same goes for the wiring as well. So when I get a call for a harmonic problem I just take measurements of the VOLTAGE on the line side. So far I have never found a "harmonic problem" doing this. I have found plenty of POWER problems often relating to undersized or failing transformers, bad connections, bad loads, and surges that are darned near impossible to locate a source for, but so far nothing in terms of harmonics. It seems to be an electrical boogie man. I have no doubt that it exists but if it does exist, it is just going to point back to some other problem such as an undersized transformer.

 

it'd be hard to overload #4 neutral on 100 amp service

 

I can't think of any basic single phase service where you'd need a full-size neutral by code.

 

What about the part that says the demand factoring doesn't apply to the first 200A?

 

I used to have a huge pile of #6 copper from repairing parking lot lighting. I would use that for the neutral on hundred amp services. They passed no problem until a guy named Steve became head inspector. We went round a little bit on it, he’s so stuck on “the 70% rule” he won’t listen to the code about only having to be as large as the GEC as long as calculated neutral load doesn’t require larger.

I told him Steve, take into consideration the Frank Adams 4 circuit fuse box. They’re very common here, 4 15 amp circuits. Even older houses have 2, yes, 2 circuits. Let’s say I goofed and put every circuit on the A phase. Even in this worst case imbalance, 100% full load, we’ve got 5 amps to spare.

“If you’re going to do this kind of thing you need to have the calculations on site for the inspector and they will become part of the permit records. No neutral smaller than #4 copper or #2 aluminum on any dwelling.”

That pile of #6 is all used up and gone at this point, and there wasn’t a windfall of money to be saved there, but who doesn’t re-use wire when we can? Just pissed me off he was so dead set on not reading the code. I never paid that re-inspection fee and he no longer returns my calls or emails. Oh well.

My #6 Cu and your #4 Al are perfectly compliant in this scenario. The maximum neutral current will only be the imbalance of the two phases, which could never exceed 100 amps with a 100 amp main. If you had 20 amps on A phase and 30 amps on B phase, your neutral current would be 10 amps. 240v loads will not provide any neutral current, except for possibly an appliance with a neutral for controls, which we would just disregard in this instance since it would be so minimal.

Let’s make a scenario for you on the house you describe:
240V loads:
Electric dryer, 30 amp
Air conditioner, 25 amp
120V loads, let’s assume we just load the breakers in a line:
Lighting. 3 bed, 1 bath let’s say. Cheap fixtures, two 60 watt bulbs per. Maybe 8 fixtures, couple of porch lights gives us two more bulbs. That’s 9 amps. A phase
Furnace. We need the blower all year, that’s what, 6 amps let’s say. B phase

so far, 3 amps on the neutral with every light on and the blower going. You follow?

Fridge, 8 amps A phase

Neutral current is now 11 amps.

This house is vacant, there’s no other appliances. Sure, you’ve got two SABCs, one A, one B. Net zero.

3 bedroom, 1 living room. A,B,A,B. Net zero.

You can get as detailed into this as you want to be, fill in all the kitchen appliances, put fans and TVs in the bedrooms. You’ll find it’s darn near impossible to reach our 65 amp limitation. Even if you goofed and put every one of your breakers on one phase, you wouldn’t do it. That’s why we still see hundreds of services a year with that bare #10 neutral serving faithful even after all this time.

 

Your renovating shell homes with out a set of approved drawings. You got more guts that I have.

 

Drawings? Hilarious! Look up Detroit Land Bank. My biggest fear is being robbed of all my tools while down there.

 

low income anything here has gone through the political machine so everyone gets their part of the pie.

 

Lots of scamming going on. The city workers skim off the best properties before the public gets to bid.