Neutral harmonics on lighting circuit with 'hi frequency' ballsast?

[ramsy]

Greetings group
For a 4 wire lighting circuit powering a HID lighting load, that uses hi frequency ballasts, do any of you up-size the neural for harmonics?
Circuit size is 20Amp 4 wire 480/277. Load is about 9500 VA and with the continuous factor ~11875 VA (or ~15A)

Here is an example ballast:

Gavita Pro 1000 DE 277-347 | Gavita – Retail

Gavita Pro 1000 DE 277-347

gavita.com

EDIT: I should add the balasts are all 277V L-N loads

Also late to this party, but I believe 210.18(B) must be used first. Nameplate Amps, rather than VA, properly sizes the hot conductors.

3.8 x 1.25 for continuous load = 4.75A each, 4 lights per 20A Circuit.

Finally harmonics are considered on the neutral, per 210.61(C) Informational note, which may require some engineering assistance.

 

[tortuga]

Also late to this party, but I believe 210.18(B) must be used first. Nameplate Amps, rather than VA, properly sizes the hot conductors.

3.8 x 1.25 for continuous load = 4.75A each, 4 lights per 20A Circuit.

4 lights per 277 leg, 12 per 4 wire circuit.

210.61(C) Informational note, which may require some engineering assistance.

Yep thats what MH forums are for

 

[ramsy]

4 lights per 277 leg, 12 per 4 wire circuit.

Yep thats what MH forums are for

I'd be tempted to see how far that #12 neutral can be adjusted for extra current before its rated ampacity is exceeded.

For example, if #12 THWN-2 is rated for 25A @ 75°C, then metered neutral load with neutral harmonics could max out <= 18 Amps x 1.25 = 22.5A, without violating 240.4(D), nor exceeding 80% breaker rating in violation of 210.20(A).

 

[tortuga]

Interesting, thanks
As fellon pointed out in post#2 the spec sheet said 10% THD max,
so I'd imagine if I were to meter it with a quality true RMS ammeter the neutral should be no more than 10% higher than the phases.

 

[ramsy]

Ok, this gets complicated with multi-phase wiring.

If thats 10% of 3.8A for 12 lights per full boat, then 0.38 x 12 = 4.56A on the balanced neutral.

However, the neutral cancels out current from 3 balanced 277/480Y phase legs, so metered #12 neutral shows Zero to perhaps 4.6Amps max.

You would need to meter neutral with only 1 hot leg connected, for worse case scenario, if lights could fail in such a manner.

With 2 legs of 277/480Y disconnected the calculated load of 3.8A x 4 lights = 15.2 x 1.25 for continuous loads = 19A.

Now lets add 10% THD harmonic load on neutral @ 0.38 x 4 lights = 1.52A + 15.2 = 16.72 metered worse case scenario.

Factoring required 1.25% for continuous loads, after harmonics, would be 20.52 Amps, which should be less than rated ampacity of THWN-2 @ 75°C

 

[Flicker Index]

Interesting, thanks
As fellon pointed out in post#2 the spec sheet said 10% THD max,
so I'd imagine if I were to meter it with a quality true RMS ammeter the neutral should be no more than 10% higher than the phases.

I don't even remember the exact computation. Non-sinusoidal, unequally loaded wye circuit neutral current is complex it's best left to power quality analyzer's computer.

The maximum neutral current occurs when it is single phased, i.e. if you have 20A 208Y120 and you have 20A on L1 to N, or 20A each on L1 to N, 20A each on L2 to N, then the N will carry 20A. If all three phases are carrying 20A, the neutral will be quite low. If they're heating elements, it will be essentially zero. If they're electronic LED ballasts, then it might be a few amps due to non-sinusoidal nature.

 

[tortuga]

Interesting discussion thanks,
What if one had the same kinda load 480V or 240V delta connected, with no neutral with THD be an issue?

 

[Flicker Index]

Interesting discussion thanks,
What if one had the same kinda load 480V or 240V delta connected, with no neutral with THD be an issue?

It would still make lesser efficient utilization of transformer, which is the reason why utility imposes poor power factor penalty.

 

[tortuga]

Right, yeah and the most distortion that could add is two phases not three.
I have seen parking lot lights 'delta' connected like that.

 

[phaset]

Right, yeah and the most distortion that could add is two phases not three.
I have seen parking lot lights 'delta' connected like that.

Pretty common in industrial settings to see Delta-configured pole lighting, with no option for Wye.

Source: I used to design light fixtures for outdoor industrial applications.

 

[Flicker Index]

Pretty common in industrial settings to see Delta-configured pole lighting, with no option for Wye.

Source: I used to design light fixtures for outdoor industrial applications.

Industrial outdoor fixtures are usually high wattage discharge lamps and you don't find transistorized ballasts in those extremely high poles. The resiliency of the core-and-coil ballast is a must in those applications as the cost to replace fried out ballast is too prohibitive.