# 208Y/120 to single phase 120

Howdy -
We're in an industrial park where our space is supplied with 208Y/120 3phase power. My understanding is that any phase taken to the fourth wire
neutral gives 120V. And any phase taken to any other phase gives 208V. My question is about the amps available on any given phase - neutral (120V) connection. The service is 120/208@100A . Is it so that three feeds of 120V (A - Neutral, B - Neutral, and C - Neutral) would each be able to carry 100A?
I guess, in other words, what I am trying to ask.. is that from a 120V perspective.. is 208Y/120 @ 100A equivalent to three 120V @100 single phase circuits?
Thanks -M
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single

Yes. Each phase can provide 100 amps, regardless of whether you connect loads as three-phase, single-phase line to line, or single-phase line to neutral (or any combination).
Ben Miller
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Benjamin D. Miller, PE
B. MILLER ENGINEERING
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| single |> phase circuits? |> |> Thanks |> -M | | Yes. Each phase can provide 100 amps, regardless of whether you connect | loads as three-phase, single-phase line to line, or single-phase line to | neutral (or any combination).
That would be for one single phase load. Multiple L-N loads work out easy because you just add up the amps separately on each phase. It gets more complicated when dealing with a couple of L-L loads because the amps don't add straight on the common wire for the loads in different phases. If all the L-L loads are equal, you can run them up to 57.735 amps and will see the 100 amps on the actual phase lines. But as long as your loads are all just 120 volt, they are all L-N, and it's simple: you get 100 amps on each wire relative to neutral, and (short of harmonics or certain weird reactive combinations) the neutral balances out.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Phil
All true, but none of that matters when considering the question of maximum supply current. For that supply system, 100 amps is the maximum total current that you can draw on any line, regardless of how you connect the loads.
Ben Miller
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Benjamin D. Miller, PE
B. MILLER ENGINEERING
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Ben Miller wrote:

I'd add one small caveat to that. Its possible that the panel feeder may have been sized to supply a group of balanced three phase in addition to single phase loads. Under certain conditions, the NEC will allow the neutral to be reduced so long as it can carry the maximum unbalanced load. In such a case, loading one phase to its maximum capacity will overload the neutral.
Generally, I don't downsize neutrals serving tennant spaces. Even if the original design includes significant 3 phase balanced loads, one can never be certain what the next tennant will modify.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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NEC is one thing and local codes are another. You won't be getting away with downsizing neutrals around here. Downsize grounds? Sure anytime.
MLR Northern CA
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Hi Phil -
Thanks for the answer. In the L-L case that you mentioned, how does one calculate the 57.735A value you gave? To clarify my understanding of that case.. I could pull 208V @57.735A from all combinations of A-B, B-C, and C-A simultaneoulsly from the given 208Y/120@100A supply?
-Jim

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supply?

------------- For a delta connected balanced 3 phase load, drawing 57.7A at 208V, the line current will be the phasor sum of the currents in the two connected legs. This would be 57.7*root(3)0A Note that the total power is 57.7*208*3 =3.6KW vs 3*120*100 36KW for a Y load. No difference.
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Don Kelly snipped-for-privacy@shawcross.ca
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36kW ?
Ben Miller
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Benjamin D. Miller, PE
B. MILLER ENGINEERING
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Over complicated.
It's simple. You can't go over 100 Amps on ANY hot leg in ANY combination.
Mike R Electrical Contractor Northern CA
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