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I'm trying to wrap my mind around how many amps 220V wire (two hot poles) can supply, when it's effectively supplying a multi wire branch circuit (sub panel)...

My question is this: if the sub panel is supplied by a single 60 amp two pole (220v) breaker (4/3 wire) circuit, can I (theoretically) put in two 60 amp single pole breakers in the sub panel (one for each pole), effectively making a multi-wire branch circuit within the sub-panel? Or, should I understand that the sub panel can only support up to two (theoretical) 30 amp 110v circuits?

thanks so much!

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Jeffrey is correct. You will have 60 amps of 240 volt power available.

As to the use of single pole breakers for a multiwire branch circuit, that is a code violation unless the breakers are tied together with a handle tie from the manufacturer. You can't use tie wire, a nail, or any other unapproved method. Better yet, you can use a common trip 2 pole breaker. This is the preferred method.

Informational Note: 110 and 220 volts are not normally available now. The voltages keep moving up. Most equipment is now rated for 125 and 250 and the National Electrical Code lists 120/240 or 120/208 as the nominal voltages for dwelling units.

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    In almost any breaker made, the handle-tie will also have the effect of common trip. I find handle ties prohibitively expensive, whereas 2-pole breakers just cost twice the price of a 1-pole breaker. Also, handle-ties are not an option if you need AFCI or GFCI. Really an MWBC needs common maintenance shutoff but not common trip. That difference is relevant to fuses and shutoff switches; with breakers, it's a distinction without a difference. – Harper - Reinstate Monica Feb 15 '18 at 20:21
  • Thanks, yes, poles are tied together... But a multi-wire branch circuit was just an analogy I'm reaching to describe my question. I guess the question is closer to this: Does a two pole breaker supply 60A to each 110V pole? thanks! – Madumi Feb 16 '18 at 1:07
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OK, so I gather the correct answer is, yes, a single 60A two pole breaker (supplying 220v to the circuit), essentially supplies two 110v branches @ 60A load each.

i.e. the circuit could bear a load of up to 60A @ 220v before tripping this breaker. Or a load of 120A @ 110v (i.e two loads of 60A @ 110v on each branch) before tripping.

Correct me if I'm wrong :-)

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    Did you mean to say "two loads of 60A @110V, one on each branch?" – ThreePhaseEel Feb 20 '18 at 3:03
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    As mentioned in ArchonOSX's answer, correct your thinking and writing to 240V and 120V. Most days my voltmeter shows 242 or 243, in fact. – Ecnerwal Feb 20 '18 at 3:05
  • @ThreePhaseEel Yes indeed, two loads of 60A @ 120V, one on each branch... I'll edit the post. – Madumi Feb 21 '18 at 2:38
  • @Ecnerwal Haha, yup, my bad 120v/240v... Don't know why I keep that in my head... – Madumi Feb 21 '18 at 2:41
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Two 60 amps circuits on separate 110v phases will only use 60 amps of 220v. The 60a current will come from one phase and return on the other one. The neutral will see no current. So, no need to split in two 30a.

When a single circuit is in use, the current will come from one phase and return via the neutral. Still, that will not go over 60amps.

In the general case, if one circuit use x amps and the other y, you'll see the following currents:

  • hot phase 1: x amps
  • hot phase 2: y amps
  • neutral: |x-y| amps

Since x and y are below 60 amps, the three values are also below 60 amps.

(I only know the theory of electricity, for the legal/code side of it, see the other answer)

(And even then, it is quite likely you'd be allowed more breakers as the breakers are there to protect the wiring. As long as the wiring after the breaker is fine for it. A typical house has all breakers summing up to more than the main breaker)

  • Good work but this might be a little bit easier. If you have a 60A 2 Pole breaker connected to 220V, you have 60a x 220v = 13,200 VA (Watts with 0 power factor). That means you have 13,200 available watts. So if you divide the available watts by 120V you get 110 amps on a perfectly balanced Panel. It doesn't matter how you divide it up so long as you never exceed your available wattage. – Retired Master Electrician Feb 15 '18 at 20:57
  • Thanks Jeffrey for your reply... Can I hone the question a bit? I'm aware that the supply provides 60A of 220V. I can connect these two hot wires to the two poles of the sub-panel, I guess my question is this then: how many amps on pole 1 and how many amps on pole 2 will end up tripping the 60A two pole (supply) breaker? – Madumi Feb 16 '18 at 1:00
  • Thanks also Retired Master Electrician... converting into Watts & back out makes it more understandable, thx! Although, that said, for wire thickness, it's the Amps that count right? – Madumi Feb 16 '18 at 1:38
  • @Madumi - Yes you size conductors per the protection of the feeder, meaning in your case,you need to furnish wire capable of 60A per phase. That would be a #6 copper or a #4 aluminum. From NEC Table 310.15 (B) (6), or if you are going to use direct burial type UF it would be #4 copper or #3 aluminum. Good luck – Retired Master Electrician Feb 16 '18 at 16:11
  • @ Retired Master Electrician - OK, so, I guess I am used to estimating the size of a circuit by the sum of the amps it can carry. Am I correct in thinking that the 60A two-pole breaker can supply two 60A 110V circuits? i.e. it would take > 120A gross load at 110V to trip the breaker (i.e. over these two branched 110V circuits)... Or, > 60A load at 220V to trip the breaker (using the two poles in tandem) – Madumi Feb 17 '18 at 2:02

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